multmap equal_range in C++

Question

This example for use of equal_range is offered on the cplusplus.com site:

 int main ()
 {
   std::multimap<char,int> mymm;

   mymm.insert(std::pair<char,int>('a',10));
   mymm.insert(std::pair<char,int>('b',20));
   mymm.insert(std::pair<char,int>('b',30));
   mymm.insert(std::pair<char,int>('b',40));
   mymm.insert(std::pair<char,int>('c',50));
   mymm.insert(std::pair<char,int>('c',60));
   mymm.insert(std::pair<char,int>('d',60));

   std::cout << "mymm contains:\n";
   for (char ch='a'; ch<='d'; ch++)
   {
      std::pair <std::multimap<char,int>::iterator, std::multimap<char,int>::iterator> ret;
      ret = mymm.equal_range(ch);
      std::cout << ch << " =>";
      for (std::multimap<char,int>::iterator it=ret.first; it!=ret.second; ++it)
         std::cout << ' ' << it->second;
      std::cout << '\n';
   }

And the output is said to be:

mymm contains:
a => 10
b => 20 30 40
c => 50 60
d => 60

But isn't this wrong? For 'd' for instance the condition it!=ret.second will immediately fail and the loop will never be executed? Or have I got this wrong? (This matters as I have based some code on this example and on a second look think it's likely to misbehave.)

Iterator intervals are half-open.

Qaz
– Qaz

2014-06-26 20:52:36 +00:00
Commented Jun 26, 2014 at 20:52 — Qaz
– Qaz, Commented Jun 26, 2014 at 20:52

Cogwheel · Accepted Answer · 2014-06-26 20:52:55Z

4

The second iterator in the range is "one-past-the-end" of where it would be just like the way mymm.end() works. If mymm.begin() and mymm.end() are the same there is nothing in the container. Here, ret.first and ret.second being the same, means there is no element in the resulting range. Since there is one element for 'd', ret.first points to that element and ret.second is the same as mymm.end().

answered Jun 26, 2014 at 20:52

Cogwheel

23.3k4 gold badges51 silver badges67 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

adrianmcmenamin Over a year ago

cheers. Sadly that rules out one reason why my code was misbehaving :(

Slava · Accepted Answer · 2014-06-26 20:56:04Z

3

No, it is not wrong. According to

Returns a range containing all elements with the given key in the container. The range is defined by two iterators, one pointing to the first element that is not less than key and another pointing to the first element greater than key.

from http://en.cppreference.com/w/cpp/container/multimap/equal_range it works as expected.

answered Jun 26, 2014 at 20:56

Slava

44.4k2 gold badges54 silver badges100 bronze badges

Collectives™ on Stack Overflow

multmap equal_range in C++

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related