C++ - How to traverse a list from end to begin?

Question

I would like to display my list from the end to the beginning. Like:

for (std::list<T>::const_iterator it = list.end(); it != list.begin(); --it)

The problem is that it doesn't enter in when it is in the first element (list.begin()). How can I do this?

_it doesn't enter_ because `list.end()` is not inside your list. — masoud, Mar 18 '13 at 13:59
Possible Duplicate: [What's the best way to do a backwards loop in C/C#/C++?](http://stackoverflow.com/q/275994/1084416) — Peter Wood, Mar 18 '13 at 13:59

stativ · Accepted Answer · 2013-03-18T13:57:29.050

21

Use rbegin and rend to get reverse iterators.

for (std::list<...>::reverse_iterator it=list.rbegin(); it!=list.rend(); ++it)

edited Mar 18 '13 at 13:57

answered Mar 18 '13 at 13:55

stativ

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and as a bonus, you can use `auto`: `for( auto it=list.rend(); it!=list.rbegn(); ++it ) {...}` – xtofl Mar 18 '13 at 13:57

score 5 · Answer 2 · answered Mar 18 '13 at 13:56

5

Use reverse iterators:

for (std::list<...>::const_reverse_iterator it = l.rbegin(); it != l.rend(); ++it)

C++11

for (auto it = l.crbegin(); it != l.crend(); ++it)

and please don't name your std::list<T> instance list.

answered Mar 18 '13 at 13:56

juanchopanza

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I believe a reverse iterator should still start with `rbegin` (the last element)... and go to `rend` (the first element), right? Your answer has it backwards. – abelenky Mar 18 '13 at 13:58
1

@abelenky Yes that is completely correct. I had a bad cut and paste fail from OP's code. – juanchopanza Mar 18 '13 at 13:59

score 1 · Answer 3 · answered Mar 18 '13 at 13:56

1

You want a reverse iterator. See rbegin and rend.

answered Mar 18 '13 at 13:56

Sean

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score 0 · Answer 4 · answered Mar 17 '14 at 20:52

Addition: Because in it = list.end(); it gets => pointer [some block of memory] which is understood as "end" (or point where to stop, right or left boundary). So output its content (cout<<*it) you will see memory[block] address. If you declare and use as this:

std::list<...>::const_iterator it = list.begin();
  std::list<...>::const_iterator iTail = list.end();
  while(it!=iTail)
  {
      //do smthing
      ++it;
  }

either loop will be skipped or you will get some garbage from heap. Its case of const_iterator (didn't read documentation but obviously for "write" protection). In case of iterator std::list<...>::iterator only .end() points to the ~~last[element]~~ => [end]. In cases:

std::list<...>::const_reverse_iterator and std::list<...>::reverse_iterator

iterator shifted automatically to the first elem, depending on start => end or end =>start you are going to run through list.

C++ - How to traverse a list from end to begin?

4 Answers4