How to find a value exists in a C++ std::map

Question

Hi, I'm trying to check if value is in a map and somewhat can't do it:

typedef map<string,string>::iterator mi;
    map<string, string> m;
    m.insert(make_pair("f","++--"));
    pair<mi,mi> p = m.equal_range("f");//I'm not sure if equal_range does what I want
    cout << p.first;//I'm getting error here

so how can I print what is in p? Thank you

Answer 1

You can use .find():

map<string,string>::iterator i = m.find("f");

if (i == m.end()) { /* Not found */ }
else { /* Found, i->first is f, i->second is ++-- */ }

Answer 2

I think you want map::find. If m.find("f") is equal to m.end(), then the key was not found. Otherwise, find returns an iterator pointing at the element found.

The error is because p.first is an iterator, which doesn't work for stream insertion. Change your last line to cout << (p.first)->first;. p is a pair of iterators, p.first is an iterator, p.first->first is the key string.

A map can only ever have one element for a given key, so equal_range isn't very useful. It's defined for map, because it's defined for all associative containers, but it's a lot more interesting for multimap.

Answer 3

What is wrong with find:

if ( m.find( "f") == m.end() ) {
  // not found
}
else {
  // found
}

Answer 4

m.find == m.end() // not found

If you want to use other API, then find go for m.count(c)>0

 if (m.count("f")>0)
      cout << " is an element of m.\n";
    else 
      cout << " is not an element of m.\n";