how to find the max element between the m-th and the n-th element of a vector in C++?

Question

It seems that the function max_element for vector would work for this purpose but I am not sure how to use it.

ForwardIterator max_element (ForwardIterator first, ForwardIterator last);

What is the type of ForwardIterator? How do I specify that it searches starting from the m-th element? And how do I find the index of the max element? Thanks.

sjrowlinson · Accepted Answer · 2016-07-07T00:41:44.170

5

Use the following:

std::max_element(vec.begin() + m, vec.begin() + n);

assuming m < n and n <= vec.size(), where vec is a std::vector object.

This will return an iterator to the max-value element in the specified range, to get the corresponding index use it - vec.begin() where it is the max-element iterator.

edited Jul 07 '16 at 00:41

answered Jul 07 '16 at 00:34

sjrowlinson

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Then how do I know which element is the max element? – Resorter Jul 07 '16 at 00:36
`std::max_element` returns the iterator to the max element in the range. – sjrowlinson Jul 07 '16 at 00:37
But I need to know the position of this element, for example, i want to know if it is the first, second, or third element. I want to know the integer which specifies the position. – Resorter Jul 07 '16 at 00:39
@Resorter [index from iterator](http://stackoverflow.com/questions/2152986/effective-way-to-get-the-index-of-an-iterator), the answer was a quick google search away. – Borgleader Jul 07 '16 at 00:40
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Just use `it - vec.begin()` to get the index, where `it` is the max-element iterator – sjrowlinson Jul 07 '16 at 00:40
Do you mean std::max_element(vec.begin() + m-1, vec.begin() + n-1); ? and it-vec.begin()+1 ? – Resorter Jul 07 '16 at 00:44
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@Resorter: `std::vector` is 0-indexed. `m` and `n` need to be 0-based values. – Remy Lebeau Jul 07 '16 at 00:45
@RemyLebeau Thanks a lot. – Resorter Jul 07 '16 at 00:56
@Resorter it would be good to edit your Question to say that you want to know the index of the element – M.M Jul 07 '16 at 01:10
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@ArchbishopOfBanterbury If the container changes, a more generic approach to get the position is `std::distance(vec.begin(), it);` – PaulMcKenzie Jul 07 '16 at 03:00

score 2 · Answer 2 · answered Jul 07 '16 at 00:35

This signature is telling you that std::max_element is a template that accepts arguments of any type so long as it meets the forward iterator requirements, and it returns the same type. You would simply pass in the iterators and it would return an iterator to the maximum element found in the range. You can dereference that to get the value itself. Hence

const auto max_iterator = std::max_element(v.begin() + m, v.begin() + n);
const auto max_value = *i;

Care must be taken to avoid the case where m is greater than or equal to n.

how to find the max element between the m-th and the n-th element of a vector in C++?

2 Answers2