printf changes my output

Question

I'm trying to solve this challenge on Hackerrank.

I've reached a problem where I cannot proceed, but I can't see where I've gone wrong - and I'm hoping someone here can help.

My current solution is as follows:

int main() {

    int n,k,p,count,total; 
    int t[n];
    scanf("%d %d",&n,&k);
    for(int i = 0; i < n; i++){
        scanf("%d",&t[i]);
    }
    p = 1;
    total=0;
    for(int x = 0; x < n; x++){
        for(int j = 1; j <= t[x]; j++, count++){
            if(count>k){
                count = 1;
                p++;
            }
            if(j==p){
                total++;
            }
            //printf("j: %d p: %d\tcount: %d\n",j,p,count);
        }
        p++;
        count=1;
    }
    printf("%d",total);

    return 0;
}

The printf that I have commented out is what changes my eventual output. For example, with an input of:

10 5

3 8 15 11 14 1 9 2 24 31

I should be getting an answer of 8. If I un-comment that printf() function, then I can see the current problem number and page number to see if it's 'special'. If I leave it un-commented, my eventual output is 8, which is what I want. But I don't want all the iterations printed out as well. The problem I have is that when I remove that line, or comment it out, the output becomes 5, not 8. What is causing this to change?

Answer 1

In your code, while defining int t[n]; , you're using n uninitialized. That , invokes undefined behavior .

To elaborate, n is an automatic local variable that is not initialized explicitly, so the content of that variable is indeterminate. Attempt to use an indeterminate value leads to UB.

Quoting C11 , chapter §6.7.9

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. [...]

and, annex §J.2, undefined behavior,

The value of an object with automatic storage duration is used while it is indeterminate

You need to move the definition of int t[n]; after you have successfully scanned the value from the user. Check the return value of scanf() to ensure the success.

Answer 2

数组必须具有固定的大小，您可以在使用calloc（），malloc（）读取元素数量n后创建