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• Movies recommendation engine conceptual database design

  - by Supyxy

  I am working at an movie recommendations engine and i'm facing a DB design issue. My actual database looks like this: MOVIES [ID,TITLE] KEYWORDS_TABLE [ID,KEY_ID] - where ID is Foreign Key for MOVIES.id and KEY_ID is a key for a text keywords table This is not the entire DB, but i showed here what's important for my problem. I have about 50,000 movies and about 1,3 milion keywords correlations, and basically my algorithm consists in extracting all the who have the same keywords with a given movie, then ordering them by the number of keywords correlations. For example i looked for a movie similar to 'Cast away' and it returned 'Six days and six nights' because it had the most keywords correlations (4 keywords): Island Airplane crash Stranded Pilot The algorithm is based on more factors, but this one is the most important and the most difficult for the approach. Basically what i do now is getting all the movies that have at least one keyword similar to the given movie and then ordering them by other factors which are not important for a moment. There wouldn't be any problem if there weren't so many records, a query lasts in many cases up to 10-20 seconds and some of them return even over 5000 movies. Someone already helped me on here (thanks Mark Byers) with optimizing the query but that's not enough because it takes too longer SELECT DISTINCT M.title FROM keywords_table K1 JOIN keywords_table K2 ON K2.key_id = K1.key_id JOIN movies M ON K2.id = M.id WHERE K1.id = 4 So i thought it would be better if i pre-made those lists with movies recommendations for each movie, but i'm not sure how to design the tables.. whatever is it a good idea or how would you take this approach?

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• Passing variables to functions

  - by Faken

  A quick question: When i pass a variable to a function, dose the program make a copy of that variable to use in the function? If it dose and I knew that the function would only read the variable and never write to it, is it possible to pass a variable to the function without creating a copy of that variable or should I just leave that up to the compiler optimizations to do that automatically for me?

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• Function-Local Static Const variable Initialization semantics.

  - by Hassan Syed

  The questions are in bold, for those that cannot be bothered reading a question in depth. This is a followup to this question. It is to do with the initialization semantics of static variables in functions. Static variables should be initialized once, and their internal state might be altered later - as I (currently) do in the linked question. However, the code in question does not require the feature to change the state of the variable later. Let me clarrify my position, since I don't require the string object's internal state to change. The code is for a trait class for meta programming, and as such would would benifit from a const char * const ptr -- thus Ideally a local cost static const variable is needed. My educated guess is that in this case the string in question will be optimally placed in memory by the link-loader, and that the code is more secure and maps to the intended semantics. This leads to the semantics of such a variable "The C++ Programming language Third Edition -- Stroustrup" does not have anything (that I could find) to say about this matter. All that is said is that the variable is initialized once when the flow of control of the thread first reaches the code. This leads me to ponder if the following code would be sensible, and if not what are the intended semantics ?. #include <iostream> const char * const GetString(const char * x_in) { static const char * const x = x_in; return x; } int main() { const char * const temp = GetString("yahoo"); std::cout << temp << std::endl; const char * const temp2 = GetString("yahoo2"); std::cout << temp2 << std::endl; } The following compiles on GCC and prints "yahoo" twice. Which is what I want -- However it might not be standards compliant (which is why I post this question). It might be more elegant to have two functions, "SetString" and "String" where the latter forwards to the first. If it is standards compliant does someone know of a templates implementation in boost (or elsewhere) ?

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• Position of least significant bit that is set

  - by peterchen

  I am looking for an efficient way to determine the position of the least significant bit that is set in an integer, e.g. for 0x0FF0 it would be 4. A trivial implementation is this: unsigned GetLowestBitPos(unsigned value) { assert(value != 0); // handled separately unsigned pos = 0; while (!(value & 1)) { value >>= 1; ++pos; } return pos; } Any ideas how to squeeze some cycles out of it? (Note: this question is for people that enjoy such things, not for people to tell me xyzoptimization is evil.) [edit] Thanks everyone for the ideas! I've learnt a few other things, too. Cool!

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• FxCop giving a warning on private constructor CA1823 and CA1053

  - by Luis Sánchez

  I have a class that looks like the following: Public Class Utilities Public Shared Function blah(userCode As String) As String 'doing some stuff End Function End Class I'm running FxCop 10 on it and it says: "Because type 'Utilities' contains only 'static' ( 'Shared' in Visual Basic) members, add a default private constructor to prevent the compiler from adding a default public constructor." Ok, you're right Mr. FxCop, I'll add a private constructor: Private Utilities() Now I'm having: "It appears that field 'Utilities.Utilities' is never used or is only ever assigned to. Use this field or remove it." Any ideas of what should I do to get rid of both warnings?

