Vector Element spontaneously changing?!

This is a discussion on Vector Element spontaneously changing?! within the C Programming forums, part of the General Programming Boards category; Hello, I am somewhat of a newbie when it comes to programming in C, although I am familiar with Fortran. ...

  1. #1
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    Question Vector Element spontaneously changing?!

    Hello,

    I am somewhat of a newbie when it comes to programming in C, although I am familiar with Fortran. I have been experiencing unrequested changes to certain elements in a vector while using a for-loop. My code for this section is as follows:

    Code:
     for (i=1;i<NZ;i++) {
       a[i]=-eta2;    // defining the entries in the tridag matrix
       b[i]=eta1;        // defining the entries in the tridag matrix
       c[i]=-eta2;     // defining the entries in the tridag matrix
       printf("%f should be %f and not %f \n",b[1],eta1,eta2);
       }
    where NZ, eta1 and eta2 are read in from a parameter file upon running the compiled program. The program compiles without any errors, but once executed I am presented with the following output:

    5.201031 should be 5.201031 and not -2.100515
    5.201031 should be 5.201031 and not -2.100515
    5.201031 should be 5.201031 and not -2.100515
    5.201031 should be 5.201031 and not -2.100515
    -2.100515 should be 5.201031 and not -2.100515
    with the final line repeating until the counter reaches NZ (in this case, 100). As can be seen from the above output, b[1] seems to change without reason when i=4 from eta1 to -eta2. It's also worth noting that a[1] and c[1] are unchanged throughout this loop.

    Does anyone have any ideas as to what might be causing this strange behaviour? A compiler bug (surely not)? I am using gcc 3.3 on Mac OS X 10.3.3.

    Any help will be greatly appreciated! Many thanks,


    -Matt

  2. #2
    ATH0 quzah's Avatar
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    Well, for array indexes, your array starts at 0, and not 1. Thus, if your array holds 4 itesm, you have valid array indexes as:
    Code:
    int array[4];
    
    /* valid */
    array[0] = 1;
    array[1] = 2;
    array[2] = 3;
    array[3] = 4;
    
    /* Not valid: */
    array[4] = 5; /* error, you're accessing beyond your array */
    Is that your problem?

    Otherwise, post your array declarations as well as what "eta1" and "eta2" are.

    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
    Registered User linuxdude's Avatar
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    make sure you read eta1 and eta2 as integers or digits not as characters. and arrays start as 0 in C not as 1make sure you aren't overwriting you bounds[edit]Beaten again[/edit]

  4. #4
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    OK, my declarations relating to the snippet of code are:

    Code:
    float *a; // vectors used in tridag
    float *b;
    float *c;
    float eta1,eta2;
    with the array sizes of a, b and c defined as:

    Code:
    	a=(float *)malloc((NZ+1) * sizeof(float));     // setting the vector size
    	b=(float *)malloc((NZ+1) * sizeof(float));
    	c=(float *)malloc((NZ+1) * sizeof(float));
    Also, while this doesn't directly relate to my problem, b[0] and b[NZ] are allocated values immediately after the for-loop, namely

    Code:
     // defining the boundary conditions in the tridag matrix
     a[0]=0.0;
     b[0]=eta3;
     c[0]=-eta4;
    
     // fix the boundary at NZ
     a[NZ]=0.0;
     b[NZ]=1.0;
     c[NZ]=0.0;
    Thanks for the responses!


    -Matt

  5. #5
    ATH0 quzah's Avatar
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    If you hard code the values for NZ and eta1 to something, in just a small program, does it work?
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    int main( void )
    {
        float *a, *b, *c;
        float eta1 = 5.201031, eta2 = -2.100515;
        int x - 0; NZ = 6;
    
        a = malloc( sizeof(*a) * NZ );
        if( a == NULL ) exit(0);
        b = malloc( sizeof(*b) * NZ );
        if( b == NULL ) exit(0);
        c = malloc( sizeof(*c) * NZ );
        if( c == NULL ) exit(0);
    
        for( x = 0; x < NZ; x++ )
        {
            a[x] = -eta2;
            printf("a[%d] is %f and should be %f\n", x, a[x], -eta2 );
            b[x] = eta1;
            printf("b[%d] is %f and should be %f\n", x, b[x], eta1 );
            c[x] = -eta2;
            printf("c[%d] is %f and should be %f\n", x, c[x], -eta2 );
    
        }
        free( a );
        free( b );
        free( c );
    
        return 0;
    }
    I think that's right... anyway, do something like that to test it by itself with hard coded values.

    Quzah.
    Hope is the first step on the road to disappointment.

  6. #6
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    The mini-program works properly (using the line ` int x, NZ = 6; ` ). However, even with hard-coded values in my larger program the same problems are occurring. For example, using the lines:

    Code:
        for( i = 1; i < NZ; i++ )
        {
            a[ i ] = -eta2;
            printf("a[%d] is %f and should be %f\n", i, a[ i ], -eta2 );
            b[ i ] = eta1;
            printf("b[1] is %f and should be %f\n", b[1], eta1 );
            printf("b[%d] is %f and should be %f\n", i, b[ i ], eta1 );
            c[ i ] = -eta2;
            printf("c[%d] is %f and should be %f\n", i, c[ i ], -eta2 );
        }
    gives

    a[1] is -2.100515 and should be -2.100515
    b[1] is 5.201031 and should be 5.201031
    b[1] is 5.201031 and should be 5.201031
    c[1] is -2.100515 and should be -2.100515
    <SNIP>
    a[4] is -2.100515 and should be -2.100515
    b[1] is 5.201031 and should be 5.201031
    b[4] is 5.201031 and should be 5.201031
    c[4] is -2.100515 and should be -2.100515
    a[5] is -2.100515 and should be -2.100515
    b[1] is -2.100515 and should be 5.201031
    b[5] is 5.201031 and should be 5.201031
    c[5] is -2.100515 and should be -2.100515
    EDIT: I forgot to mention that the last four lines of output repeat, changing with i and b[1] giving the same (incorrect) value, until i reaches NZ-1 (=99)

    The very fact that this can happen in a self-contained loop is quite disconcerting. Has anyone ever experienced anything similar to this? Anyway, I will re-write various parts of my code tomorrow and report back in the evening. Thanks to all for their help.


    -Matt
    Last edited by homgran; 05-29-2004 at 05:37 AM. Reason: Clarification of output

  7. #7
    Yes, my avatar is stolen anonytmouse's Avatar
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    Check the value of NZ with printf() right before you allocate the memory. My bet is that it is different to what you think it is.

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