map::value_comp in C++.
- From: "Graham Hardy" <graham.hardy@xxxxxxxxx>
- To: <programmingblind@xxxxxxxxxxxxx>
- Date: Thu, 1 Nov 2007 22:46:38 -0700
Hi all - What I want to do is to take a std::map <std::string, int>, and to
sort it by the int value to be sent to a file in order from highest to
lowest. Now, I suspect it has something to do with the map::value_comp
function, but I don't quite understand how the following code actually
works. (This example was taken from a website, and doesn't reflect my goal
of map <string, int>.)
#include <iostream>
#include <map>
using namespace std;
int main ()
{
map <char, int> mymap;
map <char, int> :: iterator it;
pair <char, int> highest; // I assume this 'pair' is an element of the map.
mymap ['x'] = 1001;
mymap ['y'] = 2002;
mymap ['z'] = 3003;
cout << "mymap contains:\n";
// This is where I stop understanding.
highest = * mymap.rbegin (); // last element
// What is the significance of calling it 'highest', when it is a
'reverse-end' iterator (which I take to mean an iterator to the element
before the first)?
it = mymap.begin ();
do
cout << it->first << " => " << it->second << endl;
while (mymap.value_comp () (* it ++, highest));
// What is the significance of the () before (* and after balue_comp? And
how does it accomplish this sorting that we see?
return 0;
}
Output:
mymap contains:
x => 1001
y => 2002
z => 3003
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