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• Code Golf: Evaluating Mathematical Expressions

  - by Noldorin

  Challenge Here is the challenge (of my own invention, though I wouldn't be surprised if it has previously appeared elsewhere on the web). Write a function that takes a single argument that is a string representation of a simple mathematical expression and evaluates it as a floating point value. A "simple expression" may include any of the following: positive or negative decimal numbers, +, -, *, /, (, ). Expressions use (normal) infix notation. Operators should be evaluated in the order they appear, i.e. not as in BODMAS, though brackets should be correctly observed, of course. The function should return the correct result for any possible expression of this form. However, the function does not have to handle malformed expressions (i.e. ones with bad syntax). Examples of expressions: 1 + 3 / -8 = -0.5 (No BODMAS) 2*3*4*5+99 = 219 4 * (9 - 4) / (2 * 6 - 2) + 8 = 10 1 + ((123 * 3 - 69) / 100) = 4 2.45/8.5*9.27+(5*0.0023) = 2.68... Rules I anticipate some form of "cheating"/craftiness here, so please let me forewarn against it! By cheating, I refer to the use of the eval or equivalent function in dynamic languages such as JavaScript or PHP, or equally compiling and executing code on the fly. (I think my specification of "no BODMAS" has pretty much guaranteed this however.) Apart from that, there are no restrictions. I anticipate a few Regex solutions here, but it would be nice to see more than just that. Now, I'm mainly interested in a C#/.NET solution here, but any other language would be perfectly acceptable too (in particular, F# and Python for the functional/mixed approaches). I haven't yet decided whether I'm going to accept the shortest or most ingenious solution (at least for the language) as the answer, but I would welcome any form of solution in any language, except what I've just prohibited above! My Solution I've now posted my C# solution here (403 chars). Update: My new solution has beaten the old one significantly at 294 chars, with the help of a bit of lovely regex! I suspected that this will get easily beaten by some of the languages out there with lighter syntax (particularly the funcional/dynamic ones), and have been proved right, but I'd be curious if someone could beat this in C# still. Update I've seen some very crafty solutions already. Thanks to everyone who has posted one. Although I haven't tested any of them yet, I'm going to trust people and assume they at least work with all of the given examples. Just for the note, re-entrancy (i.e. thread-safety) is not a requirement for the function, though it is a bonus. Format Please post all answers in the following format for the purpose of easy comparison: Language Number of characters: ??? Fully obfuscated function: (code here) Clear/semi-obfuscated function: (code here) Any notes on the algorithm/clever shortcuts it takes.

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• Using XStream to deserialize an XML response with separate "success" and "failure" forms?

  - by Chris Markle

  I am planning on using XStream with Java to convert between objects and XML requests and XML responses and objects, where the XML is flowing over HTTP/HTTPS. On the response side, I can get a "successful" response, which seems like it would map to one Java class, or a "failure" response, which seems like it would map to another Java class. For example, for a "file list" request, I could get an affirmative response e.g., <?xml version="1.0" encoding="UTF-8"?> <response> <success>true</success> <files> <file>[...]</file> <file>[...]</file> <file>[...]</file> </files> </response> or I could get a negative response e.g., <?xml version="1.0" encoding="UTF-8"?> <response> <success>false</success> <error> <errorCode>-502</errorCode> <systemMessage>[...]AuthenticationException</systemMessage> <userMessage>Not authenticated</userMessage> </error> </response> To handle this, should I include fields in one class for both cases or should I somehow use XStream to "conditionally" create one of the two potential classes? The case with fields from both response cases in the same object would look something like this: Class Response { boolean success; ArrayList<File> files; ResponseError error; [...] } Class File { String name; long size; [...] } Class ResponseError { int errorCode; String systemMessage; String userMessage; [...] } I don't know what the "use XStream and create different objects in case of success or error" looks like. Is it possible to do that somehow? Is it better or worse way to go? Anyway, any advice on how to handle using XStream to deal with this success vs. failure response case would be appreciated. Thanks in advance!

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• How can I turn a string of text into a BigInteger representation for use in an El Gamal cryptosystem

  - by angstrom91

  I'm playing with the El Gamal cryptosystem, and my goal is to be able to encipher and decipher long sequences of text. I have come up with a method that works for short sequences, but does not work for long sequences, and I cannot figure out why. El Gamal requires the plaintext to be an integer. I have turned my string into a byte[] using the .getBytes() method for Strings, and then created a BigInteger out of the byte[]. After encryption/decryption, I turn the BigInteger into a byte[] using the .toByteArray() method for BigIntegers, and then create a new String object from the byte[]. This works perfectly when i call ElGamalEncipher with strings up to 129 characters. With 130 or more characters, the output produced is garbled. Can someone suggest how to solve this issue? Is this an issue with my method of turning the string into a BigInteger? If so, is there a better way to turn my string of text into a BigInteger and back? Below is my encipher/decipher code. public static BigInteger[] ElGamalEncipher(String plaintext, BigInteger p, BigInteger g, BigInteger r) { // returns a BigInteger[] cipherText // cipherText[0] is c // cipherText[1] is d BigInteger[] cipherText = new BigInteger[2]; BigInteger pText = new BigInteger(plaintext.getBytes()); // 1: select a random integer k such that 1 <= k <= p-2 BigInteger k = new BigInteger(p.bitLength() - 2, sr); // 2: Compute c = g^k(mod p) BigInteger c = g.modPow(k, p); // 3: Compute d= P*r^k = P(g^a)^k(mod p) BigInteger d = pText.multiply(r.modPow(k, p)).mod(p); // C =(c,d) is the ciphertext cipherText[0] = c; cipherText[1] = d; return cipherText; } public static String ElGamalDecipher(BigInteger c, BigInteger d, BigInteger a, BigInteger p) { //returns the plaintext enciphered as (c,d) // 1: use the private key a to compute the least non-negative residue // of an inverse of (c^a)' (mod p) BigInteger z = c.modPow(a, p).modInverse(p); BigInteger P = z.multiply(d).mod(p); byte[] plainTextArray = P.toByteArray(); String output = null; try { output = new String(plainTextArray, "UTF8"); } catch (Exception e) { } return output; }

