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• How would I instruct extconf.rb to use additional g++ optimization flags, and which are advisable?

  - by mohawkjohn

  I'm using Rice to write a C++ extension for a Ruby gem. The extension is in the form of a shared object (.so) file. This requires 'mkmf-rice' instead of 'mkmf', but the two (AFAIK) are pretty similar. By default, the compiler uses the flags -g -O2. Personally, I find this kind of silly, since it's hard to debug with any optimization enabled. I've resorted to editing the Makefile to take out the flags I don't like (e.g., removing -fPIC -shared when I need to debug using main() instead of Ruby's hooks). But I figure there's got to be a better way. I know I can just do $CPPFLAGS += " -DRICE" to add additional flags. But how do I remove things without editing the Makefile directly? A secondary question: what optimizations are safe for shared objects loaded by Ruby? Can I do things like -funroll-loops? What do you all recommend? It's a scientific computing project, so the faster the better. Memory is not much of an issue. Many thanks!

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• Creating an object in the loop

  - by Jacob

  std::vector<double> C(4); for(int i = 0; i < 1000;++i) for(int j = 0; j < 2000; ++j) { C[0] = 1.0; C[1] = 1.0; C[2] = 1.0; C[3] = 1.0; } is much faster than for(int i = 0; i < 1000;++i) for(int j = 0; j < 2000; ++j) { std::vector<double> C(4); C[0] = 1.0; C[1] = 1.0; C[2] = 1.0; C[3] = 1.0; } I realize this happens because std::vector is repeatedly being created and instantiated in the loop, but I was under the impression this would be optimized away. Is it completely wrong to keep variables local in a loop whenever possible? I was under the (perhaps false) impression that this would provide optimization opportunities for the compiler. EDIT: I use VC++2005 (release mode) with full optimization (/Ox)

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• Splitting a set of object into several subsets of 'similar' objects

  - by doublep

  Suppose I have a set of objects, S. There is an algorithm f that, given a set S builds certain data structure D on it: f(S) = D. If S is large and/or contains vastly different objects, D becomes large, to the point of being unusable (i.e. not fitting in allotted memory). To overcome this, I split S into several non-intersecting subsets: S = S1 + S2 + ... + Sn and build Di for each subset. Using n structures is less efficient than using one, but at least this way I can fit into memory constraints. Since size of f(S) grows faster than S itself, combined size of Di is much less than size of D. However, it is still desirable to reduce n, i.e. the number of subsets; or reduce the combined size of Di. For this, I need to split S in such a way that each Si contains "similar" objects, because then f will produce a smaller output structure if input objects are "similar enough" to each other. The problems is that while "similarity" of objects in S and size of f(S) do correlate, there is no way to compute the latter other than just evaluating f(S), and f is not quite fast. Algorithm I have currently is to iteratively add each next object from S into one of Si, so that this results in the least possible (at this stage) increase in combined Di size: for x in S: i = such i that size(f(Si + {x})) - size(f(Si)) is min Si = Si + {x} This gives practically useful results, but certainly pretty far from optimum (i.e. the minimal possible combined size). Also, this is slow. To speed up somewhat, I compute size(f(Si + {x})) - size(f(Si)) only for those i where x is "similar enough" to objects already in Si. Is there any standard approach to such kinds of problems? I know of branch and bounds algorithm family, but it cannot be applied here because it would be prohibitively slow. My guess is that it is simply not possible to compute optimal distribution of S into Si in reasonable time. But is there some common iteratively improving algorithm?

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• Java queue and multi-dimention array question? [Beginner level]

  - by javaLearner.java

  First of all, this is my code (just started learning java): Queue<String> qe = new LinkedList<String>(); qe.add("b"); qe.add("a"); qe.add("c"); qe.add("d"); qe.add("e"); My question: Is it possible to add element to the queue with two values, like: qe.add("a","1"); // where 1 is integer So, that I know element "a" have value 1. If I want to add a number let say "2" to element a, I will have like a = 3. If this cant be done, what else in java classes that can handle this? I tried to use multi-dimention array, but its kinda hard to do the queue, like pop, push etc. (Maybe I am wrong) How to call specific element in the queue? Like, call element a, to check its value. [Note] Please don't give me links that ask me to read java docs. I was reading, and I still dont get it. The reason why I ask here is because, I know I can find the answer faster and easier.