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• Optimizing spacing of mesh containing a given set of points

  - by Feynman

  I tried to summarize the this as best as possible in the title. I am writing an initial value problem solver in the most general way possible. I start with an arbitrary number of initial values at arbitrary locations (inside a boundary.) The first part of my program creates a mesh/grid (I am not sure which is the correct nuance), with N points total, that contains all the initial values. My goal is to optimize the mesh such that the spacing is as uniform as possible. My solver seems to work half decently (it needs some more obscure debugging that is not relevant here.) I am starting with one dimension. I intend to generalize the algorithm to an arbitrary number of dimensions once I get it working consistently. I am writing my code in fortran, but feel free to reply with pseudocode or the language of your choice. Allow me to elaborate with an example: Say I am working on a closed interval [1,10] xmin=1 xmax=10 Say I have 3 initial points: xmin, 5 and xmax num_ivc=3 known(num_ivc)=[xmin,5,xmax] //my arrays start at 1. Assume "known" starts sorted I store my mesh/grid points in an array called coord. Say I want 10 points total in my mesh/grid. N=10 coord(10) Remember, all this is arbitrary--except the variable names of course. The algorithm should set coord to {1,2,3,4,5,6,7,8,9,10} Now for a less trivial example: num_ivc=3 known(num_ivc)=[xmin,5.5,xmax or just num_ivc=1 known(num_ivc)=[5.5] Now, would you have 5 evenly spaced points on the interval [1, 5.5] and 5 evenly spaced points on the interval (5.5, 10]? But there is more space between 1 and 5.5 than between 5.5 and 10. So would you have 6 points on [1, 5.5] followed by 4 on (5.5 to 10]. The key is to minimize the difference in spacing. I have been working on this for 2 days straight and I can assure you it is a lot trickier than it sounds. I have written code that only works if N is large only works if N is small only works if it the known points are close together only works if it the known points are far apart only works if at least one of the known points is near a boundary only works if none of the known points are near a boundary So as you can see, I have coded the gamut of almost-solutions. I cannot figure out a way to get it to perform equally well in all possible scenarios (that is, create the optimum spacing.)

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• Optimize code performance when odd/even threads are doing different things in CUDA

  - by Orion Nebula

  Hi all! I have two large vectors, I am trying to do some sort of element multiplication, where an even-numbered element in the first vector is multiplied by the next odd-numbered element in the second vector .... and where the odd-numbered element in the first vector is multiplied by the preceding even-numbered element in the second vector Ex. vector 1 is V1(1) V1(2) V1(3) V1(4) vector 2 is V2(1) V2(2) V2(3) V2(4) V1(1) * V2(2) V1(3) * V2(4) V1(2) * V2(1) V1(4) * V2(3) I have written a Cuda code to do this: (Pds has the elements of the first vector in shared memory, Nds the second Vector) //instead of using %2 .. i check for the first bit to decide if number is odd/even -- faster if ((tx & 0x0001) == 0x0000) Nds[tx+1] = Pds[tx] * Nds[tx+1]; else Nds[tx-1] = Pds[tx] * Nds[tx-1]; __syncthreads(); Is there anyway to further accelerate this code or avoid divergence ? Thanks

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• MySQL efficiency as it relates to the database/table size

  - by mlissner

  I'm building a system using django, Sphinx and MySQL that's very quickly becoming quite large. The database currently has about 2000 rows, and I've written a program that's going to populate it with another 40,000 rows in a couple days. Since the database is live right now, and since I've never had a database with this much information in it, I'm worried about some things: Is adding all these rows going to seriously degrade the efficiency of my django app? Will I need to go back through it and optimize all my database calls so they're doing things more cleverly? Or will this make the database slow all around to the extent that I can't do anything about it at all? If you scoff at my 40k rows, then, my next question is, at what point SHOULD I be concerned? I will likely be adding another couple hundred thousand soon, so I worry, and I fret. How is sphinx going to feel about all this? Is it going to freak out when it realizes it has to index all this data? Or will it be fine? Is this normal for it? If it is, at what point should I be concerned that it's too much data for Sphinx? Thanks for any thoughts.

