Finding the maximum index of the biggest element of an array/vector

Question

Currently I'm only aware of the following:

vector<int> nums= {3,2,1,0,2,3,3,1,0,0}; 
return max_element(nums.begin(), nums.end())-nums.begin();

But in that case it would return the lowest index of the maximum element.

A way around it would be:

vector<int> nums= {3,2,1,0,2,3,3,1,0,0}; 
int n = *max_element(nums.begin(), nums.end());
for(int i=nums.size()-1; i>-1; i--){
    if(nums[i]==n) {
       return i; 
       break; 
    }
}

But is there any simpler way that I can achieve the same result without bruteforcing it?

Pull up the documentation on std::vector and review the iterators it provides. The index from iterator calculation (which I overlooked you making at first) will also need to be adjusted when you change the iterators you use. — Avi Berger, Aug 22 '22 at 16:23
FYI, your `break` statement has no effect. Once the `return` is executed, no other statements in the function are executed (because it returned). — Thomas Matthews, Aug 22 '22 at 16:53
Sort the array by ascending order. Largest elements will be at the end. The index of the largest element will be the index of the last element. — Thomas Matthews, Aug 22 '22 at 16:55

score 2 · Accepted Answer · answered Aug 22 '22 at 16:27

2

You could iterate backwards by using reverse iterators, and get the distance:

#include <vector>
#include <algorithm>
#include <iostream>

int main()
{
    std::vector<int> nums= {3,2,1,0,2,3,3,1,0,0}; 
    auto iter = std::max_element(nums.rbegin(), nums.rend()).base();
    std::cout << std::distance(nums.begin(), std::prev(iter));
}

Output:

See what base() does when it comes to reverse iterators.

answered Aug 22 '22 at 16:27

PaulMcKenzie

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Care should be done for empty vector . `std::prev(iter)` would be UB. (but unclear which value is expected in that case anyway). – Jarod42 Aug 22 '22 at 19:14
Very interesting, thank you! @PaulMcKenzie – yokgn Aug 23 '22 at 17:17

score 1 · Answer 2 · answered Aug 22 '22 at 19:11

Whereas std::max_element returns the first largest element, std::minmax_element returns the last one (for the largest).

So you might do:

std::vector<int> nums = {3,2,1,0,2,3,3,1,0,0}; 
return std::distance(nums.begin(),
                     std::minmax_element(nums.begin(), nums.end()).second);

Finding the maximum index of the biggest element of an array/vector

2 Answers2