Thread: first number in array incorrect...

  1. #1
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    first number in array incorrect...

    Can anyone tell me if there is anything wrong with this notation?
    I have array[200];
    then this

    Code:
    for(i=0; i<100; i++){ 
    delta_x[i]=array[i]; 
    } 
    for(i=100; i<200; i++){ 
    delta_y[i-100]=array[i]; 
    }
    In my program everything is okay except that delta_y[0] is not right - it's not in array[200] at all, so it messes everything up by that amount.
    This seems right to me, but I thought maybe there was a problem, or does anyone know why this could happen?

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    That assignment statement sets delta_y[0] to array[100], so everything is okay there. Are you sure that the error is here and not some later statement?

  3. #3
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    I'm not sure of anything, actually, this is really puzzling me. I have the same notation for delta_x[100] as I do for delta_y[100] and delta_x is fine all the way.
    I printed out array[200] to make sure the mistake's not there, and it's not. I also printed out delta_y[100] by itself, and the first number is wrong.
    I don't think there's anything going on between array and delta_y besides the loop I already posted...

  4. #4
    Technical Lead QuantumPete's Avatar
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    Rather than having two loops, that essentially do the same thing, try this:

    Code:
    for(i=0; i<100; i++){ 
    delta_x[i] = array[i];
    delta_y[i] = array[i+100]; 
    }
    QuantumPete
    "No-one else has reported this problem, you're either crazy or a liar" - Dogbert Technical Support
    "Have you tried turning it off and on again?" - The IT Crowd

  5. #5
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    lol good call. My whole program's kinda like that. I did it in pieces when I was tired... :P But I just condensed my other loops, too. Unfortunately, it didn't change anything. Still same problem...

  6. #6
    Technical Lead QuantumPete's Avatar
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    How did you declare delta_x and delta_y?
    Can you post the smallest program that reproduces this problem?
    QuantumPete
    "No-one else has reported this problem, you're either crazy or a liar" - Dogbert Technical Support
    "Have you tried turning it off and on again?" - The IT Crowd

  7. #7
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    They're declared as double delta_x[100], delta_y[100].
    I'm messing with the first part of the program now. Once I get it simplified I'll post some more.

  8. #8
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    All right, I think the problem might actually be here

    Code:
         for (i=0; i<100; i++){
             x[i+1]=x[i]+delta_x[i];
             y[i+1]=y[i]+delta_y[i];
             A[i]=bluemoon(x[i], y[i]);
             }
    When I comment this out and print delta_y[100] by itself, the first number is correct.
    But how does this change delta_y?
    bluemoon is my function. :P

  9. #9
    C++ Witch laserlight's Avatar
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    I think you are still better off showing the smallest and simplest compilable program that demonstrates the problem.
    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

  10. #10
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    All right:

    Code:
    #include <stdio.h>
    
    double bluemoon (double x, double y)
    {
           double function;
           double expfunction;
           
           function=-1*x*x;-(1/2)*y*y;
           expfunction=exp(function);
           
           return expfunction;
    }
         
    main ()
    {
         double array[200];
         
         int i;
         double asta;
           
         for (i = 0; i < 200; i++){
             asta=i;
             array[i]=asta;
             }
             
         double delta_x[100], delta_y[100];
         double x[100], y[100], A[100];
                  
         for(i=0; i<100; i++){ 
                  delta_x[i] = array[i];
                  delta_y[i] = array[i+100]; 
                  }
         
         for (i=0; i<100; i++){
             x[i+1]=x[i]+delta_x[i];
             y[i+1]=y[i]+delta_y[i];
             A[i]=bluemoon(x[i], y[i]);
             }
           
           for(i=0; i<100; i++){
           printf("&#37;e\n", delta_y[i]);
           }  
           
         getchar();
         return 0;
    }
    Same problem.
    But if I take out the
    Code:
         for (i=0; i<100; i++){
             x[i+1]=x[i]+delta_x[i];
             y[i+1]=y[i]+delta_y[i];
             A[i]=bluemoon(x[i], y[i]);
             }
    section it's fine.

    EDIT: It also doesn't matter what bluemoon is defined as, still happens. Unfortunately I need that section... lol
    Last edited by sqytoad; 06-09-2008 at 11:04 AM.

  11. #11
    C++ Witch laserlight's Avatar
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    According to your code, delta_y[100] does not exist, since delta_y has 100 elements. Consequently, printing delta_y[100] by itself is accessing the array out of bounds.
    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

  12. #12
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    Sorry, I mean printing delta_y[i] from i=0 to i<100.

  13. #13
    C++ Witch laserlight's Avatar
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    I did a quick test (you really need to properly indent your code), and it seems that the problem is due to the array out of bounds when you assign to x[100] and y[100], both of which do not exist. Note that you assign to x[i+1] and y[i+1], but loop from 0 to 99 inclusive.
    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

  14. #14
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    @#$%!! I can't believe I didn't notice that! I'd been looking for that for so long, that was all I thought it could be...
    I guess I'd stared at it so long I'd convinced myself it was fine.

    Oh, well...

    Thanks so much for your help. My larger program is working now.

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