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• Fastest way to find the rotation of a vector

  - by kriss

  I have two 2D vectors, say u and v, defined by cartesian coordinates. Imagine that vectors are needles of a clock. I'm looking for the fastest way to find out, using python, if v is after or before u (or in other words find out in wich half plane is v, regarding to position of u). For the purpose of the problem if vectors are aligned answer should be before. It seems easy using some trigonometry, but I believe there should be a faster way using coordinates only. My test case: def after(u, v): """code here""" after((4,2), (6, 1)) : True after((4,2), (3, 3)) : False after((4,2), (2, 1)) : False after((4,2), (3, -3)) : True after((4,2), (-2, -5)) : True after((4,2), (-4, -2)) : False

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• MySQL query paralyzes site

  - by nute

  Once in a while, at random intervals, our website gets completely paralyzed. Looking at SHOW FULL PROCESSLIST;, I've noticed that when this happens, there is a specific query that is "Copying to tmp table" for a loooong time (sometimes 350 seconds), and almost all the other queries are "Locked". The part I don't understand is that 90% of the time, this query runs fine. I see it going through in the process list and it finishes pretty quickly most of the time. This query is being called by an ajax call on our homepage to display product recommendations based your browsing history (a la amazon). Just sometimes, randomly (but too often), it gets stuck at "copying to tmp table". Here is a caught instance of the query that was up 109 seconds when I looked: SELECT DISTINCT product_product.id, product_product.name, product_product.retailprice, product_product.imageurl, product_product.thumbnailurl, product_product.msrp FROM product_product, product_xref, product_viewhistory WHERE ( (product_viewhistory.productId = product_xref.product_id_1 AND product_xref.product_id_2 = product_product.id) OR (product_viewhistory.productId = product_xref.product_id_2 AND product_xref.product_id_1 = product_product.id) ) AND product_product.outofstock='N' AND product_viewhistory.cookieId = '188af1efad392c2adf82' AND product_viewhistory.productId IN (24976, 25873, 26067, 26073, 44949, 16209, 70528, 69784, 75171, 75172) ORDER BY product_xref.hits DESC LIMIT 10 Of course the "cookieId" and the list of "productId" changes dynamically depending on the request. I use php with PDO.

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• Compile Flex application without debug? Optimisation options for flex compiler?

  - by maoanz

  I have created a simple test application with the following code var i : int; for (i=0; i<3000000; i++){ trace(i); } When I run the application, it's very slow to load, which means the "trace" is running. I check the flash player by right-clicking, the debugger option is not enable. So I wonder if there is an option to put in compiler to exclude the trace. Otherwise, I have to remove manually all the trace in the program. Are there any other options of compiler to optimize the flex application in a maximum way? Thanks

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• Thin down jQuery

  - by Taylor Satula

  Hi, I have been optimizing my website but the one problem that stands in my way is all the jQuery functions that I do not use. The only ones that I use are for a smooth page scroller. It just seems like such a waste of download time. My question is: Is there any script or program that will remove the jQuery code that I do not need and leave the 1 or 2 functions that I do need.

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• Optimizing a "set in a string list" to a "set as a matrix" operation

  - by Eric Fournier

  I have a set of strings which contain space-separated elements. I want to build a matrix which will tell me which elements were part of which strings. For example: "" "A B C" "D" "B D" Should give something like: A B C D 1 2 1 1 1 3 1 4 1 1 Now I've got a solution, but it runs slow as molasse, and I've run out of ideas on how to make it faster: reverseIn <- function(vector, value) { return(value %in% vector) } buildCategoryMatrix <- function(valueVector) { allClasses <- c() for(classVec in unique(valueVector)) { allClasses <- unique(c(allClasses, strsplit(classVec, " ", fixed=TRUE)[[1]])) } resMatrix <- matrix(ncol=0, nrow=length(valueVector)) splitValues <- strsplit(valueVector, " ", fixed=TRUE) for(cat in allClasses) { if(cat=="") { catIsPart <- (valueVector == "") } else { catIsPart <- sapply(splitValues, reverseIn, cat) } resMatrix <- cbind(resMatrix, catIsPart) } colnames(resMatrix) <- allClasses return(resMatrix) } Profiling the function gives me this: $by.self self.time self.pct total.time total.pct "match" 31.20 34.74 31.24 34.79 "FUN" 30.26 33.70 74.30 82.74 "lapply" 13.56 15.10 87.86 97.84 "%in%" 12.92 14.39 44.10 49.11 So my actual questions would be: - Where are the 33% spent in "FUN" coming from? - Would there be any way to speed up the %in% call? I tried turning the strings into factors prior to going into the loop so that I'd be matching numbers instead of strings, but that actually makes R crash. I've also tried going for partial matrix assignment (IE, resMatrix[i,x] <- 1) where i is the number of the string and x is the vector of factors. No dice there either, as it seems to keep on running infinitely.

