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  • SQL -- How is DISTINCT so fast without an index?

    - by Jonathan
    Hi, I have a database with a table called 'links' with 600 million rows in it in SQLite. There are 2 columns in the database - a "src" column and a "dest" column. At present there are no indices. There are a fair number of common values between src and dest, but also a fair number of duplicated rows. The first thing I'm trying to do is remove all the duplicate rows, and then perform some additional processing on the results, however I've been encountering some weird issues. Firstly, SELECT * FROM links WHERE src=434923 AND dest=5010182. Now this returns one result fairly quickly and then takes quite a long time to run as I assume it's performing a tablescan on the rest of the 600m rows. However, if I do SELECT DISTINCT * FROM links, then it immediately starts returning rows really quickly. The question is: how is this possible?? Surely for each row, the row must be compared against all of the other rows in the table, but this would require a tablescan of the remaining rows in the table which SHOULD takes ages! Any ideas why SELECT DISTINCT is so much quicker than a standard SELECT?

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  • Best way to reverse a string in C# 2.0

    - by Guy
    I've just had to write a string reverse function in C# 2.0 (i.e. LINQ not available) and came up with this: public string Reverse(string text) { char[] cArray = text.ToCharArray(); string reverse = String.Empty; for (int i = cArray.Length - 1; i > -1; i--) { reverse += cArray[i]; } return reverse; } Personally I'm not crazy about the function and am convinced that there's a better way to do it. Is there?

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  • Can code formatting lead to change in object file content?

    - by kumar
    I have run though a code formatting tool to my c++ files. It is supposed to make only formatting changes. Now when I built my code, I see that size of object file for some source files have changed. Since my files are very big and tool has changed almost every line, I dont know whether it has done something disastrous. Now i am worried to check in this code to repo as it might lead to runtime error due to formatting tool. My question is , will the size of object file be changed , if code formatting is changed.?

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  • Help in optimizing a for loop in matlab

    - by HH
    I have a 1 by N double array consisting of 1 and 0. I would like to map all the 1 to symbol '-3' and '3' and all the 0 to symbol '-1' and '1' equally. Below is my code. As my array is approx 1 by 8 million, it is taking a very long time. How to speed things up? [row,ll] = size(Data); sym_zero = -1; sym_one = -3; for loop = 1 : row if Data(loop,1) == 0 Data2(loop,1) = sym_zero; if sym_zero == -1 sym_zero = 1; else sym_zero = -1; end else Data2(loop,1) = sym_one; if sym_one == -3 sym_zero = 3; else sym_zero = -3; end end end

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  • C#, Using Custom Generic Collection faster with objects than List

    - by Kaminari
    Hello, I'm using for now List< to iterate through some object collection and find matching element, The problem is that object has only 2 significant values Name and Link (strings) but has some other values wich I dont want to compare. I'm thinkig about using something like HashSet (wich 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/ But maybe there is other way? If not, can you suggest me a suitable custom collection?

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  • WPF Logical Tree - bottom up vs. top down

    - by Dor Rotman
    Hello, I've read the MSDN article about the layouts pass, that states: When a node is added or removed from the logical tree, property invalidations are raised on the node's parent and all its children. As a result, a top-down construction pattern should always be followed to avoid the cost of unnecessary invalidations on nodes that have already been validated. Now lets assume I do this. Won't the users see the control tree populate itself and the layout change several times during the control creation process? I want the whole control tree to just appear completely full. Thanks!

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  • Storing information in the DOM?

    - by John
    Im making a small private message application in the form of a phone. Ten messages are shown at the time. And the list of messages are scrolled up/down by hiding them. Just how bad is it to use the DOM to store information in this way. My main goal for doing this is to reduce calls to the database. And instead of making a new call all the time, it only checks if any new messages has arrived and ads the new message(s). Whats the alternative, cookies anyone? Thank you for the time

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  • Function defined but not used in C

    - by thetna
    I have following code: static __inline__ LIST list_List(POINTER P) { return list_Cons(P,list_Nil()); } After compilation I got following warning: inlining is unlikely but function size may grow I removed the inline and changed into the following : static LIST list_List(POINTER P) { return list_Cons(P,list_Nil()); } Now I get the following warning: list_List is defined but not used. Can anybody please suggest me how can remove that warning.

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  • regular expression for bit strings with even number of 1s

    - by equilibrium
    Let L= { w in (0+1)* | w has even number of 1s}, i.e. L is the set of all bit strings with even number of 1s. Which one of the regular expressions below represents L? A) (0*10*1)* B) 0*(10*10*)* C) 0*(10*1)* 0* D) 0*1(10*1)* 10* According to me option D is never correct because it does not represent the bit string with zero 1s. But what about the other options? We are concerned about the number of 1s(even or not) not the number of zeros doesn't matter. Then which is the correct option and why?

