Question

I have a 2D dynamic array.
I enter a line of 0’s after line which has a biggest number:

void InsertZero(int **a, int pos){
    int i, j;
    a = (int**)realloc(a, n * sizeof(*a));
    a[n-1] = (int*)calloc(n, sizeof(**a));
    d = 0;
    for(i = n-1; i > pos; i--){
        for(j = 0; j < n; j++){
            a[i][j] = a[i-1][j];
            printf("%d ", a[i][j]);
        }
    }

    for(i = 0; i < n; i++){
        a[pos][i] = 0;
    }
}

If i make a size of array 3, 5, 7, 9, … it works correctly. But if a number of lines is 2, 4, 6, … , it is an access violation error, when i try to print my array:

void Print(void){
    int i, j;
    for(i = 0; i < (n-d); i++){
        for(j = 0; j < n; j++){
            printf("%d\t", arr[i][j]);
        }
        printf("\n");
    }
}

code: http://codepad.org/JcUis6W4

Answer 1

Looking at this I cannot make sense of…. Look at comment 1, you have n set somewhere to realloc a block of memory which a is of type int ** – a double pointer, how are you calling this function? Secondly, comment 2, Why did you call calloc when the realloc on a double pointer was called previously…? Assume n has value of 5, then, realloc is called on double pointer a, meaning a[0][1]..a[4][1], now calloc is called thus a[4] has a new block of memory…

void InsertZero(int **a, int pos){
    int i, j;

    /* 1. */
    a = (int**)realloc(a, n * sizeof(*a)); 
    /* Bzzzzt....if realloc failed, a gets overwritten! */

    /* 2. */
    a[n-1] = (int*)calloc(n, sizeof(**a));

    /* 3. */
    d = 0;

    /* 4. */
    for(i = n-1; i > pos; i--){
        for(j = 0; j < n; j++){
            a[i][j] = a[i-1][j];
            printf("%d ", a[i][j]);
        }
    }

    for(i = 0; i < n; i++){
        a[pos][i] = 0;
    }
}

Comment 3, what is d used for – useless variable?
Comment 4, you are under the presumption that the block of memory has array subscripts [0][0] to [4][4] if n had a value of 5!

Can you clarify all this?

Edit: Looking at it again… it is likely that a got overwritten when the call to realloc failed! I recommend this section of code to counteract this

    int **tmpA;
    tmpA = (int**)realloc(a, n * sizeof(*a)); 
    if (tmpA != NULL){
        a = tmpA;
        ....
        a[n-1] = (int*)calloc(n, sizeof(**a));
        for(i = n-1; i > pos; i--){
            .... 
        }

        for(i = 0; i < n; i++){
            ....
        }
    }