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• Understanding PTS and DTS in video frames

  - by theateist

  I had fps issues when transcoding from avi to mp4(x264). Eventually the problem was in PTS and DTS values, so lines 12-15 where added before av_interleaved_write_frame function: 1. AVFormatContext* outContainer = NULL; 2. avformat_alloc_output_context2(&outContainer, NULL, "mp4", "c:\\test.mp4"; 3. AVCodec *encoder = avcodec_find_encoder(AV_CODEC_ID_H264); 4. AVStream *outStream = avformat_new_stream(outContainer, encoder); 5. // outStream->codec initiation 6. // ... 7. avformat_write_header(outContainer, NULL); 8. // reading and decoding packet 9. // ... 10. avcodec_encode_video2(outStream->codec, &encodedPacket, decodedFrame, &got_frame) 11. 12. if (encodedPacket.pts != AV_NOPTS_VALUE) 13. encodedPacket.pts = av_rescale_q(encodedPacket.pts, outStream->codec->time_base, outStream->time_base); 14. if (encodedPacket.dts != AV_NOPTS_VALUE) 15. encodedPacket.dts = av_rescale_q(encodedPacket.dts, outStream->codec->time_base, outStream->time_base); 16. 17. av_interleaved_write_frame(outContainer, &encodedPacket) After reading many posts I still do not understand: outStream->codec->time_base = 1/25 and outStream->time_base = 1/12800. The 1st one was set by me but I cannot figure out why and who set 12800? I noticed that before line (7) outStream->time_base = 1/90000 and right after it it changes to 1/12800, why? When I transcode from avi to avi, meaning changing the line (2) to avformat_alloc_output_context2(&outContainer, NULL, "avi", "c:\\test.avi"; , so before and after line (7) outStream->time_base remains always 1/25 and not like in mp4 case, why? What is the difference between time_base of outStream->codec and outStream? To calc the pts av_rescale_q does: takes 2 time_base, multiplies their fractions in cross and then compute the pts. Why it does this in this way? As I debugged, the encodedPacket.pts has value incremental by 1, so why changing it if it does has value? At the beginning the dts value is -2 and after each rescaling it still has negative number, but despite this the video played correctly! Shouldn't it be positive?

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• jquery anchor to html extract

  - by Benjamin Ortuzar

  I would like to implement something similar to the Google quick scroll extension with jquery for the extracts of a search result, so when the full document is opened (within the same website) it gives the user the opportunity to go straight to the extract location. Here is a sample of what I get returned from the search engine when I search for 'food'. <doc> <docid>129305</docid> <title><span class='highlighted'>Food</span></title> <summary> <summarytext>Papers subject to Negative Resolution: 4 <span class='highlighted'>Food</span> <span class='highlighted'>Food</span> Irradiation (England) Regulations 2009 (S.I., 2009, No. 1584), dated 24 June 2009 (by Act), </summarytext> </summary> <paras> <paraitemcount>2</paraitemcount> <para> <paraitem>1</paraitem> <paraid>42</paraid> <pararelevance>100</pararelevance> <paraweights>50</paraweights> <paratext>4 <span class='highlighted'>Food</span></paratext> </para> <para> <paraitem>2</paraitem> <paraid>54</paraid> <pararelevance>100</pararelevance> <paraweights>50</paraweights> <paratext><span class='highlighted'>Food</span> Irradiation (England) Regulations 2009 (S.I., 2009, No. 1584), dated 24 June 2009 (by Act), with an Explanatory Memorandum and an Impact Assessment (</paratext> </para> </paras> </doc> As you see the search engine has returned a document that contains one summary and two extracts. So let's say the user clicks on the second extract in the search resutls page, the browser would open the detailed document in the same website, and would offer the user the possibility to go to the extract as the Google quick scroll extension does. Is there an existing jquery script for this? If not, can you suggest any jquery/javascript code that would simplify my task to implement this. Notes: I can access the extracts from the document details page. I'm aware that the HTML in some cases could be slightly different in the extract than in the details page, finding no match. The search engine does not return where the extract was located. At the moment I'm trying to understand the JS code that the extension uses.

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• C++ question on prime numbers.

  - by user278330

  Hello. I am trying to make a program that determines if the number is prime or composite. I have gotten thus far. Could you give me any ideas so that it will work? All primes will , however, because composites have values that are both r0 and r==0, they will always be classified as prime. How can I fix this? int main() { int pNumber, limit, x, r; limit = 0; x = 2; cout << "Please enter any positive integer: " ; cin >> pNumber; if (pNumber < 0) { cout << "Invalid. Negative Number. " << endl; return 0; } else if (pNumber == 0) { cout << "Invalid. Zero has an infinite number of divisors, and therefore neither composite nor prime." << endl; return 0; } else if (pNumber == 1) { cout << "Valid. However, one is neither prime nor composite" << endl; return 0; } else { while (limit < pNumber) { r = pNumber % x; x++; limit++; } if (r == 0) cout << "Your number is composite" << endl; else cout << "Your number is prime" << endl; } return 0; }

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• C++ question on prime numbers.

  - by user278330

  Hello. I am trying to make a program that determines if the number is prime or composite. I have gotten thus far. Could you give me any ideas so that it will work? All primes will , however, because composites have values that are both r0 and r==0, they will always be classified as prime. How can I fix this? int main() { int pNumber, limit, x, r; limit = 0; x = 2; cout << "Please enter any positive integer: " ; cin >> pNumber; if (pNumber < 0) { cout << "Invalid. Negative Number. " << endl; return 0; } else if (pNumber == 0) { cout << "Invalid. Zero has an infinite number of divisors, and therefore neither composite nor prime." << endl; return 0; } else if (pNumber == 1) { cout << "Valid. However, one is neither prime nor composite" << endl; return 0; } else { while (limit < pNumber) { r = pNumber % x; x++; limit++; } if (r == 0) cout << "Your number is composite" << endl; else cout << "Your number is prime" << endl; } return 0; }

