array of vectors?

This is a discussion on array of vectors? within the C++ Programming forums, part of the General Programming Boards category; My small program crashes before even entering the called method, any ideas? Code: #include <stdio.h> #include <vector> #include <string> using ...

  1. #1
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    array of vectors?

    My small program crashes before even entering the called method, any ideas?
    Code:
    #include <stdio.h>
    #include <vector>
    #include <string>
    using namespace std;
    
    
    
    vector<int> primeFactors[1000001];
    
    void genPrimeFactors()
    {
    printf("IN");
    int c=0;
    int composites[1000000]={0};
    for(int i=2;i<=1000000;++i)
    {
    	if(!composites[i])
    	for(int j=2*i;j<=1000000;j+=i)
    	{
    	    c++;
    		printf("%d %d",i,j);
    		composites[j] = 1;
    		primeFactors[j].push_back(i);
    	}
    }
    
    for(int i=2;i<=1000000;++i)
    {
        c++;
    	if(!composites[i])
    		primeFactors[i].push_back(i);
    }
    printf("%d",c);
    }
    
    
    int main()
    {
    
    	printf("START");
    	genPrimeFactors();
    return 0;
    }
    OUTPUT : START
    **CRASHES**


    I am very confused

    it never even prints "IN" , it seems as though it never makes it inside the method.
    Last edited by rodrigorules; 02-09-2010 at 09:21 PM.

  2. #2
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    Im having a hard time wrapping my head around what your trying to do, and the "odd" for loop conditions.

    Anyway, not that this solves the problem, but I imagine your're not seeing "IN" because STDOUT is line buffered. Try changing "printf("IN");" to "printf("IN\n");", to force the output to be printed. Then move your debug statement (the print "IN" line) further into the function until it no longer prints before a crash. Then you have found the line that is causing the crash.

  3. #3
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    hmm still crashes without executing "printf("IN\n");"

    I AM VERY CONFUSED!

    copy and paste the code in your compiler maybe, you will see it crash for no reason.

    EDIT: after doing further testing, this is the reason for the crash: ...but it was working earlier, and I think it might have worked for me earlier as well when i ran the code on a computer with ubuntu.
    int composites[1000000]={0};
    CODE crashes:
    Code:
    #include <stdio.h>
    
    void genPrimeFactors()
    {
    int composites[1000000]={0};
    }
    
    
    int main()
    {
    
    	printf("START");
    	genPrimeFactors();
    return 0;
    }
    edit: it appears this code snip. only crashes on my computer.. it works on codepad.org
    Last edited by rodrigorules; 02-09-2010 at 11:34 PM.

  4. #4
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    You are most likely running out of stack space because of the size of the composites array. Try finding a compiler/linker switch to increase stack space; Or better, use a dynamic array.

  5. #5
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    You can make the array static and it will probably work.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  6. #6
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    It seems that i do not run into this 'problem' If i initialize dynamically
    Code:
     
    int* composites = new int[1000001];
    is there any way to have this array set every element to 0 quickly, without having to do a loop of size 1 million?

    thanks!

  7. #7
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    Code:
    memset(composites, 0, 1000001*sizeof(int));

  8. #8
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by rodrigorules
    is there any way to have this array set every element to 0 quickly, without having to do a loop of size 1 million?
    According to the C++ standard, you could write:
    Code:
    int* composites = new int[1000001]();
    which will value-initialise each int, and value initialisation for the built-in types is zero initialisation. However, if I remember correctly a bug in the MinGW port of g++ 3.4.5, and possibly some other versions of g++ and maybe even other compilers, causes it to fail to value-initialise a dynamic array of objects of built-in types with this syntax.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  9. #9
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    Quote Originally Posted by laserlight View Post
    According to the C++ standard, you could write:
    Code:
    int* composites = new int[1000001]();
    which will value-initialise each int, and value initialisation for the built-in types is zero initialisation. However, if I remember correctly a bug in the MinGW port of g++ 3.4.5, and possibly some other versions of g++ and maybe even other compilers, causes it to fail to value-initialise a dynamic array of objects of built-in types with this syntax.
    Thanks laserlight, I didn't know that.

  10. #10
    C++ Witch laserlight's Avatar
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    Actually, since std::vector can be used, just write:
    Code:
    std::vector<int> composites(1000001);
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  11. #11
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    thanks again

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