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• Slightly different execution times between python2 and python3

  - by user557634

  Hi. Lastly I wrote a simple generator of permutations in python (implementation of "plain changes" algorithm described by Knuth in "The Art... 4"). I was curious about the differences in execution time of it between python2 and python3. Here is my function: def perms(s): s = tuple(s) N = len(s) if N <= 1: yield s[:] raise StopIteration() for x in perms(s[1:]): for i in range(0,N): yield x[:i] + (s[0],) + x[i:] I tested both using timeit module. My tests: $ echo "python2.6:" && ./testing.py && echo "python3:" && ./testing3.py python2.6: args time[ms] 1 0.003811 2 0.008268 3 0.015907 4 0.042646 5 0.166755 6 0.908796 7 6.117996 8 48.346996 9 433.928967 10 4379.904032 python3: args time[ms] 1 0.00246778964996 2 0.00656183719635 3 0.01419159912 4 0.0406293644678 5 0.165960511097 6 0.923101452814 7 6.24257639835 8 53.0099868774 9 454.540967941 10 4585.83498001 As you can see, for number of arguments less than 6, python 3 is faster, but then roles are reversed and python2.6 does better. As I am a novice in python programming, I wonder why is that so? Or maybe my script is more optimized for python2? Thank you in advance for kind answer :)

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• Most efficient way of checking if Date object and Calendar object are in the same month

  - by Indigenuity

  I am working on a project that will run many thousands of comparisons between dates to see if they are in the same month, and I am wondering what the most efficient way of doing it would be. This isn't exactly what my code looks like, but here's the gist: List<Date> dates = getABunchOfDates(); Calendar month = Calendar.getInstance(); for(int i = 0; i < numMonths; i++) { for(Date date : dates) { if(sameMonth(month, date) .. doSomething } month.add(Calendar.MONTH, -1); } Creating a new Calendar object for every date seems like a pretty hefty overhead when this comparison will happen thousands of times, soI kind of want to cheat a bit and use the deprecated method Date.getMonth() and Date.getYear() public static boolean sameMonth(Calendar month, Date date) { return month.get(Calendar.YEAR) == date.getYear() && month.get(Calendar.MONTH) == date.getMonth(); } I'm pretty close to just using this method, since it seems to be the fastest, but is there a faster way? And is this a foolish way, since the Date methods are deprecated? Note: This project will always run with Java 7

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• Using Memcached in Python/Django - questions.

  - by Thomas

  I am starting use Memcached to make my website faster. For constant data in my database I use this: from django.core.cache import cache cache_key = 'regions' regions = cache.get(cache_key) if result is None: """Not Found in Cache""" regions = Regions.objects.all() cache.set(cache_key, regions, 2592000) #(2592000sekund = 30 dni) return regions For seldom changed data I use signals: from django.core.cache import cache from django.db.models import signals def nuke_social_network_cache(self, instance, **kwargs): cache_key = 'networks_for_%s' % (self.instance.user_id,) cache.delete(cache_key) signals.post_save.connect(nuke_social_network_cache, sender=SocialNetworkProfile) signals.post_delete.connect(nuke_social_network_cache, sender=SocialNetworkProfile) Is it correct way? I installed django-memcached-0.1.2, which show me: Memcached Server Stats Server Keys Hits Gets Hit_Rate Traffic_In Traffic_Out Usage Uptime 127.0.0.1 15 220 276 79% 83.1 KB 364.1 KB 18.4 KB 22:21:25 Can sombody explain what columns means? And last question. I have templates where I am getting much records from a few table (relationships). So in my view I get records from one table and in templates show it and related info from others. Generating page last a few seconds for very small table (<100records). Is it some easy way to cache queries from templates? Have I to do some big structure in my view (with all related tables), cache it and send to template?

