Finding the maximum difference between numbers

Question

Given n numbers, find maximum difference between some two of them. For example, for input data 1 2 7 -6 we have 13=7-(-6). But my code doesn't seem output correct results: 

#include <iostream>
#include <algorithm>
int main()
{
    int n, j, k;
    std::cin >> n;
    int *a;
    a = new int[n];

    for (j=0; j<n; j++)
    {
        std::cin >> a[j];
    }

   std::sort(a, a + sizeof(int));


    k=a[n-1]-a[0];
    std::cout << k;

    delete [] a;

    return 0;
}

Answer 1

As Olivier answered, your mistake is in the call to std::sort(), but I want to point out that unless this is mandatory for your task (it may be part of an homework), You don't need to sort the array nor you need an array too.

#include <iostream>
#include <climits>

int main() {
    int n, j, a;
    int max = INT_MIN;
    int min = INT_MAX;

    if ( std::cin >> n  &&  n > 0 ) {
        while ( j < n  &&  std::cin >> a ) {
            if ( a < min ) min = a;
            if ( a > max ) max = a;
            j++;
        }    
        std::cout << max - min;
    }
    return 0;
}

Answer 2

Your problem is when you call std::sort

You do:

std::sort(a, a + sizeof(int));

So you actually only sort 4 elements (sizeof(int)=4 most of the time). For correct result you could do:

std::sort(a, a+n); (better)

or

std::sort(a, &a[n]);