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• PHP Initialising strings as boolean first

  - by Anriëtte Myburgh

  I'm in the habit of initialising variables in PHP to false and then applying whatever (string, boolean, float) value to it later. Which would you reckon is better? $name = false; if (condition == true) { $name = $something_else; } if ($name) { …do something… } vs. $name =''; if (condition == true) { $name = $something_else; } if (!empty($name)) { …do something… } Which would you reckon can possibly give better performance? Which method would you use?

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• Creating an index on a view with OpenQuery

  - by Sung Meister

  SQL Server doesn't allow creating an view with schema binding where the view query uses OpenQuery as shown below. Is there a way or a walk-around to create an index on such a view?

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• What is the Fastest Way to Check for a Keyword in a List of Keywords in Delphi?

  - by lkessler

  I have a small list of keywords. What I'd really like to do is akin to: case MyKeyword of 'CHIL': (code for CHIL); 'HUSB': (code for HUSB); 'WIFE': (code for WIFE); 'SEX': (code for SEX); else (code for everything else); end; Unfortunately the CASE statement can't be used like that for strings. I could use the straight IF THEN ELSE IF construct, e.g.: if MyKeyword = 'CHIL' then (code for CHIL) else if MyKeyword = 'HUSB' then (code for HUSB) else if MyKeyword = 'WIFE' then (code for WIFE) else if MyKeyword = 'SEX' then (code for SEX) else (code for everything else); but I've heard this is relatively inefficient. What I had been doing instead is: P := pos(' ' + MyKeyword + ' ', ' CHIL HUSB WIFE SEX '); case P of 1: (code for CHIL); 6: (code for HUSB); 11: (code for WIFE); 17: (code for SEX); else (code for everything else); end; This, of course is not the best programming style, but it works fine for me and up to now didn't make a difference. So what is the best way to rewrite this in Delphi so that it is both simple, understandable but also fast? (For reference, I am using Delphi 2009 with Unicode strings.) Followup: Toby recommended I simply use the If Then Else construct. Looking back at my examples that used a CASE statement, I can see how that is a viable answer. Unfortunately, my inclusion of the CASE inadvertently hid my real question. I actually don't care which keyword it is. That is just a bonus if the particular method can identify it like the POS method can. What I need is to know whether or not the keyword is in the set of keywords. So really I want to know if there is anything better than: if pos(' ' + MyKeyword + ' ', ' CHIL HUSB WIFE SEX ') > 0 then The If Then Else equivalent does not seem better in this case being: if (MyKeyword = 'CHIL') or (MyKeyword = 'HUSB') or (MyKeyword = 'WIFE') or (MyKeyword = 'SEX') then In Barry's comment to Kornel's question, he mentions the TDictionary Generic. I've not yet picked up on the new Generic collections and it looks like I should delve into them. My question here would be whether they are built for efficiency and how would using TDictionary compare in looks and in speed to the above two lines? In later profiling, I have found that the concatenation of strings as in: (' ' + MyKeyword + ' ') is VERY expensive time-wise and should be avoided whenever possible. Almost any other solution is better than doing this.

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• explicit copy constructor or implicit parameter by value

  - by R Samuel Klatchko

  I recently read (and unfortunately forgot where), that the best way to write operator= is like this: foo &operator=(foo other) { swap(*this, other); return *this; } instead of this: foo &operator=(const foo &other) { foo copy(other); swap(*this, copy); return *this; } The idea is that if operator= is called with an rvalue, the first version can optimize away construction of a copy. So when called with a rvalue, the first version is faster and when called with an lvalue the two are equivalent. I'm curious as to what other people think about this? Would people avoid the first version because of lack of explicitness? Am I correct that the first version can be better and can never be worse?

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• How can I further optimize this color difference function?