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• A more effecient millisecond conversion method?

  - by cube

  I am currently using this method to convert milliseconds to min:sec:1/10sec. However it does not seem to be efficient at all. Would anyone know of a faster more efficient and optimized way of accomplishing the same. mills.prototype.formatTime = function(time) { var elapsedTime = (time * 1000); //Minutes var elapsedM = (elapsedTime/60000)|0; var remaining = elapsedTime - (elapsedM * 60000); //add a leading zero if it's a single digit number if (elapsedM < 10) { elapsedM = "0"+elapsedM; } //Seconds var elapsedS = ((remaining/1000)|0); remaining -= (elapsedS*1000); ////add leading zero if (elapsedS<10) { elapsedS = "0"+elapsedS; } //Hundredths var elapsedFractions = ((remaining/10)|0); if (elapsedFractions < 10) { elapsedFractions = "0"+elapsedFractions; } //display results nicely var time_data = elapsedM+":"+elapsedS+":"+elapsedFractions; //return time_data; return[time_data,elapsedM,elapsedS,elapsedFractions] };

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• Uniquing with Existing Core Data Entities

  - by warrenm

  I'm using Core Data to store a lot (1000s) of items. A pair of properties on each item are used to determine uniqueness, so when a new item comes in, I compare it against the existing items before inserting it. Since the incoming data is in the form of an RSS feed, there are often many duplicates, and the cost of the uniquing step is O(N^2), which has become significant. Right now, I create a set of existing items before iterating over the list of (possible) new items. My theory is that on the first iteration, all the items will be faulted in, and assuming we aren't pressed for memory, most of those items will remain resident over the course of the iteration. I see my options thusly: Use string comparison for uniquing, iterating over all "new" items and comparing to all existing items (Current approach) Use a predicate to filter the set of existing items against the properties of the "new" items. Use a predicate with Core Data to determine uniqueness of each "new" item (without retrieving the set of existing items). Is option 3 likely to be faster than my current approach? Do you know of a better way?

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• Move SELECT to SQL Server side

  - by noober

  Hello all, I have an SQLCLR trigger. It contains a large and messy SELECT inside, with parts like: (CASE WHEN EXISTS(SELECT * FROM INSERTED I WHERE I.ID = R.ID) THEN '1' ELSE '0' END) AS IsUpdated -- Is selected row just added? as well as JOINs etc. I like to have the result as a single table with all included. Question 1. Can I move this SELECT to SQL Server side? If yes, how to do this? Saying "move", I mean to create a stored procedure or something else that can be executed before reading dataset in while cycle. The 2 following questions make sense only if answer is "yes". Why do I want to move SELECT? First off, I don't like mixing SQL with C# code. At second, I suppose that server-side queries run faster, since the server have more chances to cache them. Question 2. Am I right? Is it some sort of optimizing? Also, the SELECT contains constant strings, but they are localizable. For instance, WHERE R.Status = "Enabled" "Enabled" should be changed for French, German etc. So, I want to write 2 static methods -- OnCreate and OnDestroy -- then mark them as stored procedures. When registering/unregistering my assembly on server side, just call them respectively. In OnCreate format the SELECT string, replacing {0}, {1}... with required values from the assembly resources. Then I can localize resources only, not every script. Question 3. Is it good idea? Is there an existing attribute to mark methods to be executed by SQL Server automatically after (un)registartion an assembly? Regards,

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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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• Why does this log output show the same answer in each iteration?