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• Best Practice: Legitimate Cross-Site Scripting

  - by Ryan

  While cross-site scripting is generally regarded as negative, I've run into several situations where it's necessary. I was recently working within the confines of a very limiting content management system. I needed to include database code within the page, but the hosting server didn't have anything usable available. I set up a couple barebones scripts on my own server, originally thinking that I could use AJAX to import the contents of my scripts directly into the template of the CMS (thus retaining dynamic images, menu items, CSS, etc.). I was wrong. Due to the limitations of XMLHttpRequest objects, it's not possible to grab content from a different domain. So I thought "iFrame" - even though I'm not a fan of frames, I thought that I could create a frame that matched the width and height of the content, so that it would appear native. Again, I was blocked by cross-site scripting "protections." While I could indeed load a remote file into the iFrame, I couldn't execute JavaScript to modify its size on either the host page or inside the loaded page. In this particular scenario, I wasn't able to point a subdomain to my server. I also couldn't create a script on the CMS server that could proxy content from my server, so my last thought was to use a remote JavaScript. A remote JavaScript works. It breaks when the user has JavaScript disabled, which is a downside; but it works. The "problem" I was having with using a remote JavaScript was that I had to use the JS function document.write() to output any content. Any output that isn't JS causes script errors. In addition to using document.write() for every line, you also have to ensure that the content is escaped - or else you end up with more script errors. My solution was as follows: My script received a GET parameter ("page") and then looked for the file ({$page}.php), and read the contents into a variable. However, I had to use awkward buffering techniques in order to actually execute the included scripts (for things like database interaction) then strip the final content of all line break characters ("\n") followed by escaping all required characters. The end result is that my original script (which outputs JavaScript) accesses seemingly "standard" scripts on my server and converts their standard output to JavaScript for displaying within the CMS template. While this solution works, it seems like there may be a better way to accomplish the same thing. What is the best way to make cross-site scripting work specifically for the purpose of including content from a completely different domain?

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• Coming Up with a Good Algorithm for a Simple Idea

  - by mkoryak

  I need to come up with an algorithm that does the following: Lets say you have an array of positive numbers (e.g. [1,3,7,0,0,9]) and you know beforehand their sum is 20. You want to abstract some average amount from each number such that the new sum would be less by 7. To do so, you must follow these rules: you can only subtract integers the resulting array must not have any negative values you can not make any changes to the indices of the buckets. The more uniformly the subtraction is distributed over the array the better. Here is my attempt at an algorithm in JavaScript + underscore (which will probably make it n^2): function distributeSubtraction(array, goal){ var sum = _.reduce(arr, function(x, y) { return x + y; }, 0); if(goal < sum){ while(goal < sum && goal > 0){ var less = ~~(goal / _.filter(arr, _.identity).length); //length of array without 0s arr = _.map(arr, function(val){ if(less > 0){ return (less < val) ? val - less : val; //not ideal, im skipping some! } else { if(goal > 0){ //again not ideal. giving preference to start of array if(val > 0) { goal--; return val - 1; } } else { return val; } } }); if(goal > 0){ var newSum = _.reduce(arr, function(x, y) { return x + y; }, 0); goal -= sum - newSum; sum = newSum; } else { return arr; } } } else if(goal == sum) { return _.map(arr, function(){ return 0; }); } else { return arr; } } var goal = 7; var arr = [1,3,7,0,0,9]; var newArray = distributeSubtraction(arr, goal); //returned: [0, 1, 5, 0, 0, 7]; Well, that works but there must be a better way! I imagine the run time of this thing will be terrible with bigger arrays and bigger numbers. edit: I want to clarify that this question is purely academic. Think of it like an interview question where you whiteboard something and the interviewer asks you how your algorithm would behave on a different type of a dataset.

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• how to align floats in IE6

  - by rei

  Good day! I am having problems displaying floated paragraphs and images in IE6. There was no problem displaying those in Opera and Firefox,though. I have three divs inside a container. Each div has its own paragraph and image either floated to the left or right. In order for me to achieve a desired layout, I set negative margins on most of the paragraphs and images. Here is how I aligned the floats: ----- CSS code for the first div ----- .row1 { float:left; width:790px; height:460px; margin:5px 0 0 40px; } .pic1 { float:right; height:460px; width:382px; margin:-100px -50px 0 -60px; } h2, p { font-family:Arial, Helvetica, sans-serif; } .row1 p { font-size:12px; text-indent:20px; font-weight:bold; text-align:justify; margin:-10px -25px 0 0; position:relative; } ----------- code for the 2nd div ------------- .row2 { float:left; width:790px; height:234px; margin:-185px 0 0 28px; position:relative; } .row2 p { float:right; font-size:12px; font-weight:bold; text-align:justify; text-indent:20px; margin:-195px 258px 0 175px; position:relative; } .pic2 { float:left; } --------- code for the 3rd div --------------- .row3 { float:left; width:790px; height:203px; margin:-10px 0 0 40px; position:relative; } .row3 p { float:left; font-size:12px; font-weight:bold; text-indent:20px; text-align:justify; margin:-180px 265px 0 10px; position:relative; } .pic3 { float:right; } ///////// The paragraphs seem to be far away from the images when viewed in IE6. Some paragraphs are overlapping with other images. I hope you can help me with this one. Thanks, Rei

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• Would someone mind giving suggestions for this new assembly language?