  - by aLfa

  I have made this function to calculate color differences in the CIE Lab colorspace, but it lacks speed. Since I'm not a Java expert, I wonder if any Java guru around has some tips that can improve the speed here. The code is based on the matlab function mentioned in the comment block. /** * Compute the CIEDE2000 color-difference between the sample color with * CIELab coordinates 'sample' and a standard color with CIELab coordinates * 'std' * * Based on the article: * "The CIEDE2000 Color-Difference Formula: Implementation Notes, * Supplementary Test Data, and Mathematical Observations,", G. Sharma, * W. Wu, E. N. Dalal, submitted to Color Research and Application, * January 2004. * available at http://www.ece.rochester.edu/~gsharma/ciede2000/ */ public static double deltaE2000(double[] lab1, double[] lab2) { double L1 = lab1[0]; double a1 = lab1[1]; double b1 = lab1[2]; double L2 = lab2[0]; double a2 = lab2[1]; double b2 = lab2[2]; // Cab = sqrt(a^2 + b^2) double Cab1 = Math.sqrt(a1 * a1 + b1 * b1); double Cab2 = Math.sqrt(a2 * a2 + b2 * b2); // CabAvg = (Cab1 + Cab2) / 2 double CabAvg = (Cab1 + Cab2) / 2; // G = 1 + (1 - sqrt((CabAvg^7) / (CabAvg^7 + 25^7))) / 2 double CabAvg7 = Math.pow(CabAvg, 7); double G = 1 + (1 - Math.sqrt(CabAvg7 / (CabAvg7 + 6103515625.0))) / 2; // ap = G * a double ap1 = G * a1; double ap2 = G * a2; // Cp = sqrt(ap^2 + b^2) double Cp1 = Math.sqrt(ap1 * ap1 + b1 * b1); double Cp2 = Math.sqrt(ap2 * ap2 + b2 * b2); // CpProd = (Cp1 * Cp2) double CpProd = Cp1 * Cp2; // hp1 = atan2(b1, ap1) double hp1 = Math.atan2(b1, ap1); // ensure hue is between 0 and 2pi if (hp1 < 0) { // hp1 = hp1 + 2pi hp1 += 6.283185307179586476925286766559; } // hp2 = atan2(b2, ap2) double hp2 = Math.atan2(b2, ap2); // ensure hue is between 0 and 2pi if (hp2 < 0) { // hp2 = hp2 + 2pi hp2 += 6.283185307179586476925286766559; } // dL = L2 - L1 double dL = L2 - L1; // dC = Cp2 - Cp1 double dC = Cp2 - Cp1; // computation of hue difference double dhp = 0.0; // set hue difference to zero if the product of chromas is zero if (CpProd != 0) { // dhp = hp2 - hp1 dhp = hp2 - hp1; if (dhp > Math.PI) { // dhp = dhp - 2pi dhp -= 6.283185307179586476925286766559; } else if (dhp < -Math.PI) { // dhp = dhp + 2pi dhp += 6.283185307179586476925286766559; } } // dH = 2 * sqrt(CpProd) * sin(dhp / 2) double dH = 2 * Math.sqrt(CpProd) * Math.sin(dhp / 2); // weighting functions // Lp = (L1 + L2) / 2 - 50 double Lp = (L1 + L2) / 2 - 50; // Cp = (Cp1 + Cp2) / 2 double Cp = (Cp1 + Cp2) / 2; // average hue computation // hp = (hp1 + hp2) / 2 double hp = (hp1 + hp2) / 2; // identify positions for which abs hue diff exceeds 180 degrees if (Math.abs(hp1 - hp2) > Math.PI) { // hp = hp - pi hp -= Math.PI; } // ensure hue is between 0 and 2pi if (hp < 0) { // hp = hp + 2pi hp += 6.283185307179586476925286766559; } // LpSqr = Lp^2 double LpSqr = Lp * Lp; // Sl = 1 + 0.015 * LpSqr / sqrt(20 + LpSqr) double Sl = 1 + 0.015 * LpSqr / Math.sqrt(20 + LpSqr); // Sc = 1 + 0.045 * Cp double Sc = 1 + 0.045 * Cp; // T = 1 - 0.17 * cos(hp - pi / 6) + // + 0.24 * cos(2 * hp) + // + 0.32 * cos(3 * hp + pi / 30) - // - 0.20 * cos(4 * hp - 63 * pi / 180) double hphp = hp + hp; double T = 1 - 0.17 * Math.cos(hp - 0.52359877559829887307710723054658) + 0.24 * Math.cos(hphp) + 0.32 * Math.cos(hphp + hp + 0.10471975511965977461542144610932) - 0.20 * Math.cos(hphp + hphp - 1.0995574287564276334619251841478); // Sh = 1 + 0.015 * Cp * T double Sh = 1 + 0.015 * Cp * T; // deltaThetaRad = (pi / 3) * e^-(36 / (5 * pi) * hp - 11)^2 double powerBase = hp - 4.799655442984406; double deltaThetaRad = 1.0471975511965977461542144610932 * Math.exp(-5.25249016001879 * powerBase * powerBase); // Rc = 2 * sqrt((Cp^7) / (Cp^7 + 25^7)) double Cp7 = Math.pow(Cp, 7); double Rc = 2 * Math.sqrt(Cp7 / (Cp7 + 6103515625.0)); // RT = -sin(delthetarad) * Rc double RT = -Math.sin(deltaThetaRad) * Rc; // de00 = sqrt((dL / Sl)^2 + (dC / Sc)^2 + (dH / Sh)^2 + RT * (dC / Sc) * (dH / Sh)) double dLSl = dL / Sl; double dCSc = dC / Sc; double dHSh = dH / Sh; return Math.sqrt(dLSl * dLSl + dCSc * dCSc + dHSh * dHSh + RT * dCSc * dHSh); }