  - by Will Hancock

  OK, I was reading an article on optimising JS for Googles V8 engine, when i saw this code example... I nearly skimmed over it, but then I saw this; |=; a[0] |= b; a = new Array(); a[0] = 0; for (var b = 0; b < 10; b++) { console.log(a, b) a[0] |= b; // Much better! 2x faster. } a[0] |= b; So I ran it, in my console, with a console.log in the loop and resulted in 15; [15] 0 [15] 1 [15] 2 [15] 3 [15] 4 [15] 5 [15] 6 [15] 7 [15] 8 [15] 9 WHAT?!?! Where the hell does it get 15 from, on every iteration?!?!?! I've been a web dev for 7 years, and this has stumped me and a fellow colleague. Can somebody talk me through this code? Cheers.

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• Fast path cache generation for a connected node graph

  - by Sukasa

  I'm trying to get a faster pathfinding mechanism in place in a game I'm working on for a connected node graph. The nodes are classed into two types, "Networks" and "Routers." In this picture, the blue circles represent routers and the grey rectangles networks. Each network keeps a list of which routers it is connected to, and vice-versa. Routers cannot connect directly to other routers, and networks cannot connect directly to other networks. Networks list which routers they're connected to Routers do the same I need to get an algorithm that will map out a path, measured in the number of networks crossed, for each possible source and destination network excluding paths where the source and destination are the same network. I have one right now, however it is unusably slow, taking about two seconds to map the paths, which becomes incredibly noticeable for all connected players. The current algorithm is a depth-first brute-force search (It was thrown together in about an hour to just get the path caching working) which returns an array of networks in the order they are traversed, which explains why it's so slow. Are there any algorithms that are more efficient? As a side note, while these example graphs have four networks, the in-practice graphs have 55 networks and about 20 routers in use. Paths which are not possible also can occur, and as well at any time the network/router graph topography can change, requiring the path cache to be rebuilt. What approach/algorithm would likely provide the best results for this type of a graph?

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• Touch draw in Quatz 2D/Core Graphics

  - by OgreSwamp

  Hello, I'm trying to implement "hand draw tool". At the moment algorythm looks like that (I don't insert any code because methods are quite big, will try to explain an idea): Drawing In touchesStarted: method I create NSMutableArray *pointsArray and add point into it. Call setNeedsDisplay: method. In touchesMoved: method I calculate points between last added point from the pointsArray and current point. Add all points to the pointsArray. Call setNeedsDisplay: method. In touchesFinished: event I calculate points between last added point from the array and current point. Set flag touchesWereFinished. Call setNeedsDisplay:. Render: drawRect: method checks is pointsArray != nil and is there any data in it. If there is - it starts to traw circles in each point of this array. If flag touchesWereFinished is set - save current context to the UIImage, release pointsArray, set it to nil and reset the flag. There are a lot disadvantages of this method: It is slow It becomes extremely slow when user touches and move finger for long time. Array becomes enormous "Lines" composed by circles are ugly I would like to change my algorithm to make it bit faster and line smoother. In result I would like to have lines like on the picture at following URL (sorry, not enough reputation to insert an image): http://2.bp.blogspot.com/_r5VzEAUYXJ4/SrOYp8tJCPI/AAAAAAAAAMw/ZwDKXiHlhV0/s320/SketchBook+Mobile(4).png Can you advice me, ho I can draw lines this way (smooth and slim on the edges)? I thought to draw circles with alpha gradient on the edges (to make lines smoother), but it will be extremely slowly IMHO. Thanks for help

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• Any utility to test expand C/C++ #define macros?