  - by Noctis Skytower

  Greetings! Last semester in college, my teacher in the Computer Languages class taught us the esoteric language named Whitespace. In the interest of learning the language better with a very busy schedule (midterms), I wrote an interpreter and assembler in Python. An assembly language was designed to facilitate writing programs easily, and a sample program was written with the given assembly mnemonics. Now that it is summer, a new project has begun with the objective being to rewrite the interpreter and assembler for Whitespace 0.3, with further developments coming afterwards. Since there is so much extra time than before to work on its design, you are presented here with an outline that provides a revised set of mnemonics for the assembly language. This post is marked as a wiki for their discussion. Have you ever had any experience with assembly languages in the past? Were there some instructions that you thought should have been renamed to something different? Did you find yourself thinking outside the box and with a different paradigm than in which the mnemonics were named? If you can answer yes to any of those questions, you are most welcome here. Subjective answers are appreciated! hold N Push the number onto the stack copy Duplicate the top item on the stack copy N Copy the nth item on the stack (given by the argument) onto the top of the stack swap Swap the top two items on the stack drop Discard the top item on the stack drop N Slide n items off the stack, keeping the top item add Addition sub Subtraction mul Multiplication div Integer Division mod Modulo save Store load Retrieve L: Mark a location in the program call L Call a subroutine goto L Jump unconditionally to a label if=0 L Jump to a label if the top of the stack is zero if<0 L Jump to a label if the top of the stack is negative return End a subroutine and transfer control back to the caller exit End the program print chr Output the character at the top of the stack print int Output the number at the top of the stack input chr Read a character and place it in the location given by the top of the stack input int Read a number and place it in the location given by the top of the stack Question: How would you redesign, rewrite, or rename the previous mnemonics and for what reasons?

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• Printf in assembler doesn't print

  - by Gaim

  Hi there, I have got a homework to hack program using buffer overflow ( with disassambling, program was written in C++, I haven't got the source code ). I have already managed it but I have a problem. I have to print some message on the screen, so I found out address of printf function, pushed address of "HACKED" and address of "%s" on the stack ( in this order ) and called that function. Called code passed well but nothing had been printed. I have tried to simulate the environment like in other place in the program but there has to be something wrong. Do you have any idea what I am doing wrong that I have no output, please? Thanks a lot EDIT: This program is running on Windows XP SP3 32b, written in C++, Intel asm there is the "hack" code CPU Disasm Address Hex dump Command Comments 0012F9A3 90 NOP ;hack begins 0012F9A4 90 NOP 0012F9A5 90 NOP 0012F9A6 89E5 MOV EBP,ESP 0012F9A8 83EC 7F SUB ESP,7F ;creating a place for working data 0012F9AB 83EC 7F SUB ESP,7F 0012F9AE 31C0 XOR EAX,EAX 0012F9B0 50 PUSH EAX 0012F9B1 50 PUSH EAX 0012F9B2 50 PUSH EAX 0012F9B3 89E8 MOV EAX,EBP 0012F9B5 83E8 09 SUB EAX,9 0012F9B8 BA 1406EDFF MOV EDX,FFED0614 ;address to jump, it is negative because there mustn't be 00 bytes 0012F9BD F7DA NOT EDX 0012F9BF FFE2 JMP EDX ;I have to jump because there are some values overwritten by the program 0012F9C1 90 NOP 0012F9C2 0090 00000000 ADD BYTE PTR DS:[EAX],DL 0012F9C8 90 NOP 0012F9C9 90 NOP 0012F9CA 90 NOP 0012F9CB 90 NOP 0012F9CC 6C INS BYTE PTR ES:[EDI],DX ; I/O command 0012F9CD 65:6E OUTS DX,BYTE PTR GS:[ESI] ; I/O command 0012F9CF 67:74 68 JE SHORT 0012FA3A ; Superfluous address size prefix 0012F9D2 2069 73 AND BYTE PTR DS:[ECX+73],CH 0012F9D5 203439 AND BYTE PTR DS:[EDI+ECX],DH 0012F9D8 34 2C XOR AL,2C 0012F9DA 2066 69 AND BYTE PTR DS:[ESI+69],AH 0012F9DD 72 73 JB SHORT 0012FA52 0012F9DF 74 20 JE SHORT 0012FA01 0012F9E1 3120 XOR DWORD PTR DS:[EAX],ESP 0012F9E3 6C INS BYTE PTR ES:[EDI],DX ; I/O command 0012F9E4 696E 65 7300909 IMUL EBP,DWORD PTR DS:[ESI+65],-6F6FFF8D 0012F9EB 90 NOP 0012F9EC 90 NOP 0012F9ED 90 NOP 0012F9EE 31DB XOR EBX,EBX ; hack continues 0012F9F0 8818 MOV BYTE PTR DS:[EAX],BL ; writing 00 behind word "HACKED" 0012F9F2 83E8 06 SUB EAX,6 0012F9F5 50 PUSH EAX ; address of "HACKED" 0012F9F6 B8 3B8CBEFF MOV EAX,FFBE8C3B 0012F9FB F7D0 NOT EAX 0012F9FD 50 PUSH EAX ; address of "%s" 0012F9FE B8 FFE4BFFF MOV EAX,FFBFE4FF 0012FA03 F7D0 NOT EAX 0012FA05 FFD0 CALL EAX ;address of printf This code is really ugly because I am new in assembler and there mustn't be null bytes because of buffer-overflow bug

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• How can I fix this NavigationController and UIToolbar offset issue in Objective-C?

  - by editor

  I'm adding a couple of buttons to an already-existing NavigationController. The two buttons are added to a UIView, which is pushed onto the NavigationItem. The buttons stop and reload a UIWebView. Problem is that there's a slight offset issue that is making it all look pretty ugly. I wish I could set the UIToolbar to transparent or clear background but that doesn't seem to be an option. Can't seem to use negative offsets either. I've got color matching, but if you look closely there's 1px or 2px of highlighting up top that's causing a visual mismatch and then a slight offset at the bottom. Some relevant code below (based on this, inbound Googlers). What are my options to resolve this? // create a toolbar for the buttons UIToolbar* toolbar = [[UIToolbar alloc] initWithFrame:CGRectMake(0, 0, 100, 45)]; [toolbar setBarStyle: UIBarStyleDefault]; UIColor *colorForBar = [[UIColor alloc] initWithRed:.72 green:0 blue:0 alpha:0]; toolbar.tintColor = colorForBar; [colorForBar release]; //[toolbar setTranslucent:YES]; // create an array for the buttons NSMutableArray* buttons = [[NSMutableArray alloc] initWithCapacity:3]; // create a standard reload button UIBarButtonItem *reloadButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemRefresh target:self action:@selector(reload)]; reloadButton.style = UIBarButtonItemStyleBordered; [buttons addObject:reloadButton]; [reloadButton release]; // create a spacer between the buttons UIBarButtonItem *spacer = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil]; [buttons addObject:spacer]; [spacer release]; // create a standard delete button with the trash icon UIBarButtonItem *stopButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemStop target:self action:@selector(stopLoading)]; stopButton.style = UIBarButtonItemStyleBordered; [buttons addObject:stopButton]; [stopButton release]; // put the buttons in the toolbar and release them [toolbar setItems:buttons animated:NO]; [buttons release]; // place the toolbar into the navigation bar self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:toolbar]; [toolbar release];

