invalid types 'int[int]' for array subscript error in C?

Question

I am trying segment my matrix 2by2 square matrices. but I'm getting a compilation fault and don't know why. My code:

    int main()
    {
        int rowsize,n,i,j,l,k, var=0,var2=0,count = 0,square2by2[2][2];
        printf("Size of square matrix: " );
        scanf("%d",&n);
        rowsize=n;
        int *matrix = (int *)malloc(rowsize * rowsize * sizeof(int));
        for (i = 0; i <  rowsize; i++)
          for (j = 0; j < rowsize; j++)
             *(matrix + i*rowsize + j) = ++count;

        for (i = 0; i <  rowsize; i++)
         {
            for (j = 0; j < rowsize; j++)
            {
                printf("%d ", *(matrix + i*rowsize + j));
            }
            printf("\n");
         }
         for( i=0, l=var2 ;i<2 , l<var2+rowsize ;i++,l++)
        {
            for( j=0 , k=var ; j<2, k<var+2 ;j++,k++)
            {
                square2by2[i][j]=matrix[l][k]; // error line
            }

            if(i==1)
            {
                var++;
                if(var==rowsize-1) 
                {
                    printf("\n");
                    for(int x=0;x<2;x++)
                    {
                        printf("\n");
                        for(int y=0;y<2;y++)
                        {
                            printf("%d\t",square2by2[x][y]);
                        }
                    }
                    var = 0;
                    i-=2;
                    l-=1;
                    if(l==rowsize+1) 
                    {
                        break;
                    }
                }
                else
                {
                    i-=2;
                    l-=2;
                    printf("\n");
                    for(int x=0;x<2;x++)
                    {
                        printf("\n");
                        for(int y=0;y<2;y++)
                        {
                            printf("%d\t",square2by2[x][y]);
                        }
                    }
                }

            }
return 0;
        }

and the error is

37 36 [Error] invalid types 'int[int]' for array subscript why am i getting this error?

Possible duplicate of c++ error: invalid types int int for array subscript — cjds
– cjds, Commented Nov 5, 2015 at 12:37
is that really a duplicate? the accepted answer draws a line between C++ and c. But it won't hurt to look into the three answers over there, @pseudocode - maybe one of them can help you in spite of it being a C++ question — Bö macht Blau
– Bö macht Blau, Commented Nov 5, 2015 at 13:17

Tom Karzes · Accepted Answer · 2015-11-05 12:56:39Z

1

The problem is your declaration for matrix:

int *matrix = ...

This declares matrix to be a pointer to an int, or equivalently a pointer into a one-dimensional int array. The expression matrix[l] is therefore int-valued. You cannot apply the second [k] index to an int.

To do what you're trying to do, you need a pointer into a two-dimensional array. Here's how you can do that:

int a[dim1][dim2];
int (*p)[dim2] = a;    // The parentheses are necessary!

Here p is a pointer into a, so you can reference p just as you would a, e.g. p[i][j].

edited Nov 5, 2015 at 12:56

answered Nov 5, 2015 at 12:47

Tom Karzes

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Comments

cup · Accepted Answer · 2015-11-05 12:49:15Z

0

matrix has been declared as int* but is being used as int[][]

To fix, use

square2by2[i][j]=matrix[l * rowsize + k];

answered Nov 5, 2015 at 12:49

cup

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