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• How can I get a COUNT(col) ... GROUP BY to use an index?

  - by thecoop

  I've got a table (col1, col2, ...) with an index on (col1, col2, ...). The table has got millions of rows in it, and I want to run a query: SELECT col1, COUNT(col2) WHERE col1 NOT IN (<couple of exclusions>) GROUP BY col1 Unfortunately, this is resulting in a full table scan of the table, which takes upwards of a minute. Is there any way of getting oracle to use the index on the columns to return the results much faster?

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• Fast find object by string property

  - by Andrew Kalashnikov

  Hello, colleagues. I've got task to fast find object by its string property. Object: class DicDomain { public virtual string Id{ get; set; } public virtual string Name { get; set; } } For storing my object I use List[T] dictionary where T is DicDomain for now . I've got 5-10 such lists, which contain about 500-20000 at each one. Task is find objects by its Name. I use next code now: List<T> entities = dictionary.FindAll(s => s.Name.Equals(word, StringComparison.OrdinalIgnoreCase)); I've got some questions: Is my search speed optimal. I think now. Data structure. It List good for this task. What about hashtable,sorted... Method Find. May be i should use string intern?? I haven't much exp at these tasks. Can u give me good advice for increase perfomance. Thanks

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• How to make if-elif-else statement in python more space-saving?

  - by Neverland

  I have a lot of if-elif-else statements in my code if message == '0' or message == '3' or message == '5' or message == '7': ... elif message == '1' or message == '2' or message == '4' or message == '6' or message == '8': ... else: ... Is it possible to format this in a more space-saving way? I tried it this way: if message == '0' or '3' or '5' or '7': ... elif message == '1' or '2' or '4' or '6' or '8': ... else: ... But without success.

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• Most efficient way to LIMIT results in a JOIN?

  - by johnnietheblack

  I have a fairly simple one-to-many type join in a MySQL query. In this case, I'd like to LIMIT my results by the left table. For example, let's say I have an accounts table and a comments table, and I'd like to pull 100 rows from accounts and all the associated comments rows for each. Thy only way I can think to do this is with a sub-select in in the FROM clause instead of simply selecting FROM accounts. Here is my current idea: SELECT a.*, c.* FROM (SELECT * FROM accounts LIMIT 100) a LEFT JOIN `comments` c on c.account_id = a.id ORDER BY a.id However, whenever I need to do a sub-select of some sort, my intermediate level SQL knowledge feels like it's doing something wrong. Is there a more efficient, or faster, way to do this, or is this pretty good? By the way... This might be the absolute simplest way to do this, which I'm okay with as an answer. I'm simply trying to figure out if there IS another way to do this that could potentially compete with the above statement in terms of speed.

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• Which is faster in memory, ints or chars? And file-mapping or chunk reading?

  - by Nick

  Okay, so I've written a (rather unoptimized) program before to encode images to JPEGs, however, now I am working with MPEG-2 transport streams and the H.264 encoded video within them. Before I dive into programming all of this, I am curious what the fastest way to deal with the actual file is. Currently I am file-mapping the .mts file into memory to work on it, although I am not sure if it would be faster to (for example) read 100 MB of the file into memory in chunks and deal with it that way. These files require a lot of bit-shifting and such to read flags, so I am wondering that when I reference some of the memory if it is faster to read 4 bytes at once as an integer or 1 byte as a character. I thought I read somewhere that x86 processors are optimized to a 4-byte granularity, but I'm not sure if this is true... Thanks!