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  • Regular Expression

    - by equilibrium
    Ohh! this regular expression thing is eating my brain up. I have been reading it from Introduction to Automata Theory, Languages and Computer by Hopcroft, Motwani and Ullman. I have solved a few exercises too but could not solve the following even after trying for almost one hr. The problem is to write a regular expression that defines a language consisting of all strings of 0s and 1s except the substring 011. Is the answer (0+1)* - 011 correct ? If not what should be the correct answer for this?

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  • optimizing oracle query

    - by deming
    I'm having a hard time wrapping my head around this query. it is taking almost 200+ seconds to execute. I've pasted the execution plan as well. SELECT user_id , ROLE_ID , effective_from_date , effective_to_date , participant_code , ACTIVE FROM CMP_USER_ROLE E WHERE ACTIVE = 0 AND (SYSDATE BETWEEN effective_from_date AND effective_to_date OR TO_CHAR(effective_to_date,'YYYY-Q') = '2010-2') AND participant_code = 'NY005' AND NOT EXISTS ( SELECT 1 FROM CMP_USER_ROLE r WHERE r.USER_ID= E.USER_ID AND r.role_id = E.role_id AND r.ACTIVE = 4 AND E.effective_to_date <= (SELECT MAX(last_update_date) FROM CMP_USER_ROLE S WHERE S.role_id = r.role_id AND S.role_id = r.role_id AND S.ACTIVE = 4 )) Explain plan ----------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 37 | 154 (2)| 00:00:02 | |* 1 | FILTER | | | | | | |* 2 | TABLE ACCESS BY INDEX ROWID | USER_ROLE | 1 | 37 | 30 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | N_USER_ROLE_IDX6 | 27 | | 3 (0)| 00:00:01 | |* 4 | FILTER | | | | | | | 5 | HASH GROUP BY | | 1 | 47 | 124 (2)| 00:00:02 | |* 6 | TABLE ACCESS BY INDEX ROWID | USER_ROLE | 159 | 3339 | 119 (1)| 00:00:02 | | 7 | NESTED LOOPS | | 11 | 517 | 123 (1)| 00:00:02 | |* 8 | TABLE ACCESS BY INDEX ROWID| USER_ROLE | 1 | 26 | 4 (0)| 00:00:01 | |* 9 | INDEX RANGE SCAN | N_USER_ROLE_IDX5 | 1 | | 3 (0)| 00:00:01 | |* 10 | INDEX RANGE SCAN | N_USER_ROLE_IDX2 | 957 | | 74 (2)| 00:00:01 | -----------------------------------------------------------------------------------------------------

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  • C++: Can't use std::wstringstream

    - by Rosarch
    For some reason, my project won't compile when I try to create a wstringstream: std::wstringstream stringstream; This causes error C2079: 'stringstream' uses undefined class 'std::basic_stringstream<_Elem, _Traits, _Alloc with [_Elem=wchar_t, _Traits=std::char_traits, _Alloc=std::allocator' What am I doing wrong?

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  • Is there a way to optimise finding text items on a page (not regex)

    - by Jeepstone
    After seeing several threads rubbishing the regexp method of finding a term to match within an HTML document, I've used the Simple HTML DOM PHP parser (http://simplehtmldom.sourceforge.net/) to get the bits of text I'm after, but I want to know if my code is optimal. It feels like I'm looping too many times. Is there a way to optimise the following loop? //Get the HTML and look at the text nodes $html = str_get_html($buffer); //First we match the <body> tag as we don't want to change the <head> items foreach($html->find('body') as $body) { //Then we get the text nodes, rather than any HTML foreach($body->find('text') as $text) { //Then we match each term foreach ($terms as $term) { //Match to the terms within the text nodes $text->outertext = str_replace($term, '<span class="highlight">'.$term.'</span>', $text->outertext); } } } For example, would it make a difference to determine check if I have any matches before I start the loop maybe?

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  • Getting confused why i dont get expected amount ?

    - by Stackfan
    I have 1 result and which i will receive in Bank account, Based on that account i have to Put a balance to user account. How can you find the Handling cost from total tried 491.50 / 0.95 = 517.36 which is wrong ? It should be 500.00 (to my expectation) User balance requires 500.00 When 500.00 selected he gets 5% discount There is a handling cost for this ex: 1) Discount: 500.00 - 5% = 475.00 2) Handling cost: (475.00 x 0.034) + 0.35 = 16.50 3) Total: 475.00 + 16.50 = 491.50 So problem is from 491.50, i have to find atleast handling cost to get promised Balance. Any solution ? Cant figure it out myself...

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  • Any tools can randomly generate the source code according to a language grammar?

    - by wbsun
    A C program source code can be parsed according to the C grammar(described in CFG) and eventually turned into many ASTs. I am considering if such tool exists: it can do the reverse thing by firstly randomly generating many ASTs, which include tokens that don't have the concrete string values, just the types of the tokens, according to the CFG, then generating the concrete tokens according to the tokens' definitions in the regular expression. I can imagine the first step looks like an iterative non-terminals replacement, which is randomly and can be limited by certain number of iteration times. The second step is just generating randomly strings according to regular expressions. Is there any tool that can do this?