  - by Randy

  It seems I often spend way too much time trying to get a #define macro to do exactly what i want. I'll post my current delemia below and any help is appreciated. But really the bigger question is whether there is any utility someone could reccomend, to quickly display what a macro is actually doing? It seems like even the slow trial and error process would go much faster if I could see what is wrong. Currently, I'm dynamically loading a long list of functions from a DLL I made. The way I've set things up, the function pointers have the same nanes as the exported functions, and the typedef(s) used to prototyp them have the same names, but with a prepended underscor. So I want to use a define to simplfy assignments of a long long list of function pointers. For example, In the code statement below, 'hexdump' is the name of a typdef'd function point, and is also the name of the function, while _hexdump is the name of the typedef. If GetProcAddress() fails, a failure counter in incremented. if (!(hexdump = (_hexdump)GetProcAddress(h, "hexdump"))) --iFail; So lets say I'd like to rplace each line like the above with a macro, like this... GETADDR_FOR(hexdump ) Well this is the best I've come up with so far. It doesn't work (my // comment is just to prevent text formatting in the message)... // #define GETADDR_FOR(a) if (!(a = (#_#a)GetProcAddress(h, "/""#a"/""))) --iFail; And again, while I'd APPRECIATE an insight into what silly mistake I've made, it would make my day to have a utility that would show me the error of my ways, by simply plugging in my macro

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• How slow are bit fields in C++

  - by Shane MacLaughlin

  I have a C++ application that includes a number of structures with manually controlled bit fields, something like #define FLAG1 0x0001 #define FLAG2 0x0002 #define FLAG3 0x0004 class MyClass { ' ' unsigned Flags; int IsFlag1Set() { return Flags & FLAG1; } void SetFlag1Set() { Flags |= FLAG1; } void ResetFlag1() { Flags &= 0xffffffff ^ FLAG1; } ' ' }; For obvious reasons I'd like to change this to use bit fields, something like class MyClass { ' ' struct Flags { unsigned Flag1:1; unsigned Flag2:1; unsigned Flag3:1; }; ' ' }; The one concern I have with making this switch is that I've come across a number of references on this site stating how slow bit fields are in C++. My assumption is that they are still faster than the manual code shown above, but is there any hard reference material covering the speed implications of using bit fields on various platforms, specifically 32bit and 64bit windows. The application deals with huge amounts of data in memory and must be both fast and memory efficient, which could well be why it was written this way in the first place.

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• F# ref-mutable vars vs object fields

  - by rwallace

  I'm writing a parser in F#, and it needs to be as fast as possible (I'm hoping to parse a 100 MB file in less than a minute). As normal, it uses mutable variables to store the next available character and the next available token (i.e. both the lexer and the parser proper use one unit of lookahead). My current partial implementation uses local variables for these. Since closure variables can't be mutable (anyone know the reason for this?) I've declared them as ref: let rec read file includepath = let c = ref ' ' let k = ref NONE let sb = new StringBuilder() use stream = File.OpenText file let readc() = c := stream.Read() |> char // etc I assume this has some overhead (not much, I know, but I'm trying for maximum speed here), and it's a little inelegant. The most obvious alternative would be to create a parser class object and have the mutable variables be fields in it. Does anyone know which is likely to be faster? Is there any consensus on which is considered better/more idiomatic style? Is there another option I'm missing?

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• Batch insert mode with hibernate and oracle: seems to be dropping back to slow mode silently

  - by Chris

  I'm trying to get a batch insert working with Hibernate into Oracle, according to what i've read here: http://docs.jboss.org/hibernate/core/3.3/reference/en/html/batch.html , but with my benchmarking it doesn't seem any faster than before. Can anyone suggest a way to prove whether hibernate is using batch mode or not? I hear that there are numerous reasons why it may silently drop into normal mode (eg associations and generated ids) so is there some way to find out why it has gone non-batch? My hibernate.cfg.xml contains this line which i believe is all i need to enable batch mode: <property name="jdbc.batch_size">50</property> My insert code looks like this: List<LogEntry> entries = ..a list of 100 LogEntry data classes... Session sess = sessionFactory.getCurrentSession(); for(LogEntry e : entries) { sess.save(e); } sess.flush(); sess.clear(); My 'logentry' class has no associations, the only interesting field is the id: @Entity @Table(name="log_entries") public class LogEntry { @Id @GeneratedValue public Long id; ..other fields - strings and ints... However, since it is oracle, i believe the @GeneratedValue will use the sequence generator. And i believe that only the 'identity' generator will stop bulk inserts. So if anyone can explain why it isn't running in batch mode, or how i can find out for sure if it is or isn't in batch mode, or find out why hibernate is silently dropping back to slow mode, i'd be most grateful. Thanks