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• Using Custom Generic Collection faster with objects than List

  - by Kaminari

  I'm iterating through a List<> to find a matching element. The problem is that object has only 2 significant values, Name and Link (both strings), but has some other values which I don't want to compare. I'm thinking about using something like HashSet (which is exactly what I'm searching for -- fast) from .NET 3.5 but target framework has to be 2.0. There is something called Power Collections here: http://powercollections.codeplex.com/, should I use that? But maybe there is other way? If not, can you suggest me a suitable custom collection?

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• Creating objects makes the VM faster?

  - by Sudhir Jonathan

  Look at this piece of code: MessageParser parser = new MessageParser(); for (int i = 0; i < 10000; i++) { parser.parse(plainMessage, user); } For some reason, it runs SLOWER (by about 100ms) than for (int i = 0; i < 10000; i++) { MessageParser parser = new MessageParser(); parser.parse(plainMessage, user); } Any ideas why? The tests were repeated a lot of times, so it wasn't just random. How could creating an object 10000 times be faster than creating it once?

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• Execute a method less times possible - PHP

  - by serhio

  I have a site in multiple languages. I have a method that returns me the today currencies in a array. I display that currencies in a table then. // --- en/index.php <?php include_once "../exchangeRates.php"; $currencies = ReadExchangeRates(); // --- fr/index.php <?php include_once "../exchangeRates.php"; $currencies = ReadExchangeRates(); ... // somewhere in the page <td><?php echo $currencies["eur"]["today"]; ?></td> So, every time I load, en/ or fr/ or other language, I request the exchange rates from a external site. Can I optimize this behavior (reading once per day or session)? maybe to store a global variable and check the update date?

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• Which way to store this data is effective?

  - by Tattat

  I am writing a game, which need a map, and I want to store the map. The first thing I can think of, is using a 2D-array. But the problem is what data should I store in the 2D-array. The player can tap different place to have different reaction. So, I am thinking store a 2D-array with objects, when player click some position, and I find it in the array, and use the object in that array to execute a cmd. But I have a concern that storing lots of object may use lots of memory. So, I am think storing char/int only. But it seems that not enough for me. I want to store the data like that: { Type:1 Color:Green } No matter what color is, if they are all type 1, the have same reactions in logic, but the visual effect is based on the color. So, it is not easy to store using a prue char/int data, unless I make something like this: 1-5 --> all type 1. 1=color green , 2=color red, 3 = color yellow.... ... 6-10 --> all type 2. 2 = color green, 2 = color red ... ... So, do you have any ideas on how to minimize the ram use, but also easy for me to read... ...thx

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• Is count(*) really expensive ?

  - by Anil Namde

  I have a page where I have 4 tabs displaying 4 different reports based off different tables. I obtain the row count of each table using a select count(*) from <table> query and display number of rows available in each table on the tabs. As a result, each page postback causes 5 count(*) queries to be executed (4 to get counts and 1 for pagination) and 1 query for getting the report content. Now my question is: are count(*) queries really expensive -- should I keep the row counts (at least those that are displayed on the tab) in the view state of page instead of querying multiple times? How expensive are COUNT(*) queries ?

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• File IO with Streams - Best Memory Buffer Size

  - by AJ

  I am writing a small IO library to assist with a larger (hobby) project. A part of this library performs various functions on a file, which is read / written via the FileStream object. On each StreamReader.Read(...) pass, I fire off an event which will be used in the main app to display progress information. The processing that goes on in the loop is vaired, but is not too time consuming (it could just be a simple file copy, for example, or may involve encryption...). My main question is: What is the best memory buffer size to use? Thinking about physical disk layouts, I could pick 2k, which would cover a CD sector size and is a nice multiple of a 512 byte hard disk sector. Higher up the abstraction tree, you could go for a larger buffer which could read an entire FAT cluster at a time. I realise with today's PC's, I could go for a more memory hungry option (a couple of MiB, for example), but then I increase the time between UI updates and the user perceives a less responsive app. As an aside, I'm eventually hoping to provide a similar interface to files hosted on FTP / HTTP servers (over a local network / fastish DSL). What would be the best memory buffer size for those (again, a "best-case" tradeoff between perceived responsiveness vs. performance).

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• C++ pimpl idiom wastes an instruction vs. C style?