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• Best Practices / Patterns for Enterprise Protection/Remediation of SSNs (Social Security Numbers)

  - by Erik Neu

  I am interested in hearing about enterprise solutions for SSN handling. (I looked pretty hard for any pre-existing post on SO, including reviewing the terriffic SO automated "Related Questions" list, and did not find anything, so hopefully this is not a repeat.) First, I think it is important to enumerate the reasons systems/databases use SSNs: (note—these are reasons for de facto current state—I understand that many of them are not good reasons) Required for Interaction with External Entities. This is the most valid case—where external entities your system interfaces with require an SSN. This would typically be government, tax and financial. SSN is used to ensure system-wide uniqueness. SSN has become the default foreign key used internally within the enterprise, to perform cross-system joins. SSN is used for user authentication (e.g., log-on) The enterprise solution that seems optimum to me is to create a single SSN repository that is accessed by all applications needing to look up SSN info. This repository substitutes a globally unique, random 9-digit number (ASN) for the true SSN. I see many benefits to this approach. First of all, it is obviously highly backwards-compatible—all your systems "just" have to go through a major, synchronized, one-time data-cleansing exercise, where they replace the real SSN with the alternate ASN. Also, it is centralized, so it minimizes the scope for inspection and compliance. (Obviously, as a negative, it also creates a single point of failure.) This approach would solve issues 2 and 3, without ever requiring lookups to get the real SSN. For issue #1, authorized systems could provide an ASN, and be returned the real SSN. This would of course be done over secure connections, and the requesting systems would never persist the full SSN. Also, if the requesting system only needs the last 4 digits of the SSN, then that is all that would ever be passed. Issue #4 could be handled the same way as issue #1, though obviously the best thing would be to move away from having users supply an SSN for log-on. There are a couple of papers on this: UC Berkely: http://bit.ly/bdZPjQ Oracle Vault: bit.ly/cikbi1

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• Opinion on "loop invariants", and are these frequently used in the industry?

  - by Michael Aaron Safyan

  I was thinking back to my freshman year at college (five years ago) when I took an exam to place-out of intro-level computer science. There was a question about loop invariants, and I was wondering if loop invariants are really necessary in this case or if the question was simply a bad example... the question was to write an iterative definition for a factorial function, and then to prove that the function was correct. The code that I provided for the factorial function was as follows: public static int factorial(int x) { if ( x < 0 ){ throw new IllegalArgumentException("Parameter must be = 0"); }else if ( x == 0 ){ return 1; }else{ int result = 1; for ( int i = 1; i <= x; i++ ){ result*=i; } return result; } } My own proof of correctness was a proof by cases, and in each I asserted that it was correct by definition (x! is undefined for negative values, 0! is 1, and x! is 1*2*3...*x for a positive value of x). The professor wanted me to prove the loop using a loop invariant; however, my argument was that it was correct "by definition", because the definition of "x!" for a positive integer x is "the product of the integers from 1... x", and the for-loop in the else clause is simply a literal translation of this definition. Is a loop invariant really needed as a proof of correctness in this case? How complicated must a loop be before a loop invariant (and proper initialization and termination conditions) become necessary for a proof of correctness? Additionally, I was wondering... how often are such formal proofs used in the industry? I have found that about half of my courses are very theoretical and proof-heavy and about half are very implementation and coding-heavy, without any formal or theoretical material. How much do these overlap in practice? If you do use proofs in the industry, when do you apply them (always, only if it's complicated, rarely, never)?

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• Creating a spam list with a web crawler in python

  - by user313623

  Hey guys, I'm not trying to do anything malicious here, I just need to do some homework. I'm a fairly new programmer, I'm using python 3.0, and I having difficulty using recursion for problem-solving. I've been stuck on this question for quite a while. Here's the assignment: Write a recursive method spam(url, n) that takes a url of a web page as input and a non-negative integer n, collects all the email address contained in the web page and adds them to a global dictionary variable spam_dict, and then recursively calls itself on every http hyperlink contained in the web page. You will use a dictionary so only one copy of every email address is save; your dictionary will store (key,value) pairs (email, email). The recursive call should use the parameter n-1 instead of n. If n = 0, you should collect the email addresses but no recursive calls should be made. The parameter n is used to limit the recursion to at most depth n. You will need to use the solutions of the two above problems; you method spam() will call the methods links2() and emails() and possibly other functions as well. Notes: 1. running spam() directly will produce no output on the screen; to find your spam_dict, you will need to read the value of spam_dict, and you will also need to reset it to the empty dictionary before every run of spam. 2. Recall how global variables are used. Usage: spam_dict = {} spam('http://reed.cs.depaul.edu/lperkovic/csc242/test1.html',0) spam_dict.keys() dict_keys([]) spam_dict = {} spam('http://reed.cs.depaul.edu/lperkovic/csc242/test1.html',1) spam_dict.keys() dict_keys(['[email protected]', '[email protected]']) So far, I've written a function that traverses web pages and puts all the links in a nice little list, and what I wanted to do was call that functions. And why would I use recursion on a dictionary? And how? I don't understand how n ties into all of this. def links2(url): content = str(urlopen(url).read()) myparser = MyHTMLParser() myparser.feed(content) lst = myparser.get() mergelst = [] for link in lst: mergelst.append(urljoin(lst[0],link)) print(mergelst) Any input (except why spam is bad) would be greatly appreciated. Also, I realize that the above function could probably look better, if you have a way to do it, I'm all ears. However, all I need is the point is for the program to produce the proper output.

