STL MAP should use find() or [n] identifier to find element in map?

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离开以前 2020-12-05 20:37

I am confused which is more efficient?

As we can access map directly, why do we need to use find?

I just need to know which way is more efficient.

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夕颜 (楼主)

2020-12-05 21:27

This code and doc is picked from cplusplus.com

// accessing mapped values
#include 
#include 
#include 
using namespace std;

int main ()
{
  map mymap;

  mymap['a']="an element";
  mymap['b']="another element";
  mymap['c']=mymap['b'];

  cout << "mymap['a'] is " << mymap['a'] << endl;
  cout << "mymap['b'] is " << mymap['b'] << endl;
  cout << "mymap['c'] is " << mymap['c'] << endl;
  cout << "mymap['d'] is " << mymap['d'] << endl;

  cout << "mymap now contains " << (int) mymap.size() << " elements." << endl;

  return 0;
}

OP:
mymap['a'] is an element
mymap['b'] is another element
mymap['c'] is another element
mymap['d'] is
mymap now contains 4 elements.

Notice how the last access (to element 'd') inserts a new element in the map with that key and initialized to its default value (an empty string) even though it is accessed only to retrieve its value. Member function map::find does not produce this effect.

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