  - by Rob

  (Yes, I know that one machine instruction usually doesn't matter. I'm asking this question because I want to understand the pimpl idiom, and use it in the best possible way; and because sometimes I do care about one machine instruction.) In the sample code below, there are two classes, Thing and OtherThing. Users would include "thing.hh". Thing uses the pimpl idiom to hide it's implementation. OtherThing uses a C style – non-member functions that return and take pointers. This style produces slightly better machine code. I'm wondering: is there a way to use C++ style – ie, make the functions into member functions – and yet still save the machine instruction. I like this style because it doesn't pollute the namespace outside the class. Note: I'm only looking at calling member functions (in this case, calc). I'm not looking at object allocation. Below are the files, commands, and the machine code, on my Mac. thing.hh: class ThingImpl; class Thing { ThingImpl *impl; public: Thing(); int calc(); }; class OtherThing; OtherThing *make_other(); int calc(OtherThing *); thing.cc: #include "thing.hh" struct ThingImpl { int x; }; Thing::Thing() { impl = new ThingImpl; impl->x = 5; } int Thing::calc() { return impl->x + 1; } struct OtherThing { int x; }; OtherThing *make_other() { OtherThing *t = new OtherThing; t->x = 5; } int calc(OtherThing *t) { return t->x + 1; } main.cc (just to test the code actually works...) #include "thing.hh" #include <cstdio> int main() { Thing *t = new Thing; printf("calc: %d\n", t->calc()); OtherThing *t2 = make_other(); printf("calc: %d\n", calc(t2)); } Makefile: all: main thing.o : thing.cc thing.hh g++ -fomit-frame-pointer -O2 -c thing.cc main.o : main.cc thing.hh g++ -fomit-frame-pointer -O2 -c main.cc main: main.o thing.o g++ -O2 -o $@ $^ clean: rm *.o rm main Run make and then look at the machine code. On the mac I use otool -tv thing.o | c++filt. On linux I think it's objdump -d thing.o. Here is the relevant output: Thing::calc(): 0000000000000000 movq (%rdi),%rax 0000000000000003 movl (%rax),%eax 0000000000000005 incl %eax 0000000000000007 ret calc(OtherThing*): 0000000000000010 movl (%rdi),%eax 0000000000000012 incl %eax 0000000000000014 ret Notice the extra instruction because of the pointer indirection. The first function looks up two fields (impl, then x), while the second only needs to get x. What can be done?

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• optimize a string.Format + replace.

  - by acidzombie24

  I have this function. The visual studio profile marked the line with string.Format as hot and were i spend much of my time. How can i write this loop more efficiently? public string EscapeNoPredicate(string sz) { var s = new StringBuilder(sz); s.Replace(sepStr, sepStr + sepStr); foreach (char v in IllegalChars) { string s2 = string.Format("{0}{1:X2}", seperator, (Int16)v); s.Replace(v.ToString(), s2); } return s.ToString(); }

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• Why isn't the copy constructor elided here?

  - by Jesse Beder

  (I'm using gcc with -O2.) This seems like a straightforward opportunity to elide the copy constructor, since there are no side-effects to accessing the value of a field in a bar's copy of a foo; but the copy constructor is called, since I get the output meep meep!. #include <iostream> struct foo { foo(): a(5) { } foo(const foo& f): a(f.a) { std::cout << "meep meep!\n"; } int a; }; struct bar { foo F() const { return f; } foo f; }; int main() { bar b; int a = b.F().a; return 0; }

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• Specific template for the first element.

  - by Kalinin

  I have a template: <xsl:template match="paragraph"> ... </xsl:template> I call it: <xsl:apply-templates select="paragraph"/> For the first element I need to do: <xsl:template match="paragraph[1]"> ... <xsl:apply-templates select="."/><!-- I understand that this does not work --> ... </xsl:template> How to call <xsl:apply-templates select="paragraph"/> (for the first element paragraph) from the template <xsl:template match="paragraph[1]">? So far that I have something like a loop. I solve this problem so (but I do not like it): <xsl:for-each select="paragraph"> <xsl:choose> <xsl:when test="position() = 1"> ... <xsl:apply-templates select="."/> ... </xsl:when> <xsl:otherwise> <xsl:apply-templates select="."/> </xsl:otherwise> </xsl:choose> </xsl:for-each>

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• Inline functions in C#?