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• How do I print out objects in an array in python?

  - by Jonathan

  I'm writing a code which performs a k-means clustering on a set of data. I'm actually using the code from a book called collective intelligence by O'Reilly. Everything works, but in his code he uses the command line and i want to write everything in notepad++. As a reference his line is >>>kclust=clusters.kcluster(data,k=10) >>>[rownames[r] for r in k[0]] Here is my code: from PIL import Image,ImageDraw def readfile(filename): lines=[line for line in file(filename)] # First line is the column titles colnames=lines[0].strip( ).split('\t')[1:] rownames=[] data=[] for line in lines[1:]: p=line.strip( ).split('\t') # First column in each row is the rowname rownames.append(p[0]) # The data for this row is the remainder of the row data.append([float(x) for x in p[1:]]) return rownames,colnames,data from math import sqrt def pearson(v1,v2): # Simple sums sum1=sum(v1) sum2=sum(v2) # Sums of the squares sum1Sq=sum([pow(v,2) for v in v1]) sum2Sq=sum([pow(v,2) for v in v2]) # Sum of the products pSum=sum([v1[i]*v2[i] for i in range(len(v1))]) # Calculate r (Pearson score) num=pSum-(sum1*sum2/len(v1)) den=sqrt((sum1Sq-pow(sum1,2)/len(v1))*(sum2Sq-pow(sum2,2)/len(v1))) if den==0: return 0 return 1.0-num/den class bicluster: def __init__(self,vec,left=None,right=None,distance=0.0,id=None): self.left=left self.right=right self.vec=vec self.id=id self.distance=distance def hcluster(rows,distance=pearson): distances={} currentclustid=-1 # Clusters are initially just the rows clust=[bicluster(rows[i],id=i) for i in range(len(rows))] while len(clust)>1: lowestpair=(0,1) closest=distance(clust[0].vec,clust[1].vec) # loop through every pair looking for the smallest distance for i in range(len(clust)): for j in range(i+1,len(clust)): # distances is the cache of distance calculations if (clust[i].id,clust[j].id) not in distances: distances[(clust[i].id,clust[j].id)]=distance(clust[i].vec,clust[j].vec) #print 'i' #print i #print #print 'j' #print j #print d=distances[(clust[i].id,clust[j].id)] if d<closest: closest=d lowestpair=(i,j) # calculate the average of the two clusters mergevec=[ (clust[lowestpair[0]].vec[i]+clust[lowestpair[1]].vec[i])/2.0 for i in range(len(clust[0].vec))] # create the new cluster newcluster=bicluster(mergevec,left=clust[lowestpair[0]], right=clust[lowestpair[1]], distance=closest,id=currentclustid) # cluster ids that weren't in the original set are negative currentclustid-=1 del clust[lowestpair[1]] del clust[lowestpair[0]] clust.append(newcluster) return clust[0] def kcluster(rows,distance=pearson,k=4): # Determine the minimum and maximum values for each point ranges=[(min([row[i] for row in rows]),max([row[i] for row in rows])) for i in range(len(rows[0]))] # Create k randomly placed centroids clusters=[[random.random( )*(ranges[i][1]-ranges[i][0])+ranges[i][0] for i in range(len(rows[0]))] for j in range(k)] lastmatches=None for t in range(100): print 'Iteration %d' % t bestmatches=[[] for i in range(k)] # Find which centroid is the closest for each row for j in range(len(rows)): row=rows[j] bestmatch=0 for i in range(k): d=distance(clusters[i],row) if d<distance(clusters[bestmatch],row): bestmatch=i bestmatches[bestmatch].append(j) # If the results are the same as last time, this is complete if bestmatches==lastmatches: break lastmatches=bestmatches # Move the centroids to the average of their members for i in range(k): avgs=[0.0]*len(rows[0]) if len(bestmatches[i])>0: for rowid in bestmatches[i]: for m in range(len(rows[rowid])): avgs[m]+=rows[rowid][m] for j in range(len(avgs)): avgs[j]/=len(bestmatches[i]) clusters[i]=avgs return bestmatches

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• Memory efficient int-int dict in Python

  - by Bolo

  Hi, I need a memory efficient int-int dict in Python that would support the following operations in O(log n) time: d[k] = v # replace if present v = d[k] # None or a negative number if not present I need to hold ~250M pairs, so it really has to be tight. Do you happen to know a suitable implementation (Python 2.7)? EDIT Removed impossible requirement and other nonsense. Thanks, Craig and Kylotan! To rephrase. Here's a trivial int-int dictionary with 1M pairs: >>> import random, sys >>> from guppy import hpy >>> h = hpy() >>> h.setrelheap() >>> d = {} >>> for _ in xrange(1000000): ... d[random.randint(0, sys.maxint)] = random.randint(0, sys.maxint) ... >>> h.heap() Partition of a set of 1999530 objects. Total size = 49161112 bytes. Index Count % Size % Cumulative % Kind (class / dict of class) 0 1 0 25165960 51 25165960 51 dict (no owner) 1 1999521 100 23994252 49 49160212 100 int On average, a pair of integers uses 49 bytes. Here's an array of 2M integers: >>> import array, random, sys >>> from guppy import hpy >>> h = hpy() >>> h.setrelheap() >>> a = array.array('i') >>> for _ in xrange(2000000): ... a.append(random.randint(0, sys.maxint)) ... >>> h.heap() Partition of a set of 14 objects. Total size = 8001108 bytes. Index Count % Size % Cumulative % Kind (class / dict of class) 0 1 7 8000028 100 8000028 100 array.array On average, a pair of integers uses 8 bytes. I accept that 8 bytes/pair in a dictionary is rather hard to achieve in general. Rephrased question: is there a memory-efficient implementation of int-int dictionary that uses considerably less than 49 bytes/pair?

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• The Math of a Jump in a 2D game.

