CODEWITHSUNDEEP
c++
Invictus
Invictus

Reputation: 4328

How can i directly find the number of elements between any two iterator pointing to two different key in map without the for loop?

How can i directly find the number of elements between any two iterator pointing to two different key in map without the for loop?

     #include <string>
     #include <iostream>
     #include <map>
     #include <utility>
     #include <iterator>
     #include <stdio.h>
     using namespace std;
     int main()
   {

     std::map<string, string> m;
     m.insert(pair< string, string>("4-2"," 61-7" ));
     m.insert(pair< string, string>("5-2"," 61-7" ));
     m.insert(pair< string, string>("5-3"," 61-7" ));
     m.insert(pair< string, string>("5-4"," 61-7" ));
     m.insert(pair< string, string>("5-5"," 61-7" ));
     m.insert(pair< string, string>("5-6"," 61-7" ));

     map<string, string>::iterator it4;
     map<string, string>::iterator it5;
     it4=m.find("5-2");
     it5=m.find("5-5");
     cout << " value of it4" << it4->first << it4->second <<  endl;
     cout << " value of it5" << it5->first << it5->second <<  endl;
     m.clear();
     getchar();

  }

My desired output is difference of elements betwen it5 and it4 which here is 2 as elements with key "5-3" and "5-4" lies between them . I dont want to iterate in for loop as in my case number of elements in map are large enough . And I have to regularly know the difference . using a for loop will take lot of time if two iterator are far away .

Upvotes: 0

Views: 179

Answers (2)

Stack Overflow is garbage
Stack Overflow is garbage

Reputation: 247919

Given your requirements, it can't be done. std::map does not offer random-access iterators, which are necessary for determining iterator distance in constant time.

You can still use std::distance, but the cost will be the same as if you wrote the loop yourself.

If you need to generate this information more efficiently, you'll have to replace the map with a different data structure (or at least, augment it with some additional bookkeeping).

The only standard-library containers which have random-access iterators are the vector, deque and `array.

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 476990

You can use std::distance(it4, it5) from <iterator>. The cost will be linear in the result, though, so that doesn't really save you. If you must have quick access to this information, consider a Boost.Multi_index container so you can add a random-access index to your map.

Upvotes: 5

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