STL map search

[Thomas Fazekas]

Hi forum,

If you follow the output of the current code snipet
(is supposed to be compilable with VC6 or gcc)
you'll see that the search stops after the second key...
So what is wrong with my code ?

#ifdef _MSC_VER
#include <functional>
#else
#include <bits/stl_function.h>
#endif

#include <iostream>
#include <map>

class CCharPComp : public std::binary_function < const char *, const char *, bool > {
public :
bool operator()( const char *arg1, const char * arg2) const {
std::cout << "First str : " << arg1 << std::endl
<< "Second str : " << arg2 << std::endl
<< "Result : " << strcmp( arg1, arg2 ) << std::endl;
return strcmp( arg1, arg2 );
}
};

std::map < const char *, const char *, CCharPComp > trMap;
std::map < const char *, const char *, CCharPComp >::const_iterator mapIter;

void justPrint(const char *actKey){

mapIter = trMap.find(actKey);

if ( mapIter == trMap.end() )
std::cout << "Not Found" << std::endl;
else
std::cout << "Found : " << mapIter->second << std::endl;
}

int main(void) {
const char * strs[8] = { "firstKey", "firstVal", "secondKey", "secondVal", "thirdKey", "thirdVal", "fourthKey", "fourthVal"};

trMap.insert(std::pair<const char *, const char*>(strs[0], strs[1]));
trMap.insert(std::pair<const char *, const char*>(strs[2], strs[3]));
trMap.insert(std::pair<const char *, const char*>(strs[4], strs[5]));
trMap.insert(std::pair<const char *, const char*>(strs[6], strs[7]));

std::cout << std::endl << "Start searching" << std::endl;

//justPrint("secondKey");

justPrint("fourthKey");
}

[Stefan Näwe]

Thomas Fazekas wrote:

Hi forum,

If you follow the output of the current code snipet
(is supposed to be compilable with VC6 or gcc)
you'll see that the search stops after the second key...
So what is wrong with my code ?

#ifdef _MSC_VER
#include <functional>
#else
#include <bits/stl_function.h>
#endif
Why this?
#include <functional> should do it.

#include <iostream>
#include <map>

class CCharPComp : public std::binary_function < const char *, const char *, bool > {
public :
bool operator()( const char *arg1, const char * arg2) const {
std::cout << "First str : " << arg1 << std::endl
<< "Second str : " << arg2 << std::endl
<< "Result : " << strcmp( arg1, arg2 ) << std::endl;
return strcmp( arg1, arg2 );
return strcmp( arg1, arg2 ) < 0;
[SNIP]

Worked for me.

Stefan

[Karl Heinz Buchegger]

Thomas Fazekas wrote:

class CCharPComp : public std::binary_function < const char *, const char *, bool > {
public :
bool operator()( const char *arg1, const char * arg2) const {
std::cout << "First str : " << arg1 << std::endl
<< "Second str : " << arg2 << std::endl
<< "Result : " << strcmp( arg1, arg2 ) << std::endl;
return strcmp( arg1, arg2 );
}
};
What you need to implement is a binary predicate, that tells if one argument
*is less* then the other. That means:

return value meaning
------------------------------------------
true arg1 *is less then* arg2
false arg1 *is not less then* arg2

now look at the return values of strcmp

return value meaning
------------------------------------------- 0 arg1 *is greater then* arg2

== 0 arg1 *is equal to* arg2
< 0 arg1 *is less then* arg2

Now combine that with the usual C++ convention that everything
not 0 is regarded as true and you get:
your operator does not implement the required *is less then* functionality,
but implements *is equal to*.

a simple

return strcmp( arg1, arg2 ) < 0;

cures this one.

--
Karl Heinz Buchegger
kb******@gascad.at

[Thomas]

Thnaks a lot, it works just fine now :)

Thomas Fazekas wrote:

Hi forum,

If you follow the output of the current code snipet
(is supposed to be compilable with VC6 or gcc)
you'll see that the search stops after the second key...
So what is wrong with my code ?

#ifdef _MSC_VER
#include <functional>
#else
#include <bits/stl_function.h>
#endif

#include <iostream>
#include <map>

class CCharPComp : public std::binary_function < const char *, const char
*, bool > {
public :
bool operator()( const char *arg1, const char * arg2)
const {
std::cout << "First str : " << arg1 << std::endl
<< "Second str : " << arg2 <<
std::endl << "Result : " <<
strcmp( arg1, arg2 ) << std::endl;
return strcmp( arg1, arg2 );
}
};

std::map < const char *, const char *, CCharPComp > trMap;
std::map < const char *, const char *, CCharPComp >::const_iterator
mapIter;

void justPrint(const char *actKey){

mapIter = trMap.find(actKey);

if ( mapIter == trMap.end() )
std::cout << "Not Found" << std::endl;
else
std::cout << "Found : " << mapIter->second << std::endl;
}

int main(void) {
const char * strs[8] = { "firstKey", "firstVal", "secondKey",
"secondVal", "thirdKey", "thirdVal", "fourthKey", "fourthVal"};

trMap.insert(std::pair<const char *, const char*>(strs[0],
strs[1])); trMap.insert(std::pair<const char *, const
char*>(strs[2], strs[3])); trMap.insert(std::pair<const char *,
const char*>(strs[4], strs[5])); trMap.insert(std::pair<const char
*, const char*>(strs[6], strs[7]));

std::cout << std::endl << "Start searching" << std::endl;

//justPrint("secondKey");

justPrint("fourthKey");
}