Why would you ever want to use 'int index = min_element(vector.begin(), vector.end()) - vector.begin();' (c++)

Question

I found some c++ code that I would like to understand. In this code they use

int airplane = min_element(min_cost_airplane.begin(),
min_cost_airplane.end()) - min_cost_airplane.begin();

But I don't know what this line of code exactly accomplishes. min_cost_airplane is a vector. I understand the min_element function, but I can't wrap my head around the -vector.begin at the end. Is the structure of this line of code common used? The thing I understand is that this line of code returns an iterator to the smallest element in the vector minus an iterator to the first element of the vector. So what does the iterator point to? Can someone please help me?

what did you not understand when consulting [some documentation](https://en.cppreference.com/w/cpp/algorithm/min_element) ? — 463035818_is_not_an_ai, Dec 16 '20 at 08:02

score 3 · Answer 1 · answered Dec 16 '20 at 08:12

The std::min_element algorithm returns an iterator. You can dereference that iterator to access the "minimum" element of the container. If, instead, you want to know the index of the element you need to compute it as the distance from the beginning of the container.

For random-access iterators you can subtract the iterators to get the offset, or index value. That's what your example does. There's also a std::distance function that computes the index but it also works for non-random access iterators. For example:

auto iter = std::min_element(min_cost_airplane.begin(), min_cost_airplane.end());
int index = std::distance(min_cost_airplane.begin(), iter);

Bathsheba · Answer 2 · 2020-12-16T08:55:52.600

3

std::min_element returns an iterator to the first instance of the minimum value in the given range.

begin returns an iterator to the first element in the container.

The std::vector iterators are special (it's a random access iterator) in that you can subtract one from another which yields the distance between them in terms of elements (it's pointer arithmetic under the hood). To be more generic and clearer, write

auto airplane = std::distance(
    min_cost_airplane.begin(),
    std::min_element(min_cost_airplane.begin(), min_cost_airplane.end())
);

edited Dec 16 '20 at 08:55

answered Dec 16 '20 at 08:15

Bathsheba

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nitpick: it isnt `std::vector`s iterators that are special, but all random access iterators support `-` – 463035818_is_not_an_ai Dec 16 '20 at 08:20
@largest_prime_is_463035818: Yes, it's worth adding that in. – Bathsheba Dec 16 '20 at 08:56

Villance · Answer 3 · 2020-12-16T08:23:17.077

-3

std::min_element is used to finds the smallest element in the range [first, last].

std::vector<int> v{3, 8, 4, 2, 5, 9};
 
std::vector<int>::iterator result = std::min_element(v.begin(), v.end());
 //result iterator to the minimum value in vector v.
std::cout << "min element is: " << *result;

output: min element is: 2

Note : The smallest value in vector v is 2.

edited Dec 16 '20 at 08:23

answered Dec 16 '20 at 08:06

Villance

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This is not the answer, because the result of the line in the question is offset, not a minimal element. – Suthiro Dec 16 '20 at 08:07
1

OPs code finds the index of the min element, not the value – 463035818_is_not_an_ai Dec 16 '20 at 08:08
This doesn't answer the question. Perhaps you can explain what you get when you subtract `begin()` from `result`. – Retired Ninja Dec 16 '20 at 08:08
OK but i am telling the use of `std::min_element` because i thought he is confuse in algorithm. – Villance Dec 16 '20 at 08:10
1

explaining something else than the code actually does might be confusing ;) – 463035818_is_not_an_ai Dec 16 '20 at 08:10
I am not explaining some thing else, it is exactly what he asked befor edit:). – Villance Dec 16 '20 at 08:26
1

OP wrote in the question `I understand the min_element function`, but not what is next to the function. So, your answer only explains stuff, which OP already knows. – mch Dec 16 '20 at 08:28
Firstly his question question was not so clear, after third edit it is clear now but i answered to the first one. May be little bit confusion. – Villance Dec 16 '20 at 08:32
already the original question was not using the value of the minimum element, but its index. Nevermind, just saying – 463035818_is_not_an_ai Dec 16 '20 at 08:57

Why would you ever want to use 'int index = min_element(vector.begin(), vector.end()) - vector.begin();' (c++)

3 Answers3