  - by Dinah

  How do you do "inline functions" in C#? I don't think I understand the concept. Are they like anonymous methods? Like lambda functions?

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• PHP programmers: I would like to optimize the speed at which my PHP scripts execute. What could I do

  - by racl101

  I have zlib and Zend Optimizer enabled on my server. I have read about the zlib.output_compression directive. Are there any caveats with turning this directive on in my server?

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• Will an optimizing compiler remove calls to a method whose result will be multiplied by zero?

  - by Tim R.

  Suppose you have a computationally expensive method, Compute(p), which returns some float, and another method, Falloff(p), which returns another float from zero to one. If you compute Falloff(p) * Compute(p), will Compute(p) still run when Falloff(p) returns zero? Or would you need to write a special case to prevent Compute(p) from running unnecessarily? Theoretically, an optimizing compiler could determine that omitting Compute when Falloff returns zero would have no effect on the program. However, this is kind of hard to test, since if you have Compute output some debug data to determine whether it is running, the compiler would know not to omit it because of that debug info, resulting in sort of a Schrodinger's cat situation. I know the safe solution to this problem is just to add the special case, but I'm just curious.

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• C++ performance, optimizing compiler, empty function in .cpp

  - by Dodo

  I've a very basic class, name it Basic, used in nearly all other files in a bigger project. In some cases, there needs to be debug output, but in release mode, this should not be enabled and be a NOOP. Currently there is a define in the header, which switches a makro on or off, depending on the setting. So this is definetely a NOOP, when switched off. I'm wondering, if I have the following code, if a compiler (MSVS / gcc) is able to optimize out the function call, so that it is again a NOOP. (By doing that, the switch could be in the .cpp and switching will be much faster, compile/link time wise). --Header-- void printDebug(const Basic* p); class Basic { Basic() { simpleSetupCode; // this should be a NOOP in release, // but constructor could be inlined printDebug(this); } }; --Source-- // PRINT_DEBUG defined somewhere else or here #if PRINT_DEBUG void printDebug(const Basic* p) { // Lengthy debug print } #else void printDebug(const Basic* p) {} #endif

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• Optimizing sparse dot-product in C#

  - by Haggai

  Hello. I'm trying to calculate the dot-product of two very sparse associative arrays. The arrays contain an ID and a value, so the calculation should be done only on those IDs that are common to both arrays, e.g. <(1, 0.5), (3, 0.7), (12, 1.3) * <(2, 0.4), (3, 2.3), (12, 4.7) = 0.7*2.3 + 1.3*4.7 . My implementation (call it dict) currently uses Dictionaries, but it is too slow to my taste. double dot_product(IDictionary<int, double> arr1, IDictionary<int, double> arr2) { double res = 0; double val2; foreach (KeyValuePair<int, double> p in arr1) if (arr2.TryGetValue(p.Key, out val2)) res += p.Value * val2; return res; } The full arrays have about 500,000 entries each, while the sparse ones are only tens to hundreds entries each. I did some experiments with toy versions of dot products. First I tried to multiply just two double arrays to see the ultimate speed I can get (let's call this "flat"). Then I tried to change the implementation of the associative array multiplication using an int[] ID array and a double[] values array, walking together on both ID arrays and multiplying when they are equal (let's call this "double"). I then tried to run all three versions with debug or release, with F5 or Ctrl-F5. The results are as follows: debug F5: dict: 5.29s double: 4.18s (79% of dict) flat: 0.99s (19% of dict, 24% of double) debug ^F5: dict: 5.23s double: 4.19s (80% of dict) flat: 0.98s (19% of dict, 23% of double) release F5: dict: 5.29s double: 3.08s (58% of dict) flat: 0.81s (15% of dict, 26% of double) release ^F5: dict: 4.62s double: 1.22s (26% of dict) flat: 0.29s ( 6% of dict, 24% of double) I don't understand these results. Why isn't the dictionary version optimized in release F5 as do the double and flat versions? Why is it only slightly optimized in the release ^F5 version while the other two are heavily optimized? Also, since converting my code into the "double" scheme would mean lots of work - do you have any suggestions how to optimize the dictionary one? Thanks! Haggai

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