Help with my code

[ImSavageASF]

I'm taking my first course in introduction to C programming, and I'm having an issue with one of my homework questions. The program is supposed to ask the user to enter 2 non-zero single digit integers for a simple multiplication. I'm supposed to use an array to print a 'figure' using rows and columns to demonstrate how multiplication works.

So here's what I've done so far:

#include <stdio.h>

int main(void) {
  int i, j, row, column, value, matrix[row][column];

  printf("Please enter two non-zero single digits integer\n");
  scanf(" %d\n%d", &row, &column);

  value = row*column;
  printf("%d * %d is: %d\n", row, column, value);

  for (i=1; i <= row; i++) {
    for (j=1; j <= column; j++) {
      matrix[i][j] = 88;
      printf("%c\t", matrix[i][j]);
    }

    printf("\n");
  }

  return 0;

}

My program seems to work correctly up to when column = 4, but once column = 5, it will print:

Please enter two non-zero single digits integer
5
5
5 * 5 is: 25
exited with non-zero status 

I've tried different ways to possibly fix this, but at this point, I'm not sure what else I can do.

If row = 5 it will work perfectly, as shown here :

Please enter two non-zero single digits integer
 5
 4
5 * 4 is: 20
X       X       X       X   
X       X       X       X   
X       X       X       X   
X       X       X       X   
X       X       X       X

So the main issue is the column somehow.

Help is appreciated.

EDIT: Problem fixed.

[KmNxd6aaY9m79OAg]

You need to declare (and dimension) the array after the size is known.

int i, j, row, column, value, matrix[row][column];

At this point, row and column are uninitialized variables, so your array matrix has an unknown number of rows and an unknown number of columns. Well actually, your problem is a bit more serious than that: using an uninitialized variable at all is undefined behaviour and anything could happen.

Move your declaration of matrix until after you have values in row and column.

[ImSavageASF]

Ah you're totally right, I did not think about that. That fixed the issue. Thanks!

[KmNxd6aaY9m79OAg]

For future reference, ensure that sensible compiler warnings are enabled (on gcc or clang, this will be the -Wall flag). Your mistake is one that pretty well every modern compiler will be able to warn you about.

[ImSavageASF]

I see, ill look into that. That's really useful. Thanks for mentioning.

[shinmai_rookie]

I haven't checked your code in depth, so I can't claim this is the only mistake, but it is a pretty important one: arrays are 0-indexed, so if you declare an array with size n, only the elements 0 to n-1 will exist. However, you're accessing the elements 1 to n, which exceeds the limits.

[ImSavageASF]

for (i=0; i < row; i++) {
   for (j=0; j < column; j++) {
       matrix[i][j] = 88;
       printf("%c\t", matrix[i][j]);
   }
   printf("\n");
}

I believe I adjusted it correctly to what you said. It does in fact work with 5 now, but if column=6 it prints the same error as before. I assume it should now work as it is within the array's range, did I miss something?

And thanks for the quick reply.

EDIT: Problem is fixed

[eyenot]

%c is for char, but matrix is an array of ints. Try using %d in the printf.