Finding index of minimum value in array, USING min(a,b)

I don't think there is anything wrong with your solution for an array of floating-point elements (I assume that there are no NaNs). std::min returns a reference to its argument with a lower value (which is defined for non-nan FP numbers). Thus std::min(arr[i], arr[min_index]):

1. either returns a reference to arr[i] if it is less or equal to arr[min_index],
2. or, returns a reference to arr[min_index], if this is greater than arr[i].

In 1., when you compare the result of std::min with arr[i], you are comparing two very same FP objects, which is guaranteed to be evaluated ot true.

In 2., you are comparing two FP objects with different values, which is guaranteed to be evaluated to false.

Therefore, I think your solution works well.

Just be aware that rounding errors outside of your this solution can seemingly break it. Consider the following example:

float arr[3] = { 1.0f / 3.0f, 0.33333334, 2.0f };
size_t min_index = 0;
for (size_t i = 1; i < 3; i++)
   min_index = std::min(arr[i], arr[min_index]) == arr[i] ? i : min_index;
std::cout << min_index;

1/3 is the smallest number, but this printed out 1 in my experiment, since what your code is comparing is a computer representation of 1.0f/3.0f with a computer representation of 0.33333334, which both are subjects of rounding. Consequently, the numbers actually stored in arr, are different.

#include <iostream>
#include <algorithm>

using namespace std;

int main()
{
    int n;
    cin>>n;
    float arr[n];
    float min_index = 0;
    float mini = 0;


    for(int i=0; i<n; i++)
    {
        cin>>arr[i];
    }

    mini = arr[0];

    for(int i=0; i<n; i++)
    {
        mini = min(arr[i], mini);
    }

    for(int i=0; i<n; i++)
    {
        if(arr[i] == mini)
            min_index = i;
    }

    cout<<min_index<<endl;

    return 0;
}

working fine with float! could u be more specific about ur precision?