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  • Create a java executable with Eclipse

    - by Micah
    This is a totally newbie question. I'm running Eclipse on Ubuntu. I created a test project that I want to compile to an executable (whataver the linux equivalent is of a Windows .exe file). Here's the contents of my program: public class MyTest { public static void main(String[] args) { System.out.println("You passed in: " + args[0]); } } I want to know how to compile it and then how to execute it from the command line. Thanks!

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  • Nested loop traversing arrays

    - by alecco
    There are 2 very big series of elements, the second 100 times bigger than the first. For each element of the first series, there are 0 or more elements on the second series. This can be traversed and processed with 2 nested loops. But the unpredictability of the amount of matching elements for each member of the first array makes things very, very slow. The actual processing of the 2nd series of elements involves logical and (&) and a population count. I couldn't find good optimizations using C but I am considering doing inline asm, doing rep* mov* or similar for each element of the first series and then doing the batch processing of the matching bytes of the second series, perhaps in buffers of 1MB or something. But the code would be get quite messy. Does anybody know of a better way? C preferred but x86 ASM OK too. Many thanks! Sample/demo code with simplified problem, first series are "people" and second series are "events", for clarity's sake. (the original problem is actually 100m and 10,000m entries!) #include <stdio.h> #include <stdint.h> #define PEOPLE 1000000 // 1m struct Person { uint8_t age; // Filtering condition uint8_t cnt; // Number of events for this person in E } P[PEOPLE]; // Each has 0 or more bytes with bit flags #define EVENTS 100000000 // 100m uint8_t P1[EVENTS]; // Property 1 flags uint8_t P2[EVENTS]; // Property 2 flags void init_arrays() { for (int i = 0; i < PEOPLE; i++) { // just some stuff P[i].age = i & 0x07; P[i].cnt = i % 220; // assert( sum < EVENTS ); } for (int i = 0; i < EVENTS; i++) { P1[i] = i % 7; // just some stuff P2[i] = i % 9; // just some other stuff } } int main(int argc, char *argv[]) { uint64_t sum = 0, fcur = 0; int age_filter = 7; // just some init_arrays(); // Init P, P1, P2 for (int64_t p = 0; p < PEOPLE ; p++) if (P[p].age < age_filter) for (int64_t e = 0; e < P[p].cnt ; e++, fcur++) sum += __builtin_popcount( P1[fcur] & P2[fcur] ); else fcur += P[p].cnt; // skip this person's events printf("(dummy %ld %ld)\n", sum, fcur ); return 0; } gcc -O5 -march=native -std=c99 test.c -o test

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  • Minimizing distance to a weighted grid

    - by Andrew Tomazos - Fathomling
    Lets suppose you have a 1000x1000 grid of positive integer weights W. We want to find the cell that minimizes the average weighted distance.to each cell. The brute force way to do this would be to loop over each candidate cell and calculate the distance: int best_x, best_y, best_dist; for x0 = 1:1000, for y0 = 1:1000, int total_dist = 0; for x1 = 1:1000, for y1 = 1:1000, total_dist += W[x1,y1] * sqrt((x0-x1)^2 + (y0-y1)^2); if (total_dist < best_dist) best_x = x0; best_y = y0; best_dist = total_dist; This takes ~10^12 operations, which is too long. Is there a way to do this in or near ~10^8 or so operations?

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  • serving cached files based upon cookie?

    - by matthewsteiner
    So I realized something today. In my application, you really can't get anywhere (except the front page) unless you're logged in. And you can't be logged in without a cookie. So my front page could be cached, except the problem is if you are logged in (have a cookie set) then it should just redirect into the application. Is there a way for nginx to look for a cookie and if it finds it then deliver a cached file? Just an idea...

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  • Why use one dimensional array instead of a two dimensional arrray?

    - by user3869145
    I was doing some work handling a lot of information and my partner told me that I was using too many matrices to manipulate the variables of the problem. The idea was to use one dimension arrays int a[] instead of the 2 dimensional arrays int b[][], to save memory and processing speed of the algorithm. How certain is that this change will accelerate the speed of execution or compilation of my code in c ++?

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  • error in c, but not in c++

    - by eswar
    const int t=5; char buf[t+5]; When I compile this gives error in C but not in C++!! Can anybody please explain me the reason? Note: I know the const defaults to internal linkage in 'C++', where as in 'C' it defaults to external linkage. Does it has any relation to the above case??

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  • How do I remove the alert: "...may not respond to..."

    - by balexandre
    In my .m file I call a method that is inside the same .m file. In the header I have the correct import for the header but I keep getting this alert: What am I doing wrong? What should I do in order to make this error disappear? I'm kinda lost here :-( Even if I changed this to: NSString *path = [[NSString alloc] initWithString:@"...."]; [self parseXMLFileAtURL:path]; [path release];

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