Question

I have the following C code:

int main(int argc, char *argv[])
{

    int n = argc - 1;
    int array[n];
    int m[n][n];

    int i = 0;
    for(i = 1; i<=n;i++)
    {    
             array[i] = atoi(argv[i]);
            printf("%d\n",array[i]);
    }   
    printf("array[4] = %d\n",array[4]);
    for(i = 1; i<=n;i++)
    {   
             m[i][i] = 0;
            printf("address of m[i][i] = %p\n",&m[i][i]);
    }   

    printf("value of array[4] =%d pointer = %p\n",array[4],&array[4]);

    for(i=1;i<=n;i++) printf("After %d\n",array[i]);

    return 0;
}

If i run with the following command: “./program 30 35 15 5 10 20 15” the output is:

30
35
15
5
10
20
25
array[4] = 5
address of m[i][i] = 0xbf93070c
address of m[i][i] = 0xbf93072c
address of m[i][i] = 0xbf93074c
address of m[i][i] = 0xbf93076c
address of m[i][i] = 0xbf93078c
address of m[i][i] = 0xbf9307ac
address of m[i][i] = 0xbf9307cc
value of array[4] =0 pointer = 0xbf9307cc
After 30
After 35
After 15
After 0
After 10
After 20
After 25

Notice how array[4] has the same pointer as m[n][n]. And i really don’t understand how this is possible. What is wrong with the code. Why does array[4] = m[n][n]?

Answer 1

Your loops are wrong, arrays in C are zero-based. Meaning the first element is a[0] and the last is a[N-1] where N is the size of the array.

This:

for(i = 1; i <= n; i++)

Should be this:

for(i = 0; i < n; i++)

Otherwise you overstep the array boundries.

As a side note, you are using VLA but didn’t specify a C99 tag. Be sure you know what you are doing.