std::transform for std::map

[Ralf Goertz]

Hi,

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

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

using namespace std;

int doit(int w) {
return w+1;
}

int main() {
map<int,int> m;
vector<int> v;
m[0]=42;
v.push_back(42);
//transform(m.begin(),m.end(),m.begin(),doit); (*)
transform(v.begin(),v.end(),v.begin(),doit);
copy(v.begin(),v.end(),ostream_iterator<int>(cout," "));
cout<<endl;
}

I understand syntactically why the line (*) does not compile. But
conceptually is doesn't make much sense (IMHO) to not be able to use
maps in std::tranform. Even changing doit to

pair<int, int> doit(pair<int, int> w) {
pair<int,int> result(w);
w.second++;
return result;
}

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

Thanks,

Ralf

 

[Pascal J. Bourguignon]

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:air<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<<endl;
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.

 

[Ralf Goertz]

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:air<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<<endl; 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.

Okay, thanks. But can I also do it *in place* (like I did it with the
vector in the OP).

 

[red floyd]

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:air<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<<endl;
    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;
}

 

[James Kanze]

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

That's because maps aren't really containers, in the usual
sense.

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

using namespace std;

int doit(int w) {
return w+1;
}

int main() {
map<int,int> m;
vector<int> v;
m[0]=42;
v.push_back(42);
//transform(m.begin(),m.end(),m.begin(),doit); (*)
transform(v.begin(),v.end(),v.begin(),doit);
copy(v.begin(),v.end(),ostream_iterator<int>(cout," "));
cout<<endl;
}

I understand syntactically why the line (*) does not compile.
But conceptually is doesn't make much sense (IMHO) to not be
able to use maps in std::tranform. Even changing doit to

pair<int, int> doit(pair<int, int> w) {
pair<int,int> result(w);
w.second++;
return result;
}

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

Probably using an iterator adaptor or an iterator facade from
Boost. Basically, what you want is an iterator which only
"shows" the second element of the pair.

 

[James Kanze]

What about using std::for_each? map<K,V>::value_type is
typedef'ed as std:air<const K, V>, meaning that as long as
you have a non-const map, the iterator you get from begin()
will allow you to change the value.

Sure, but transform and the special iterators express the intent
a lot better. He wants to iterate over the mapped objects, and
transform them.

 

[Ralf Goertz]

Sure, but transform and the special iterators express the intent
a lot better. He wants to iterate over the mapped objects, and
transform them.

Yes, that was the reasoning. I will check the special iterator approach
but I'm also happy with the for_each solution.

Thanks,

Ralf