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• Combine static files or load in parallel

  - by Niall Collins

  I am at present introducing code to my site to combine css and javascript files. Is there a way without having to include an external library to load javascript asynchronously or in parallel? I have read on some blogs that combining of files can be counter productive as the load of the http request can be large and its better to load multiple files in parallel. Opinions on this? I am caching my javascript/css. And would have thought it was better to combine rather than have multiple http requests.

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• Complicated idea - how to create car racing for my RPG game's players

  - by Donator

  So, I want to create car racing for my RPG game's players. Player can create race and choose how many participants can participate in race. After race is being created, other people can join it. When the maximum participants are collected, race begins. My idea, when the last participant joins, then instantly choose the winner (who's car is the best, that person wins), but how can I do it? If I choose to pick the winner after the last participant joins, then I have to put many queries in one page (select data from table, then delete the race, then select players' cars' statistics and pick the winner and then again, using mysql, send message to everyone). But this idea is really not optimal and it will lag cruelly for that last person. Maybe you have any ideas how I can avoid lag and make it more optimal. Thank you very much.

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• gcc memory alignment pragma

  - by aaa

  hello. Does gcc have memory alignment pragma, akin #pragma vector aligned in Intel compiler? I would like to tell compiler to optimize particular loop using aligned loads/store instructions. Thanks

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• How do you control what your C compiler Optimizes?

  - by Jordan S

  I am writing the firmware for an embedded device in C using the Silicon Labs IDE and the SDCC compiler. The device architecture is based on the 8051 family. The function in question is shown below. The function is used to set the ports on my MCU to drive a stepper motor. It gets called in by an interrupt handler. The big switch statement just sets the ports to the proper value for the next motor step. The bottom part of the function looks at an input from a hall effect sensor and a number of steps moved in order to detect if the motor has stalled. The problem is, for some reason the second IF statement that looks like this if (StallDetector > (GapSize + 20)) { HandleStallEvent(); } always seems to get optimized out. If I try to put a breakpoint at the HandleStallEvent() call the IDE gives me a message saying "No Address Correlation to this line number". I am not really good enough at reading assembly to tell what it is doing but I have pasted a snippet from the asm output below. Any help would be much appreciated. void OperateStepper(void) { //static bit LastHomeMagState = HomeSensor; static bit LastPosMagState = PosSensor; if(PulseMotor) { if(MoveDirection == 1) // Go clockwise { switch(STEPPER_POSITION) { case 'A': STEPPER_POSITION = 'B'; P1 = 0xFD; break; case 'B': STEPPER_POSITION = 'C'; P1 = 0xFF; break; case 'C': STEPPER_POSITION = 'D'; P1 = 0xFE; break; case 'D': STEPPER_POSITION = 'A'; P1 = 0xFC; break; default: STEPPER_POSITION = 'A'; P1 = 0xFC; } //end switch } else // Go CounterClockwise { switch(STEPPER_POSITION) { case 'A': STEPPER_POSITION = 'D'; P1 = 0xFE; break; case 'B': STEPPER_POSITION = 'A'; P1 = 0xFC; break; case 'C': STEPPER_POSITION = 'B'; P1 = 0xFD; break; case 'D': STEPPER_POSITION = 'C'; P1 = 0xFF; break; default: STEPPER_POSITION = 'A'; P1 = 0xFE; } //end switch } //end else MotorSteps++; StallDetector++; if(PosSensor != LastPosMagState) { StallDetector = 0; LastPosMagState = PosSensor; } else { if (PosSensor == ON) { if (StallDetector > (MagnetSize + 20)) { HandleStallEvent(); } } else if (PosSensor == OFF) { if (StallDetector > (GapSize + 20)) { HandleStallEvent(); } } } } //end if PulseMotor } ... and the asm output for the the bottom part of this function... ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:653: if(PosSensor != LastPosMagState) mov c,_P1_4 jb _OperateStepper_LastPosMagState_1_1,00158$ cpl c 00158$: jc 00126$ C$MotionControl.c$655$3$7 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:655: StallDetector = 0; clr a mov _StallDetector,a mov (_StallDetector + 1),a C$MotionControl.c$657$3$7 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:657: LastPosMagState = PosSensor; mov c,_P1_4 mov _OperateStepper_LastPosMagState_1_1,c ret 00126$: C$MotionControl.c$661$2$8 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:661: if (PosSensor == ON) jb _P1_4,00123$ C$MotionControl.c$663$4$9 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:663: if (StallDetector > (MagnetSize + 20)) mov a,_MagnetSize mov r2,a rlc a subb a,acc mov r3,a mov a,#0x14 add a,r2 mov r2,a clr a addc a,r3 mov r3,a clr c mov a,r2 subb a,_StallDetector mov a,r3 subb a,(_StallDetector + 1) jnc 00130$ C$MotionControl.c$665$5$10 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:665: HandleStallEvent(); ljmp _HandleStallEvent 00123$: C$MotionControl.c$668$2$8 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:668: else if (PosSensor == OFF) jnb _P1_4,00130$ C$MotionControl.c$670$4$11 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:670: if (StallDetector > (GapSize + 20)) mov a,#0x14 add a,_GapSize mov r2,a clr a addc a,(_GapSize + 1) mov r3,a clr c mov a,r2 subb a,_StallDetector mov a,r3 subb a,(_StallDetector + 1) jnc 00130$ C$MotionControl.c$672$5$12 ==. ; C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:672: HandleStallEvent(); C$MotionControl.c$678$2$1 ==. XG$OperateStepper$0$0 ==. ljmp _HandleStallEvent 00130$: ret It looks to me like the compiler is NOT optimizing out this second if statement from the looks of the asm but if that is the case why does the IDE not allow me so set a breakpoint there? Maybe it's just a dumb IDE!

