除非存在另一个printf，否则Printf不起作用

Question

I was trying to solve a challenge in C on HackerRank. There is a square matrix with n rows and columns.The challenge is to print the absolute difference between the two sums of the matrix's diagonals as a single integer.

Here's the link: https://www.hackerrank.com/challenges/diagonal-difference

I'm able to get the correct answer but the printf() statement at the end of the code does not work unless I add an unnecessary printf("") before the for loop statement. Could someone explain why this is happening?

Thanks in advance

Here's the code:

#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <stdbool.h>

int main()
{
    int n; 
    scanf("%d",&n);
    int a[n][n],sum_d1=0,sum_d2=0,a_i,a_j;
    for( a_i = 1; a_i <= n; a_i++)
    {
       for(a_j = 1; a_j <= n; a_j++)
       {
           scanf("%d",&a[a_i][a_j]);
           if(a_j==a_i)
               sum_d1=sum_d1+a[a_i][a_j];
       }
    }
    printf("");// the last printf works only when this statement is present
    for(a_i=1;a_i<=n;a_i++)
    {
        for(a_j=n;a_j>0;a_j--)
        {
            if((a_i+a_j)==(n+1))
                sum_d2=sum_d2+a[a_i][a_j];
        }
    }
    printf("%d",abs(sum_d1-sum_d2)); //this doesn't work if there is no printf above
    return 0;
}

Answer 1

The trailing newline is the key here. Standard out only flushes when it encounters a line ending for performance reasons. If you change to fprintf(stderr, ...) then your code will work as is. You could also use fflush(stdout) after your printf's.

Answer 2

As a couple of other people have mentioned, your main issue is your array declaration / use. Arrays in C have their first index from 0, not from 1. As such, when you try and store things in your array, or access them again, it is out of the boundary of the array, and may act in a way unintended.

Array example: Array[2][3]

    0  1 
0  a|  b | 
1  c|  d | 
2  e|  f |

Array[0][0] == a 
Array[1][1] == d 
Array[1][2] == f

In your code, if someone enters "1", an array of size [1][1] is created - a single element. This should be accessed using a[0][0], but in the code is being accessed by a[1][1], which is an invalid element, as it is not a part of the array.

Without knowing exactly what you intend your code to do, I cannot fix it entirely, but a good start would be as below:

#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <stdbool.h>

int main(){
    int n; 
    scanf("%d",&n);
    int a[n][n],sum_d1=0,sum_d2=0,a_i,a_j;
    for( a_i = 0; a_i < n; a_i++){
       for(a_j = 0; a_j < n; a_j++){
           scanf("%d",&a[a_i][a_j]);
           if(a_j==a_i)
               sum_d1=sum_d1+a[a_i][a_j];
       }
    }
    //printf("");// the last printf works only when this statement is present
    for(a_i=0;a_i<n;a_i++)
    {
        for(a_j=n-1;a_j>=0;a_j--)
        {
            if((a_i+a_j)==(n+1))
                sum_d2=sum_d2+a[a_i][a_j];
        }
    }
    printf("%d",abs(sum_d1-sum_d2)); //this doesn't work if there is no printf above
    return 0;
}