How do i use the for loop for a specified range while using maps in c++

Question

I want to use the for loops with iterators while using maps and want to run it for a specified range not from begin() end end(). I would want to use it for a range like from 3rd element to 5th element

So you know what you want to do, where's the problem? Where is the code that's causing an issue? — UnholySheep
– UnholySheep, Commented Feb 13, 2018 at 15:11
You know those "old" examples of for(auto it = m.begin(); it != m.end(); ++it)? You can in fact use other iterator values besides begin and end, if you have them. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Feb 13, 2018 at 15:12

Mateusz Grzejek · Accepted Answer · 2018-02-13 15:39:40Z

6

I would want to use it for a range like from 3rd element to 5th element

Since std::map's iterator is not a RandomAccessIterator, but only a BidirectionalIterator (you cannot write .begin() + 3), you can use std::next for this purpose:

for (auto it = std::next(m.begin(), 2); it != std::next(m.begin(), 5); ++it)
{
  // ...
}

And remember - check your ranges to ensure, that you iterate over a valid scope.

edited Feb 13, 2018 at 15:39

answered Feb 13, 2018 at 15:18

Mateusz Grzejek

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4 Comments

john Over a year ago

Worth saving the value of std::next(m.begin(), 5); instead of recalculating it each time round the loop

patatahooligan Over a year ago

I wonder, would there be gains in storing the end condition iterator or would compilers be aware enough to optimize the std::next away if the map is not modified withing the loop?

john Over a year ago

I doubt compilers would be aware enough.

patatahooligan Over a year ago

It's important to note that if the map doesn't have at least 5 elements, this will result in undefined behavior. Depending on OP's circumstances, a size check or an assert statement might be appropriate.

Slava · Accepted Answer · 2018-02-13 15:41:42Z

1

This code should be pretty optimal and safe for corner cases:

int count = 0;
for( auto it = m.begin(); it != m.end(); ++it ) {
    if( ++count <= 3 ) continue;
    if( count > 5 ) break;
    // use iterator
}

but the fact you are iterating an std::map this way shows most probably you are using a wrong container (or your logic for 3rd to 5th element is wrong)

answered Feb 13, 2018 at 15:41

Slava

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2 Comments

Rahul malawadkar Over a year ago

I just wanted to know if it is possible i m not implementing it anywhere✌

Slava Over a year ago

@Rahulmalawadkar everything is possible, question is what is the price.

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