Problem with maps

[2kaud]

Have a look at http://www.cplusplus.com/reference/algorithm/find/

[flex567]

Maybe I should rather use pair instead of map?

[laserlight]

How can I search for a non-key value of a map?

You have to do linear search, i.e., loop over the mapped values one by one and compare. I believe that std::find_if can be used (i.e., you need to provide a comparator that will compare the mapped values). If you are often searching for the elements of a map by value, then perhaps something like boost::bimap would be more appropriate.

[2kaud]

Maybe I should rather use pair instead of map?

Have you considered reversing the mapping ie

	map<char, int> test;
	test['d'] = 100;
	test['e'] = 101;
	test['f'] = 102;
	test['*'] = 32;

How you organize a map depends a lot on how you intend to use it.

[flex567]

how can I reverse the entire map? Is there a function that can do that?

[laserlight]

Create a second map or use boost::bimap.

[flex567]

Have a look at http://www.cplusplus.com/reference/algorithm/find/

I don't know why my program cannot find the key of the map?

#include <iostream>
#include <map>
#include <algorithm>
using namespace std;

int main() {

    map<int,char> m1;
    m1[3] = 'a';
    m1[4] = 'b';

    map<int,char>::iterator it;

    it = find(m1.begin(),m1.end(),'b');
    cout << (*it).second << '\n';

    return 0;
}

[Philip Nicoletti]

1) find looks for equality of elements in the container. The elements are pairs, not a single char.

2) you want find_if ... it starts to get more complicated using this.

a) using find_if with a lamda (which I doubt you have covered):

    it = find_if(m1.begin(),m1.end(),[] (const std::pair<const int,char> & p) { return p.second == 'b'; });

b) with a function object (which you probably also have not covered)

struct ValueEquals
{
    bool operator () (const std::pair<const int,char> & p) const
    {
        return p.second == value;
    }

    ValueEquals(char c) : value(c) {}

private:
    char value;
};


// use :

    it = find_if(m1.begin(),m1.end(),ValueEquals('b'));

[Philip Nicoletti]

And if you want to use find() it would be something like this:

    std::pair<const int,char> p(4,'b');

    it = find(m1.begin(),m1.end(),p);

But this really does not make sense, since if you know the key, you would just use the std::map's find() member function.

Also: you check that it != m1.end() before dereferencing it.

[2kaud]

For your specific case, you may find this easier to understand

#include <iostream>
#include <map>
#include <algorithm>
using namespace std;

typedef int Mkey;
typedef char Mval;
typedef map<Mkey, Mval> Mic;
typedef Mic::const_iterator ciMic;

bool findval(const Mic& mp, const Mval& fnd, Mkey& key)
{
ciMic cit;

	for (cit = mp.cbegin(); cit != mp.cend() && cit->second != fnd; ++cit);
	return (cit != mp.cend()) ? key = cit->first, true : false;
}

int main() {
Mic m1;
Mkey ky;

	m1[3] = 'a';
	m1[4] = 'b';

	if (findval(m1, 'b', ky))
		cout << "found: " << ky << endl;
	else
		cout << "not found" << endl;

	return 0;
}