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• SQL Table Setup Advice

  - by Ozzy

  Hi all. Basically I have an xml feed from an offsite server. The xml feed has one parameter ?value=n now N can only be between 1 and 30 What ever value i pick, there will always be 4000 rows returned from the XML file. My script will call this xml file 30 times for each value once a day. So thats 120000 rows. I will be doing quite complicated queries on these rows. But the main thing is I will always filter by value first so SELECT * WHERE value = 'N' etc. That will ALWAYS be used. Now is it better to have one table where all 120k rows are stored? or 30 tables were 4k rows are stored? EDIT: the SQL database in question will be MySQL

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• How to optimize neural network by using genetic algorithm?

  - by Billy Coen

  I'm quite new with this topic so any help would be great. What i need is to optimize a neural network in MATLAB by using GA. My network has [2x98] input and [1x98] target, i've tried consulting matlab help but im still kind of clueless about what to do :( so, any help would be appreciated. Thanks in advance. edit: i guess i didn't say what is there to be optimized as Dan said in the 1st answer. I guess most important thing is number of hidden neurons. And maybe number of hidden layers and training parameters like number of epochs or so. Sorry for not providing enough info, i'm still learning about this.

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• Why doesn't gcc remove this check of a non-volatile variable?

  - by Thomas

  This question is mostly academic. I ask out of curiosity, not because this poses an actual problem for me. Consider the following incorrect C program. #include <signal.h> #include <stdio.h> static int running = 1; void handler(int u) { running = 0; } int main() { signal(SIGTERM, handler); while (running) ; printf("Bye!\n"); return 0; } This program is incorrect because the handler interrupts the program flow, so running can be modified at any time and should therefore be declared volatile. But let's say the programmer forgot that. gcc 4.3.3, with the -O3 flag, compiles the loop body (after one initial check of the running flag) down to the infinite loop .L7: jmp .L7 which was to be expected. Now we put something trivial inside the while loop, like: while (running) putchar('.'); And suddenly, gcc does not optimize the loop condition anymore! The loop body's assembly now looks like this (again at -O3): .L7: movq stdout(%rip), %rsi movl $46, %edi call _IO_putc movl running(%rip), %eax testl %eax, %eax jne .L7 We see that running is re-loaded from memory each time through the loop; it is not even cached in a register. Apparently gcc now thinks that the value of running could have changed. So why does gcc suddenly decide that it needs to re-check the value of running in this case?

