How to use find(itr ,itr ,const tp) algorithm with map in C++

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Hi, I was wondering how to use find(itr,itr,key) with a map. This code generates an error message with me.

C++

#include <iostream>
#include <string>
#include <algorithm>
#include <map>

using namespace std;

int main()
{
    map<string, string> student_roster;
    student_roster.insert(pair<string,string>("John","a"));
    map<string, string>::iterator itr;
    itr = find( student_roster.begin() , student_roster.end() , "John" );//the error
    if (itr == student_roster.end())
        cerr << "John is not found" << endl;
    else
        cout << "Student : " << itr->first << " Grade : " << itr->second << endl;
}

However, is it not legal to use that algorithm ?

Thanks, Sam.

Posted 7-Jul-15 21:28pm

Samuel Shokry

Updated 8-Jul-15 3:51am

v3

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Comments

Suvendu Shekhar Giri 8-Jul-15 4:22am

What is the exact error message?

Samuel Shokry 8-Jul-15 4:32am

That's it , I guess :
e:\programs\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\stl_algo.h|135|error: no match for 'operator==' in '__first.std::_Rb_tree_iterator<_Tp>::operator*<std::pair<const std::basic_string<char="">, std::basic_string<char> > >() == __val'|

Richard MacCutchan 8-Jul-15 5:23am

See https://msdn.microsoft.com/en-us/library/92cwhskb.aspx.

Samuel Shokry 8-Jul-15 9:43am

Oh thanks, but what about this ?
"itr = find( student_roster.begin() , student_roster.end() , "John" );"
I'm just curious about this line, is it not legal in C++ ?

Richard MacCutchan 8-Jul-15 13:58pm

Yes it's legal C++, but it does not conform to the definition of find, as shown in the documentation. You cannot expect to call functions/methods with random parameters that the method has no knowledge of.

Samuel Shokry 8-Jul-15 17:37pm

Umm thanks.

2 solutions

Solution 1

You are not using find correctly. Please see below

C++

itr = student_roster.find( "John" );

Posted 7-Jul-15 22:25pm

_Asif_

Comments

Samuel Shokry 8-Jul-15 4:30am

I know how to use this about what something like that in my code, it works with vectors by the way.

[no name] 8-Jul-15 4:53am

Just replace your line with this. What's the question? All of this is very clear in the documentation: http://www.cplusplus.com/reference/map/map/find/

Samuel Shokry 8-Jul-15 9:42am

Have you read the code ? Do you know why this line of code doesn't work ?
"itr = find( student_roster.begin() , student_roster.end() , "John" );"
Of course, I know the usage of the member function find of the container map, I'm just curious about the line that I have written above in this comment.
is it not legal in C++ ?

Richard MacCutchan 8-Jul-15 10:42am

Go to the link I gave you and read the documentation for the find method.

[no name] 9-Jul-15 8:41am

Stefan_Lang has given the explanation for the error and this is the solution.

Samuel Shokry 9-Jul-15 21:23pm

Oh right, He gave the solution. Thanks

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Stefan_Lang · Accepted Answer · 2015-07-08T21:35:00

Solution 2

You are using std::find[^]. This is a generic algorithm that works for any container, but it has no knowledge of the properties of the elements of the container. The elements of a map are (key,value) pairs, not strings. Therefore, find fails to compare the elements with the string you pass as the search value. It would need an operator to compare a map element type to string, which doesn't exist.

Posted 8-Jul-15 21:35pm

Stefan_Lang

Comments

[no name] 9-Jul-15 4:03am

Not the problem at all. The key and value can both be std::string and there is an equality operator for those. Try it.

Stefan_Lang 9-Jul-15 8:12am

Yes it is the problem. The element type of a map<string,string> is pair<string,string> , and there is no comparison operator for the operands pair<string,string> and string.

I did implement a short test function, and the compiler pointed me to the following internal template function with exactly the error message reported by the OP:

		// TEMPLATE FUNCTION find
template<class _InIt,
	class _Ty> inline
	_InIt _Find(_InIt _First, _InIt _Last, const _Ty& _Val)
	{	// find first matching _Val
	for (; _First != _Last; ++_First)
		if (*_First == _Val) // <-- error message points to this line!
			break;
	return (_First);
	}

[no name] 9-Jul-15 8:40am

Thanks for putting us on track. Yes that is the source of error in the supplied code and the reason - which is why we have std::map::find. My apologies.

Samuel Shokry 9-Jul-15 21:24pm

Thank you so much Stefan, I have got it now.