Re: std::transform for std::map

From:

red floyd <redfloyd@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 16 Mar 2009 09:00:01 -0700 (PDT)

Message-ID:

<fed6e6f6-0cba-48fc-950d-88fcce1959cb@e38g2000yqa.googlegroups.com>

On Mar 16, 7:46 am, p...@informatimago.com (Pascal J. Bourguignon)
wrote:

Ralf Goertz <r_goe...@expires-2006-11-30.arcornews.de> writes:

it doesn't seem to be possible to use transform for maps.

Yes it is possible:

#include <algorithm>
#include <map>
#include <iterator>
#include <vector>
#include <iostream>

using namespace std;

int doit(std::pair<const int,int>& w) {
    return w.second+1;

}

int main() {
    map<int,int> m;
    map<int,int> o;
    m[0]=42;
    m[3]=33;
    m[6]=66;
    m[2]=22;
    m[4]=44;
    vector<int> v(m.size());
    transform(m.begin(),m.end(),v.begin(),doit);
    copy(v.begin(),v.end(),ostream_iterator<int>(cout," ")); cout<<en=

dl;

return(0);

}

/*
-*- mode: compilation; default-directory: "/tmp/" -*-
Compilation started at Mon Mar 16 15:45:08

SRC="/tmp/tm.c++" ; EXE="tm" ; g++ -g3 -ggdb3 -o ${EXE} ${SRC} && =

./${EXE} && echo status = $?

43 23 34 45 67
status = 0

Compilation finished at Mon Mar 16 15:45:09
*/>

doesn't help. What would be the right way to do it?

You need to declare the key const in the pair.

Even better would be:

typedef std::map<int, int> intmap;

int doit(intmap::value_type w)
{
return w.second + 1;
}