C program printing wrong output

Question

I am trying to create a program which prints a matrix of integers, but the output returns weird numbers before the actual matrix. There are no compiling errors.

This is what my code looks like: //ignore void function for now, focus on main function::

#include <stdio.h> #include <stdlib.h> //array[30][30] = 2D array of max 30 rows and 30 columns //n = number of rows and columns void printmatrix(int array[30][30], int n){ for(int i = 0; i < n; i++){ for(int j = 0; j < n; j++){ printf("%d", array[i][j]); } printf("\n"); } return; } int main(){ int n; scanf("%d", &n); int ints2D[n][n]; for(int i = 0; i < n; i++){ for(int j = 0; j < n; j++){ scanf("%d", &ints2D[i][j]); } } printmatrix(ints2D, n); for(int i = 0; i < n; i++){ for(int j = 0; j < n; j++){ printf("%d ", ints2D[i][j]); } printf("\n"); } return 0; }

And this is my output (I only want the last three lines)

123 -514159984327663 -51415932632766-514159305 1 2 3 4 5 6 7 8 9

"ignore void function for now" -- why? it's the function that's printing the numbers you don't want. It's not possible to answer your question if we have to ignore that function. Anyway: your bug is that the function you're calling, printmatrix(), is expecting an array of arrays, where each array has 30 elements. But the array you pass has only three arrays, and each of those has only three elements. This means that once the loop gets past the first row, you're just printing random data. Also, you didn't include the extra ' ' character in the function, so all the numbers run together. — Peter Duniho
– Peter Duniho, Commented Sep 28, 2019 at 2:22

Eric Postpischil · Accepted Answer · 2019-09-28 02:24:58Z

You are missing a space in "%d" in printmatrix, and, more importantly, it is not proper to pass an int [n][n] array for an int [30][30] parameter unless n is 30.

Change void printmatrix(int array[30][30], int n) to void printmatrix(int n, int array[n][n]), and change printmatrix(ints2D, n); to printmatrix(n, ints2D);. That makes the type of the argument you are passing match the type of the parameter.

niry · Accepted Answer · 2019-09-28 02:43:45Z

In your function args you defined the array as fixed size ([30][30]) but you are passing a VLA ([3][3] in your example) which makes it find uninitialized memory and why you are seeing the strange numbers.

@Eric Postpischil answer is spot on. Another way to solve it: 2d arrays could be flatten into 1d. Here is a working code for you:

#include <stdio.h> #include <stdlib.h> //array[30][30] = 2D array of max 30 rows and 30 columns //n = number of rows and columns void printmatrix(int *array, int n){ for(int i = 0; i < n; i++){ for(int j = 0; j < n; j++){ printf("%d ", array[i * n + j]); } printf("\n"); } return; } int main(){ int n; scanf("%d", &n); int ints2D[n * n]; for(int i = 0; i < n; i++){ for(int j = 0; j < n; j++){ scanf("%d", &ints2D[i * n + j]); } } printmatrix(ints2D, n); for(int i = 0; i < n; i++){ for(int j = 0; j < n; j++){ printf("%d ", ints2D[i * n + j]); } printf("\n"); } return 0; }

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