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• Time gaps between host clEnqueue_xxx calls

  - by dialer

  Consider these OpenCL calls (3 memcpy DtoH, 4313 cl_float elements each): clEnqueueReadBuffer(CommandQueue, SpectrumAbsMem, CL_FALSE, 0, SpectrumMemSize, SpectrumAbs, 0, NULL, NULL); clEnqueueReadBuffer(CommandQueue, SpectrumReMem, CL_FALSE, 0, SpectrumMemSize, SpectrumRe, 0, NULL, NULL); clEnqueueReadBuffer(CommandQueue, SpectrumImMem, CL_FALSE, 0, SpectrumMemSize, SpectrumIm, 0, NULL, NULL); When I analyze these with the NVIDIA visual profiler, I see that the actual memcpy operation only takes 8 us, but there is a significant gap of around 130 us after each memcpy. I'm already using the supposedly asynchronous method (the CL_FALSE in the argument list). When I use only one operation, but with three times the size, the operation is way faster. Why is the time gap between the actual memcpy operations so huge, whereas the gap between the kernel execution (exactly before these three operations) and the first memcpy is only 7us? Can I get rid of it, or do I need to accumulate more data before starting a memcpy? If so, is there a convenient way how I could combine mutliple arrays into a single contiguous block of memory, but still have a cl_mem object as a separate device memory pointer to each section?

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• cross-platform development for mobile devices

  - by user924

  What language/framework is best worth learning for mobile application development? My specific situation is that I'm very familiar with Java and C++ (I especially love Qt), but have limited experience with other languages. Some options I'm considering: 1) Learn Objective-C and all the iPhone-specific tools I do have access to a mac. The downside here is I'm restricted to the iPhone, so I'd have to rewrite almost everything if I wanted to branch off into another mobile device (or move later to a cross-platform framework). Even after knowing Objective-C, it seems like other frameworks might be more efficient/faster to code in? 2) Use some existing cross-platform framework for development I've looked at rhomobile, but I only have limited experience with Ruby (and at first glance, it might be a little pricey comapred to other options). Appcelerator also looks popular and nice, but it uses html/css/javascript. Airplaysdk looks good, but it's new and I haven't been able to see much written about it (is it worth going for?). 3) Wait for something better to come along How far away is Qt for the iPhone? That would be ideal, but it isn't available now. So what do you recommend? Productivity/efficiency is my top priority, although learning a useful language for the long term would also be okay. Thanks

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• finding N contiguous zero bits in an integer to the left of the MSB from another

  - by James Morris

  First we find the MSB of the first integer, and then try to find a region of N contiguous zero bits within the second number which is to the left of the MSB from the first integer. Here is the C code for my solution: typedef unsigned int t; unsigned const t_bits = sizeof(t) * CHAR_BIT; _Bool test_fit_within_left_of_msb( unsigned width, t val1, t val2, unsigned* offset_result) { unsigned offbit = 0; unsigned msb = 0; t mask; t b; while(val1 >>= 1) ++msb; while(offbit + width < t_bits - msb) { mask = (((t)1 << width) - 1) << (t_bits - width - offbit); b = val2 & mask; if (!b) { *offset_result = offbit; return true; } if (offbit++) /* this conditional bothers me! */ b <<= offbit - 1; while(b <<= 1) offbit++; } return false; } Aside from faster ways of finding the MSB of the first integer, the commented test for a zero offbit seems a bit extraneous, but necessary to skip the highest bit of type t if it is set. I have also implemented similar algorithms but working to the right of the MSB of the first number, so they don't require this seemingly extra condition. How can I get rid of this extra condition, or even, are there far more optimal solutions?

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• Parsing large txt files in ruby taking a lot of time?