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• set difference in SQL query

  - by TheObserver

  I'm trying to select records with a statement SELECT * FROM A WHERE LEFT(B, 5) IN (SELECT * FROM (SELECT LEFT(A.B,5), COUNT(DISTINCT A.C) c_count FROM A GROUP BY LEFT(B,5) ) p1 WHERE p1.c_count = 1 ) AND C IN (SELECT * FROM (SELECT A.C , COUNT(DISTINCT LEFT(A.B,5)) b_count FROM A GROUP BY C ) p2 WHERE p2.b_count = 1) which takes a long time to run ~15 sec. Is there a better way of writing this SQL?

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• Does Delphi really handle dynamic classes better than static?

  - by John

  Hello, I was told more than once that Delphi handles dynamic classes better than static.Thereby using the following: type Tsomeclass=class(TObject) private procedure proc1; public someint:integer; procedure proc2; end; var someclass:TSomeclass; implementation ... initialization someclass:=TSomeclass.Create; finalization someclass.Free; rather than type Tsomeclass=class private class procedure proc1; public var someint:integer; class procedure proc2; end; 90% of the classes in the project I'm working on have and need only one instance.Do I really have to use the first way for using those classes? Is it better optimized,handled by Delphi? Sorry,I have no arguments to backup this hypothesis,but I want an expert's opinion. Thanks in advance!

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• Optimize date query for large child tables: GiST or GIN?

  - by Dave Jarvis

  Problem 72 child tables, each having a year index and a station index, are defined as follows: CREATE TABLE climate.measurement_12_013 ( -- Inherited from table climate.measurement_12_013: id bigint NOT NULL DEFAULT nextval('climate.measurement_id_seq'::regclass), -- Inherited from table climate.measurement_12_013: station_id integer NOT NULL, -- Inherited from table climate.measurement_12_013: taken date NOT NULL, -- Inherited from table climate.measurement_12_013: amount numeric(8,2) NOT NULL, -- Inherited from table climate.measurement_12_013: category_id smallint NOT NULL, -- Inherited from table climate.measurement_12_013: flag character varying(1) NOT NULL DEFAULT ' '::character varying, CONSTRAINT measurement_12_013_category_id_check CHECK (category_id = 7), CONSTRAINT measurement_12_013_taken_check CHECK (date_part('month'::text, taken)::integer = 12) ) INHERITS (climate.measurement) CREATE INDEX measurement_12_013_s_idx ON climate.measurement_12_013 USING btree (station_id); CREATE INDEX measurement_12_013_y_idx ON climate.measurement_12_013 USING btree (date_part('year'::text, taken)); (Foreign key constraints to be added later.) The following query runs abysmally slow due to a full table scan: SELECT count(1) AS measurements, avg(m.amount) AS amount FROM climate.measurement m WHERE m.station_id IN ( SELECT s.id FROM climate.station s, climate.city c WHERE -- For one city ... -- c.id = 5182 AND -- Where stations are within an elevation range ... -- s.elevation BETWEEN 0 AND 3000 AND 6371.009 * SQRT( POW(RADIANS(c.latitude_decimal - s.latitude_decimal), 2) + (COS(RADIANS(c.latitude_decimal + s.latitude_decimal) / 2) * POW(RADIANS(c.longitude_decimal - s.longitude_decimal), 2)) ) <= 50 ) AND -- -- Begin extracting the data from the database. -- -- The data before 1900 is shaky; insufficient after 2009. -- extract( YEAR FROM m.taken ) BETWEEN 1900 AND 2009 AND -- Whittled down by category ... -- m.category_id = 1 AND m.taken BETWEEN -- Start date. (extract( YEAR FROM m.taken )||'-01-01')::date AND -- End date. Calculated by checking to see if the end date wraps -- into the next year. If it does, then add 1 to the current year. -- (cast(extract( YEAR FROM m.taken ) + greatest( -1 * sign( (extract( YEAR FROM m.taken )||'-12-31')::date - (extract( YEAR FROM m.taken )||'-01-01')::date ), 0 ) AS text)||'-12-31')::date GROUP BY extract( YEAR FROM m.taken ) The sluggishness comes from this part of the query: m.taken BETWEEN /* Start date. */ (extract( YEAR FROM m.taken )||'-01-01')::date AND /* End date. Calculated by checking to see if the end date wraps into the next year. If it does, then add 1 to the current year. */ (cast(extract( YEAR FROM m.taken ) + greatest( -1 * sign( (extract( YEAR FROM m.taken )||'-12-31')::date - (extract( YEAR FROM m.taken )||'-01-01')::date ), 0 ) AS text)||'-12-31')::date The HashAggregate from the plan shows a cost of 10006220141.11, which is, I suspect, on the astronomically huge side. There is a full table scan on the measurement table (itself having neither data nor indexes) being performed. The table aggregates 237 million rows from its child tables. Question What is the proper way to index the dates to avoid full table scans? Options I have considered: GIN GiST Rewrite the WHERE clause Separate year_taken, month_taken, and day_taken columns to the tables What are your thoughts? Thank you!

