C - find prime numbers in randomly generated 2d array

Question

In C, I have a randomly generated 20x20 array. I can sort and display it fine, but I need to take the prime numbers from each row and put them into their own array. It needs to be done with a function outside of main.

Problem is, no matter what I try I can't find the correct syntax to perform the arithmetic on the array to find the prime numbers, I always get "invalid operands to binary % (have 'int *' and 'int')" error. I'm not sure if there's a way to dereference the array to perform arithmetic on it. Here's some code:

#include <stdio.h>
#include <stdlib.h>

#define TOTAL_ROWS 20
#define TOTAL_COLUMNS 20

void fillMatrix(int A[TOTAL_ROWS][TOTAL_COLUMNS], int *set_rows, int *set_columns)
{
    int rows = 20, columns = 20;
    for(int i = 0; i < rows; i++)
        for(int j = 0; j < columns; j++)
            A[i][j] = rand() % 500;

    *set_rows = rows;
    *set_columns = columns;
}




void sortMatrix(int A[TOTAL_ROWS][TOTAL_COLUMNS], int rowsize, int colsize)
{
    for(int r = 0; r < rowsize; r++)
        qsort(A[r], colsize, sizeof(int), compare);
    }



void displayArray(int A[TOTAL_ROWS][TOTAL_COLUMNS], int rows, int columns)
{
    for(int i = 0; i < rows; i++)
    {
        for(int j = 0; j < columns; j++)
            printf("%3i ", A[i][j]);
        printf("\n");
    }
}

void findPrimes(int A[TOTAL_ROWS][TOTAL_COLUMNS], int rows, int columns)  
{
int n,c,sum,count;
    for(int i = 0; i < rows; i++)
        {
            for(int j = 0; j < columns; j++){
                for(i=0;i<n;i++)
                    {
                        c=0;
                        for(j=2;j<A[i];j++)
                        {
                            if(A[i]%j==0) //where "invalid operands to binary % (have 'int *' and 'int')" happens 
                            {
                                   c=1;
                                   break;
                            }
                        }
                         if(c==0)
                         {
                            printf("%d\t",A[i]);
                            sum=sum+A[i];
                            count++;
                         }
                    }
            }
        }
}



int main(void)
{
    int A[TOTAL_ROWS][TOTAL_COLUMNS];
    int rows, columns;
    fillMatrix(A, &rows, &columns);
    sortMatrix(A, rows, columns);
    displayArray(A, rows, columns);
    findPrimes(A, rows, columns);
    return  0;
}

Answer 1

The issue is that A[i] is one entire row of the 2D array and is therefore itself a standard (1D) array (which decays to int* when used in most ways, hence the message).

You want to use A[i][j] to get the int value of the specific cell.

But while this allows your code to compile, it still won't work properly. Here is a list of further problems to fix:

• Nested loops both iterating on i . This is clearly a mistake, and will produce unexpected results.
• Using the uninitialized automatic variables n , sum , and count . The latter two should be initialized to 0 ; you need to do this explicitly. I'm not sure what n is trying to do.
• Code style. While this won't cause your code to fail directly, the mess of inconsistent indent and lack of encapsulation present makes it much easier to make mistakes.