% symbols keep printing when trying to print out a char 2D array in C

Question

I'm new to C and I'm just trying to print out a two 2 array. This bug has been annoying me all day and I'm not really sure whats going on.

#include<stdio.h>

void run(int);

main()
{
    run(5);
    return 0;
}

//Have to make it a character array as it needs to 
//store numbers AND commas. 
run(int x)
{
    int size = 2*x -1;
    char array[size][size];
    int i = 0;
    int j = 0;
    for( i; i < size; i++){
        for(j; j< size; j++){
            array[i][j] = '1';
        }
    }

    int k = 0;
    int l = 0;
    for( k; k < size; k++){
        for(l; l< size; l++){
            printf( "%c" , array[l][k]);
        }
        printf("%\n", "");
    } 
}

This is the output I get:

1%
%
%
%
%
%
%
%
%

What are you expecting `printf("%\n", "")` to print? You don't have a formatting operator after `%`. — Barmar, Oct 26 '15 at 22:15
I was expecting just a new line to print - I guess I should just format that as a character then... — , Oct 26 '15 at 22:18
`for(j; j< size; j++){` - the `(j;` doesn't do anything, probably meant `for(j = 0; j< size; j++){`. Same problem with `l`, and similar (although harmless) with `i` & `k`. — Amit, Oct 26 '15 at 22:19
You should really learn how to use a debugger and step through your code. It will be the most important leasson you'll learn as a programmer. — Amit, Oct 26 '15 at 22:21

Amit · Accepted Answer · 2015-10-26T23:34:46.740

You code has several mistakes:

The biggest problem is that your not initializing your loop counters where you should:

for(i; i < size; i++){
  for(j; j < size; j++){

With that, i & j are left as they were prior to the for statement. The first section of these statements does nothing at all. While that's harmless for i (since it's initialized to 0 before the for), that's devastating for j, which never goes back to 0. Your code should be:

for(i = 0; i < size; i++){
  for(j = 0; j < size; j++){

The same issue exists with k & l, and the same fix should be applied:

for(k = 0; k < size; k++){
  for(l = 0; l < size; l++){

Next, you're "rotating" access in your array. When you fill the array with values, you have i in your outer loop and j in the inner loop, and you use them as [i][j]:

array[i][j] = '1';

Think of that as Out & In --> [Out][In].

When you print the array, you "rotate" that, k is outer & l is inner, and you use them as [l][k]:

printf("%c", array[l][k]);

That's like doing [In][Out].
While that's not a problem with all values being identical ('1'), and the matrix being square (width == height), it won't work with other values or dimensions, and is confusing.

Last, you're attempt to print a new line is wrong. You have a % specifier, but your not really using any valid character after that, and you don't need that anyway, just print:

printf("\n");

So, all together, here's what the code should be:

run(int x)
{
    int size = 2*x -1;
    char array[size][size];
    int i,j;
    for(i = 0; i < size; i++){
        for(j = 0; j < size; j++){
            array[i][j] = '1';
        }
    }

    int k, l;
    for(k = 0; k < size; k++){
        for(l = 0; l < size; l++){
            printf("%c", array[k][l]);
        }
        printf("\n");
    } 
}

(And as a side note, k & l are not really required, you can simply reuse i & j)

% symbols keep printing when trying to print out a char 2D array in C

1 Answers1