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• How can i benchmark method execution time in java?

  - by KP65

  I have a program which i have myself written in java, but I want to test method execution times and get timings for specific methods. I was wondering if this is possible, by maybe somehow an eclipse plug-in? or maybe inserting some code? Many thanks

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• PostgreSQL - fetch the row which has the Max value for a column

  - by Joshua Berry

  I'm dealing with a Postgres table (called "lives") that contains records with columns for time_stamp, usr_id, transaction_id, and lives_remaining. I need a query that will give me the most recent lives_remaining total for each usr_id There are multiple users (distinct usr_id's) time_stamp is not a unique identifier: sometimes user events (one by row in the table) will occur with the same time_stamp. trans_id is unique only for very small time ranges: over time it repeats remaining_lives (for a given user) can both increase and decrease over time example: time_stamp|lives_remaining|usr_id|trans_id ----------------------------------------- 07:00 | 1 | 1 | 1 09:00 | 4 | 2 | 2 10:00 | 2 | 3 | 3 10:00 | 1 | 2 | 4 11:00 | 4 | 1 | 5 11:00 | 3 | 1 | 6 13:00 | 3 | 3 | 1 As I will need to access other columns of the row with the latest data for each given usr_id, I need a query that gives a result like this: time_stamp|lives_remaining|usr_id|trans_id ----------------------------------------- 11:00 | 3 | 1 | 6 10:00 | 1 | 2 | 4 13:00 | 3 | 3 | 1 As mentioned, each usr_id can gain or lose lives, and sometimes these timestamped events occur so close together that they have the same timestamp! Therefore this query won't work: SELECT b.time_stamp,b.lives_remaining,b.usr_id,b.trans_id FROM (SELECT usr_id, max(time_stamp) AS max_timestamp FROM lives GROUP BY usr_id ORDER BY usr_id) a JOIN lives b ON a.max_timestamp = b.time_stamp Instead, I need to use both time_stamp (first) and trans_id (second) to identify the correct row. I also then need to pass that information from the subquery to the main query that will provide the data for the other columns of the appropriate rows. This is the hacked up query that I've gotten to work: SELECT b.time_stamp,b.lives_remaining,b.usr_id,b.trans_id FROM (SELECT usr_id, max(time_stamp || '*' || trans_id) AS max_timestamp_transid FROM lives GROUP BY usr_id ORDER BY usr_id) a JOIN lives b ON a.max_timestamp_transid = b.time_stamp || '*' || b.trans_id ORDER BY b.usr_id Okay, so this works, but I don't like it. It requires a query within a query, a self join, and it seems to me that it could be much simpler by grabbing the row that MAX found to have the largest timestamp and trans_id. The table "lives" has tens of millions of rows to parse, so I'd like this query to be as fast and efficient as possible. I'm new to RDBM and Postgres in particular, so I know that I need to make effective use of the proper indexes. I'm a bit lost on how to optimize. I found a similar discussion here. Can I perform some type of Postgres equivalent to an Oracle analytic function? Any advice on accessing related column information used by an aggregate function (like MAX), creating indexes, and creating better queries would be much appreciated! P.S. You can use the following to create my example case: create TABLE lives (time_stamp timestamp, lives_remaining integer, usr_id integer, trans_id integer); insert into lives values ('2000-01-01 07:00', 1, 1, 1); insert into lives values ('2000-01-01 09:00', 4, 2, 2); insert into lives values ('2000-01-01 10:00', 2, 3, 3); insert into lives values ('2000-01-01 10:00', 1, 2, 4); insert into lives values ('2000-01-01 11:00', 4, 1, 5); insert into lives values ('2000-01-01 11:00', 3, 1, 6); insert into lives values ('2000-01-01 13:00', 3, 3, 1);

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