  - by ONi

  I have been all around with this question and I can't find the correct answer! So, behold, the description of my question: I'm working in J2ME, I have my gameloop that do the following: public void run() { Graphics g = this.getGraphics(); while (running) { long diff = System.currentTimeMillis() - lastLoop; lastLoop = System.currentTimeMillis(); input(); this.level.doLogic(); render(g, diff); try { Thread.sleep(10); } catch (InterruptedException e) { stop(e); } } } so it's just a basic gameloop, the doLogic() function calls for all the logic functions of the characters in the scene and render(g, diff) calls the animateChar function of every character on scene, following this, the animChar function in the Character class sets up everything in the screen as this: protected void animChar(long diff) { this.checkGravity(); this.move((int) ((diff * this.dx) / 1000), (int) ((diff * this.dy) / 1000)); if (this.acumFrame > this.framerate) { this.nextFrame(); this.acumFrame = 0; } else { this.acumFrame += diff; } } This ensures me that everything must to move according to the time that the machine takes to go from cycle to cycle (remember it's a phone, not a gaming rig). I'm sure it's not the most efficient way to achieve this behavior so I'm totally open for criticism of my programming skills in the comments, but here my problem: When I make I character jump, what I do is that I put his dy to a negative value, say -200 and I set the boolean jumping to true, that makes the character go up, and then I have this function called checkGravity() that ensure that everything that goes up has to go down, checkGravity also checks for the character being over platforms so I will strip it down a little for the sake of your time: public void checkGravity() { if (this.jumping) { this.jumpSpeed += 10; if (this.jumpSpeed > 0) { this.jumping = false; this.falling = true; } this.dy = this.jumpSpeed; } if (this.falling) { this.jumpSpeed += 10; if (this.jumpSpeed > 200) this.jumpSpeed = 200; this.dy = this.jumpSpeed; if (this.collidesWithPlatform()) { this.falling = false; this.standing = true; this.jumping = false; this.jumpSpeed = 0; this.dy = this.jumpSpeed; } } } So, the problem is, that this function updates the dy regardless of the diff, making the characters fly like Superman in slow machines, and I have no idea how to implement the diff factor so that when a character is jumping, his speed decrement in a proportional way to the game speed. Can anyone help me fix this issue? or give me pointers on how to make a 2D Jump in J2ME the right way. Thank you very much for your time.

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• How to manage multiple versions of the same record

  - by Darvis Lombardo

  I am doing short-term contract work for a company that is trying to implement a check-in/check-out type of workflow for their database records. Here's how it should work... 1) A user creates a new entity within the application. There are about 20 related tables that will be populated in addition to the main entity table. 2) Once the entity is created the user will mark it as the master. 3) Another user can make changes to the master only by "checking out" the entity. Multiple users can checkout the entity at the same time. 4) Once the user has made all the necessary changes to the entity, they put it in a "needs approval" status. 5) After an authorized user reviews the entity, they can promote it to master which will put the original record in a tombstoned status. The way they are currently accomplishing the "check out" is by duplicating the entity records in all the tables. The primary keys include EntityID + EntityDate, so they duplicate the entity records in all related tables with the same EntityID and an updated EntityDate and give it a status of "checked out". When the record is put into the next state (needs approval), the duplication occurs again. Eventually it will be promoted to master at which time the final record is marked as master and the original master is marked as dead. This design seems hideous to me, but I understand why they've done it. When someone looks up an entity from within the application, they need to see all current versions of that entity. This was a very straightforward way for making that happen. But the fact that they are representing the same entity multiple times within the same table(s) doesn't sit well with me, nor does the fact that they are duplicating EVERY piece of data rather than only storing deltas. I would be interested in hearing your reaction to the design, whether positive or negative. I would also be grateful for any resoures you can point me to that might be useful for seeing how someone else has implemented such a mechanism. Thanks! Darvis

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• Is the recent trend toward widescreen (16:9) computer monitors a plus or minus for programmers?

  - by DanM

  It's almost gotten to the point where you can't buy a conventional (4:3) monitor anymore. Pretty much everything is widescreen. This is fine for watching movies or TV, but is it good or bad for programming? My initial thoughts on the issue are that widescreens are a net negative for programmers. Here are some of the disadvantages I see: Poor space utiliziation One disadvantage of widescreens you can't argue with is that they offer poor space utilization for the amount of total pixels you get. For example, my Thinkpad, which I bought just before the widescreen craze, has a 15" monitor with a native resolution of 1600 x 1200. The newer 15.4" Thinkpads run at most 1680 x 1050. So (if you do the math) you get fewer pixels in a wider (but not shorter) package. With desktop monitors, you pay a price in terms of desk space used. Two 1680 x 1050 monitors will simply take up more of your desk than two 1600 x 1200 monitors (assuming equal dot pitch). More scrolling If you compare a 1680 x 1050 monitor to a 1600 x 1200 monitor, you get 80 extra pixels of width but 150 fewer pixels of height. The height reduction means you lose approximately 11 lines of code. That's less you can see on the screen at one time and more scrolling you have to do. This harms productivity, maybe not dramatically, but insidiously. Less room for wide panels Widescreens also mean you lose space for wide but short panels common in programming environments. If you use Visual Studio, for example, your code window will be that much shorter when viewing the Find Results, Task List, or Error List (all of which I use frequently). This isn't to say the 80 pixels of extra width you get with widescreen would never be useful, but I tend to keep my lines of code short, so seeing more lines would be more valuable to me than seeing fewer, longer lines. What do you think? Do you agree/disagree? Are you now using one or more widescreen monitors for development? What resolution are you running on each? Do you ever miss the height of the traditional 4:3 monitor? Would you complain if your monitors were one inch narrower but two inches taller?

