Difference between memset() and initializinng array elements using a for loop?

Question

When I use memset inside the printNGE function I get the correct result also when I initialize the array elements to -1 in the main() , I get the correct result.

But when I initialize the same in the printNGE function using a for loop I get a wrong answer. It seems the initial value of the array elements in not changed inside the else part of the while loop for some reason? Please tell me what could be the possible reason for such discrepancies.

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<stack>
using namespace std;

/* arr[] of size n */

void printNGE(int arr[], int n)
{
    int i = 0;
    stack <int> s;
    int ans[n];
    for(i=0;i<n;i++)//Set all the elements to -1
        ans[i] = -1;

    //memset(ans,-1,sizeof(ans));

    while (i<n)
    {
        if(s.empty() || arr[s.top()]>arr[i]) 
            s.push(i++);
        else
        {
            ans[s.top()]=arr[i];    
            s.pop(); //pop till incoming element is greater
        }
    }
    for(i=0;i<n;i++)
        printf("%d -> %d\n",arr[i],ans[i]);
}

int main()
{
    int arr[]= {11, 13, 21, 3};
    int i;
    int n = sizeof(arr)/sizeof(arr[0]);
    //int ans[n];
    //for(i=0;i<n;i++)//Set all the elements to -1
    // ans[i] = -1;
    printNGE(arr, n);
    getchar();
    return 0;
}

You could find our by using a debugger and stepping through the code. — Roland Illig
– Roland Illig, Commented Dec 1, 2016 at 7:59
Additionally is -1 an exception. memset performs bytewise initialization of a data-block and a loop iterates over the elements. That means that other values than -1 (0xffffffff) would result in different states of the array. — h4x0r
– h4x0r, Commented Dec 1, 2016 at 8:01
To expand a bit on the comment by @h4x0r - this use of memset only works by accident. Try initializing the array to 2 instead of -1. Don't use memset here. In fact, in general, don't use memset. — Pete Becker
– Pete Becker, Commented Dec 1, 2016 at 13:49

Mathieu · Accepted Answer · 2016-12-01 07:51:12Z

5

The index variable i of the for loop is not reset. Thus, the while loop won't get executed.

edited Dec 1, 2016 at 7:51

Mathieu

10.1k7 gold badges38 silver badges50 bronze badges

answered Dec 1, 2016 at 7:48

h4x0r

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2 Comments

Persixty Over a year ago

Just to murder the point. when you use memset() you're not incrementing i so the loop behaves as you expect.

newcoder191 Over a year ago

Thanks purplepsycho!

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