getting array index from a function

i've been assigned to write a program in C++ that, given an array of integers, finds the smallest (including its index) and largest number, and their subtraction. To do this, i decided to write three functions for each operation and it works perfectly. The problem is i don't know how to get the index of the smallest number.

I tried solving the problem by creating a variable and putting in inside a for loop and then print it. However, the program always says the index is 0.

Here you can see an example of how the output should be:

int myvector[200] = {40, 250, 9, 50} // Array containing 4 numbers

It should output: 

The smallest number is: 9 at index 2
The largest number is: 250
Their subtraction is: 241

Here you can see my code:

#include <iostream>
#include <stdlib.h>
#include <stdio.h>

using namespace std;

int find_smallest (int myvector[], int smallest, int index){

    smallest = myvector[0];
    index = myvector[0];
    for(int i=0; i<4; i++) {
        if(myvector[i] < smallest) {
            smallest = myvector[i];
            index = i;
        }
    }

    return smallest;
}


int find_largest (int myvector[], int largest) {

    largest = myvector[0];
     for(int i=0; i<4; i++) {
        if(myvector[i] > largest) {
            largest = myvector[i];
        }
    }

    return largest;
}

int sub_minmax (int myvector[], int subtraction, int smallest, int largest){

    subtraction = largest - smallest;
    return subtraction;
}

int main()
{

    int myvector[200], smallest, largest, subtraction, index;

    for(int i=0;i<4; i++) {
        cout<<"Enter the number " <<i+1<<endl;
        cin>>myvector[i];
    }

    smallest = find_smallest(myvector, smallest, index);
    largest = find_largest(myvector, largest);
    subtraction = sub_minmax(myvector, subtraction, smallest, largest);

    cout<<endl;
    cout<<"the smallest number is: "<<smallest<<" at index "<<index<<endl;
    cout<<"the largest number is: "<<largest<<endl;
    cout<<"the substraction is: "<<subtraction<<endl;

    return 0;
}

Any help will be appreciated, thanks