  - by hershey92

  below is the code to download a txt file from internet approx 9000 lines and populate the database, I have tried a lot but it takes a lot of time more than 7 minutes. I am using win 7 64 bit and ruby 1.9.3. Is there a way to do it faster ?? require 'open-uri' require 'dbi' dbh = DBI.connect("DBI:Mysql:mfmodel:localhost","root","") #file = open('http://www.amfiindia.com/spages/NAV0.txt') file = File.open('test.txt','r') lines = file.lines 2.times { lines.next } curSubType = '' curType = '' curCompName = '' lines.each do |line| line.strip! if line[-1] == ')' curType,curSubType = line.split('(') curSubType.chop! elsif line[-4..-1] == 'Fund' curCompName = line.split(" Mutual Fund")[0] elsif line == '' next else sCode,isin_div,isin_re,sName,nav,rePrice,salePrice,date = line.split(';') sCode = Integer(sCode) sth = dbh.prepare "call mfmodel.populate(?,?,?,?,?,?,?)" sth.execute curCompName,curSubType,curType,sCode,isin_div,isin_re,sName end end dbh.do "commit" dbh.disconnect file.close 106799;-;-;HDFC ARBITRAGE FUND RETAIL PLAN DIVIDEND OPTION;10.352;10.3;10.352;29-Jun-2012 This is the format of data to be inserted in the table. Now there are 8000 such lines and how can I do an insert by combining all that and call the procedure just once. Also, does mysql support arrays and iteration to do such a thing inside the routine. Please give your suggestions.Thanks.

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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 to speed up a slow UPDATE query

  - by Mike Christensen

  I have the following UPDATE query: UPDATE Indexer.Pages SET LastError=NULL where LastError is not null; Right now, this query takes about 93 minutes to complete. I'd like to find ways to make this a bit faster. The Indexer.Pages table has around 506,000 rows, and about 490,000 of them contain a value for LastError, so I doubt I can take advantage of any indexes here. The table (when uncompressed) has about 46 gigs of data in it, however the majority of that data is in a text field called html. I believe simply loading and unloading that many pages is causing the slowdown. One idea would be to make a new table with just the Id and the html field, and keep Indexer.Pages as small as possible. However, testing this theory would be a decent amount of work since I actually don't have the hard disk space to create a copy of the table. I'd have to copy it over to another machine, drop the table, then copy the data back which would probably take all evening. Ideas? I'm using Postgres 9.0.0. UPDATE: Here's the schema: CREATE TABLE indexer.pages ( id uuid NOT NULL, url character varying(1024) NOT NULL, firstcrawled timestamp with time zone NOT NULL, lastcrawled timestamp with time zone NOT NULL, recipeid uuid, html text NOT NULL, lasterror character varying(1024), missingings smallint, CONSTRAINT pages_pkey PRIMARY KEY (id ), CONSTRAINT indexer_pages_uniqueurl UNIQUE (url ) ); I also have two indexes: CREATE INDEX idx_indexer_pages_missingings ON indexer.pages USING btree (missingings ) WHERE missingings > 0; and CREATE INDEX idx_indexer_pages_null ON indexer.pages USING btree (recipeid ) WHERE NULL::boolean; There are no triggers on this table, and there is one other table that has a FK constraint on Pages.PageId.

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• Protecting my apps security from deassembling

  - by sandis

  So I recently tested deassembling one of my android apps, and to my horror I discovered that the code was quite readable. Even worse, all my variable names where intact! I thought that those would be compressed to something unreadable at compile time. The app is triggered to expire after a certain time. However, now it was trivial for me to find my function named checkIfExpired() and find the variable "expired". Is there any good way of making it harder for a potential hacker messing with my app? Before someone states the obvious: Yes, it is security through obscurity. But obviously this is my only option since the user always will have access to all my code. This is the same for all apps. The details of my deactivation-thingy is unimportant, the point is that I dont want deassembler to understand some of the things I do. side questions: Why are the variable names not compressed? Could it be the case that my program would run faster if I stopped using really long variable names, as are my habit?

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