C not printing 2D dynamic array content

Question

I am trying to make a 2D dynamic array that calculates the derteminant of 4 or 9 numbers.

While using pointers and a dynamic array worked flawlessly in a 1D array, I can't wrap my head around what I have to do to make it owrk in 2D. So far, the program compiles properly and receives input. However, it does not print anything. I even tried printing a single cell content. Nothing.

Here is the code:

  #include <stdio.h>
    #include <stdlib.h>
    
    int size; int i; int j; int **arDet; int a; int b; int c; int d; int e; int f; int g; int h;
    
    int input()
    {
        do
        {
            scanf("%d", &size);
        }while(size > 3 || size < 2);
        arDet = (int**)malloc(sizeof(int)*size);
        for(i = 0 ; i < size; i++)
        {
            for(j = 0 ; j < size; j++)
            {
                scanf("%d", &arDet[i][j]);
            }
        }
    }
    
    int main()
    {
        input();
        if(size == 2)
        {
            printf("%d\n", arDet[0][0]*arDet[1][1] - arDet[0][1]*arDet[1][0]); 
            return 0;
        }else if(size == 3)
        {
           // printf("%d", arDet[0]*(arDet[4]*arDet[8] - arDet[7]*arDet[5]) - arDet[1]*(arDet[3]*arDet[8] - arDet[5]*arDet[6]) + arDet[2]*(arDet[3]*arDet[7] - arDet[6]*arDet[4]));
            return 0;
        }else
        {
            return -1;
        }
    }

MikeCAT · Accepted Answer · 2020-10-30 14:11:02Z

1

You only allocated an array to store pointers to each rows. You have to allocate arrays to store values for each rows.
The size calculation for arDet is wrong. The elements of arDet are int*. int (4 bytes for example) has shorter size than int* (8 bytes for example) in some environments and your code will cause trouble in such environments.
Casting results of malloc() in C is discouraged.

    arDet = malloc(sizeof(arDet[0])*size); /* fix size calculation */
    for(i = 0 ; i < size; i++)
    {
        arDet[i] = malloc(sizeof(arDet[i][0]) * size); /* add this */
        for(j = 0 ; j < size; j++)
        {
            scanf("%d", &arDet[i][j]);
        }
    }

Also note that the commented line

       // printf("%d", arDet[0]*(arDet[4]*arDet[8] - arDet[7]*arDet[5]) - arDet[1]*(arDet[3]*arDet[8] - arDet[5]*arDet[6]) + arDet[2]*(arDet[3]*arDet[7] - arDet[6]*arDet[4]));

doesn't make sense because C doesn't support multiplying two pointers and indice above 2 is out-of-range when size is 3.

answered Oct 30, 2020 at 14:11

MikeCAT

75.1k11 gold badges50 silver badges71 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

DionGR Over a year ago

Yes, that was exactly the issue. I am completely new with C and I can't wrap my head around pointers, but your solution makes perfect sense.

Collectives™ on Stack Overflow

C not printing 2D dynamic array content

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related