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• Box Shadow on only 3 sides

  - by Connor

  I have two overlapping divs that have css3 box shadows. The trouble is that even when I set the z-index I will still need to eliminate one of the div's box-shadow. I have seen cases where negative spreads and zero values are used but I don't think that would work here. The code I have now is: #bulb-top { position: relative; width: 280px; height: 280px; background-color: #E5F7A3; -webkit-border-radius: 280px; -moz-border-radius: 280px; border-radius: 280px; border: 8px solid #FFF40C; top: -430px; margin-left: auto; margin-right: auto; -webkit-box-shadow: 0px 0px 15px 1px #FFF40C; -moz-box-shadow: 0px 0px 15px 1px #FFF40C; box-shadow: 0px 0px 15px 1px #FFF40C; z-index: 4; } #bulb-bottom { position: relative; width: 140px; height: 120px; background-color: #E5F7A3; -moz-border-radius-topleft: 0px; -moz-border-radius-topright: 0px; -moz-border-radius-bottomright: 30px; -moz-border-radius-bottomleft: 30px; -webkit-border-radius: 0px 0px 30px 30px; border-radius: 0px 0px 30px 30px; border-left: 8px solid #FFF40C; border-right: 8px solid #FFF40C; border-bottom: 8px solid #FFF40C; top: -455px; margin-left: auto; margin-right: auto; -webkit-box-shadow: 0px 0px 15px 1px #FFF40C; -moz-box-shadow: 0px 0px 15px 1px #FFF40C; box-shadow: 0px 0px 15px 1px #FFF40C; z-index: 5; } http://jsfiddle.net/minitech/g42vq/3/

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• Checking to see if a number is evenly divisible by other numbers with recursion in Python

  - by Ernesto

  At the risk of receiving negative votes, I will preface this by saying this is a midterm problem for a programming class. However, I have already submitted the code and passed the question. I changed the name of the function(s) so that someone can't immediately do a search and find the correct code, as that is not my purpose. I am actually trying to figure out what is actually MORE CORRECT from two pieces that I wrote. The problem tells us that a certain fast food place sells bite-sized pieces of chicken in packs of 6, 9, and 20. It wants us to create a function that will tell if a given number of bite-sized piece of chicken can be obtained by buying different packs. For example, 15 can be bought, because 6 + 9 is 15, but 16 cannot be bought, because no combination of the packs will equal 15. The code I submitted and was "correct" on, was: def isDivisible(n): """ n is an int Returns True if some integer combination of 6, 9 and 20 equals n Otherwise returns False. """ a, b, c = 20, 9, 6 if n == 0: return True elif n < 0: return False elif isDivisible(n - a) or isDivisible(n - b) or isDivisible(n - c): return True else: return False However, I got to thinking, if the initial number is 0, it will return True. Would an initial number of 0 be considered "buying that amount using 6, 9, and/or 20"? I cannot view the test cases the grader used, so I don't know if the grader checked 0 as a test case and decided that True was an acceptable answer or not. I also can't just enter the new code, because it is a midterm. I decided to create a second piece of code that would handle an initial case of 0, and assuming 0 is actually False: def isDivisible(n): """ n is an int Returns True if some integer combination of 6, 9 and 20 equals n Otherwise returns False. """ a, b, c = 20, 9, 6 if n == 0: return False else: def helperDivisible(n): if n == 0: return True elif n < 0: return False elif helperDivisible(n - a) or helperDivisible(n - b) or helperDivisible(n - c): return True else: return False return helperDivisible(n) As you can see, my second function had to use a "helper" function in order to work. My overall question, though, is which function do you think would provide the correct answer, if the grader had tested for 0 as an initial input?

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• Measure width() with jQuery after DOM refresh

  - by o_O Tync

  My script dynamically creates a <ul> width left-floating <li>s inside: it's a paginator. Afterwards, the script measures width of all <li>s and summs them up. The problem is that after the nodes are injected into the document — the browser refreshed DOM and applies CSS styles which takes a while. It has a negative effect on my script: when these operations are not complete before I measure the width — my script gets a wrong value. If I perform the measure in a second — everything is ok. The thing I'm looking for is a way to detect the moment when the <ul> is fully drawn, styles applied and the width has stabilizes. Or at least a way to detect every dimensions changes. Of course I can use setTimeout(..., 100) but it's ugly and I guess — not a solution at all. If there's a way to detect width stabilization — I would do the measuring right after it to get the correct values. HTML code generated by the DOM <div> <ul> <li><a href="...">1</a></li> <li><a href="...">2</a></li> .... </ul> </div> P.S. Why I need this. My paginator's left-floating <li> items tend to move to the next line when the <ul> tries to become wider than the page itself. Even though most of <li>s are invisible because of parent <div>'s width restriction: div { width: 500px; overflow: hidden; } div ul { width: 100%; white-space: nowrap; } div ul li { display: block; float: left; } they still go down unless I specify the actual summed width of the <ul> with the script.

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• Are there known problems with >= and <= and the eval function in JS?

  - by Augier

  I am currently writing a JS rules engine which at one point needs to evaluate boolean expressions using the eval() function. Firstly I construct an equation as such: var equation = "relation.relatedTrigger.previousValue" + " " + relation.operator + " " + "relation.value"; relation.relatedTrigger.previousValue is the value I want to compare. relation.operator is the operator (either "==", "!=", <=, "<", "", ="). relation.value is the value I want to compare with. I then simply pass this string to the eval function and it returns true or false as such: return eval(equation); This works absolutely fine (with words and numbers) or all of the operators except for = and <=. E.g. When evaluating the equation: relation.relatedTrigger.previousValue <= 100 It returns true when previousValue = 0,1,10,100 & all negative numbers but false for everything in between. I would greatly appreciate the help of anyone to either answer my question or to help me find an alternative solution. Regards, Augier. P.S. I don't need a speech on the insecurities of the eval() function. Any value given to relation.relatedTrigger.previousValue is predefined. edit: Here is the full function: function evaluateRelation(relation) { console.log("Evaluating relation") var currentValue; //if multiple values if(relation.value.indexOf(";") != -1) { var values = relation.value.split(";"); for (x in values) { var equation = "relation.relatedTrigger.previousValue" + " " + relation.operator + " " + "values[x]"; currentValue = eval(equation); if (currentValue) return true; } return false; } //if single value else { //Evaluate the relation and get boolean var equation = "relation.relatedTrigger.previousValue" + " " + relation.operator + " " + "relation.value"; console.log("relation.relatedTrigger.previousValue " + relation.relatedTrigger.previousValue); console.log(equation); return eval(equation); } } Answer: Provided by KennyTM below. A string comparison doesn't work. Converting to a numerical was needed.

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