How to use map

[infotek]

Could some one give me a quick tutorial on using map I have been using CMap but I'd like to use map.

[Andreas Masur]

#include <map>
#include <string>
#include <sstream>
#include <iostream>

using namespace std;

int main()
{
stringstream strsTemp;
map<int, string> Map;

// Fill map
for(int iCnt = 0; iCnt < 10; ++iCnt)
{
strsTemp.str("");
strsTemp << "This is string " << iCnt + 1;

// Add to map
pair<map<int, string>::iterator, bool> pairmapRC = Map.insert(map<int, string>::value_type(iCnt, strsTemp.str()));
if(pairmapRC.second == true)
// String added to map
else
// Could not add string to map
}

// Write string number 5
map<int, string>::iterator iter = Map.find(5);
if(iter != Map.end())
cout << iter->second << endl;
else
// String not found in map

return 0;
}



This example generates 10 different strings and adds them to the map. After that it will print out the fifth string. I didn't test it but it should work...

Ciao, Andreas

"Software is like sex, it's better when it's free." - Linus Torvalds

[Stefan Svensson]

I have another problem concerning maps. I do not know how to iterate through the elements of a map. What I would need is an example of that. Do you know? // Stefan

[kjetil7]

Iterate through elements in map:


#pragma warning (disable : 4786) //disable stl warning

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

using namespace std;
typedef map<int, string, less<int> > myMapType;

int main()
{

myMapType myMap;
myMapType::iterator iter;
char tmp[128];

// insert items
for(int i=0; i<16; i++) {
sprintf(tmp, "string no. %02d", i);
myMap.insert(myMapType::value_type(i, tmp) );
}

//iterate through items
for(iter=myMap.begin(); iter!=myMap.end(); ++iter) {
cout << (*iter).first << " --- "; //first element in map (int)
cout << (*iter).second.data() << endl; //second element in map (std::string)
}

return 0;
}





Regards,

Kjetil Haga

[NMTop40]

you've answered but made it very complicated.

And you can do iter->first and iter->second (instead of (*iter).first etc. )



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[NMTop40]

extern std::map< T1, T2 > t12Map; // declared externally with values
//
std::map< T1, T2 >::iterator it = t12Map.begin();
for (; it != t12Map.end(); ++it )
{
T1 key = it->first;
T2 value = it->second;
}



The method of iteration is the same as for all STL collections.

You can use const_iterator instead of iterator, which will make it->first and it->second both const.

Note that the prefix ++ is slightly more efficient than the suffix ++ because the latter has to make a copy of the iterator.

You can add a value to an iterator eg it + 2 but you can't do it += 2 (well not in Microsoft anyway).


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[Stefan Svensson]

Thanks!

[Chizl]

In my Header file I put this:

public:
int m_iMapCount;

protected:
typedef std::map< long, std::string, std::less< long > > ANYNAME_I_WANT;
ANYNAME_I_WANT m_mapANYNAME_I_WANT;

Then in my CPP I have:
mapANYNAME_I_WANT[m_iMapCount] = szNewVal; //passing in an std::string
m_iMapCount += 1;


--
Chizl
[email protected]
http://www.chizl.com/

[NMTop40]

1. There is no need for you to put in std::less< long >. You only normally use this parameter on a user-defined class which either does not have operator<, or does not have the one you wish to use - eg std::string where you want to compare case-insensitive instead of case-sensitive, therefore you want to use a different compare function (Scott Meyers discusses this case, although often I find I use map with upper-case or lower-case key, where the key is often a member of the second element anyway, i.e. I use it instead of "set" with my own predicate. I do this because I'm sure it is quicker)

2. You seem to be sticking them in in the order in which they are added - so what you are actually creating is simply a vector: why not, instead, use a vector and push_back()



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[Chizl]

Well, I kind of did a cut and paste. I've been using C++ on and off for some time, mostly ATL, but I don't use it every day. Maybe once a month. hehe Here is what I got my code from:


//header.h has:
protected:
typedef std::map< long, std::string, std::less< long > > LONG2STRING;
LONG2STRING m_mapErrorLookup;

struct response_code
{
UINT nResponse; // Response we're looking for
TCHAR* sMessage; // Error message if we don't get it
};

static response_code response_table[];




Then in my CPP I have this:


CInit::response_code CInit::response_table[] =
{
//error codes found at:
//http://sourcesite.geeksatwork.com/cplusplus/files/Winsock_Error_Codes.txt
//if site is down, search google.com for, "Windows Sockets Error Codes, Values, and Meanings"
{ 0, "OK." },
{ 10004, "Interrupted system call." },
{ 10009, "Bad file number." },
{ 10013, "Permission denied."},
{ 10014, "Bad address."},
{ 10022, "Invalid argument."},
{ 10024, "Too many open files."},
{ 10035, "Operation would block."},
{ 10036, "Operation now in progress."},
{ 10037, "Operation already in progress."},
{ 10038, "Socket operation on nonsocket."},
{ 10039, "Destination address required."},
{ 10040, "Message too long."},
{ 10041, "Protocol wrong type for socket."},
{ 10042, "Protocol not available."},
{ 10043, "Protocol not supported."},
{ 10044, "Socket type not supported."},
{ 10045, "Operation not supported on socket."},
{ 10046, "Protocol family not supported."},
{ 10047, "Address family not supported by protocol family."},
{ 10048, "Address already in use."},
{ 10049, "Cannot assign requested address."},
{ 10050, "Network is down."},
{ 10051, "Network is unreachable."},
{ 10052, "Network dropped connection on reset."},
{ 10053, "Software caused connection abort."},
{ 10054, "Connection reset by peer."},
{ 10055, "No buffer space available."},
{ 10056, "Socket is already connected."},
{ 10057, "Socket is not connected."},
{ 10058, "Cannot send after socket shutdown."},
{ 10059, "Too many references: cannot splice."},
{ 10060, "Connection timed out."},
{ 10061, "Connection refused."},
{ 10062, "Too many levels of symbolic links."},
{ 10063, "File name too long."},
{ 10064, "Host is down."},
{ 10065, "No route to host."},
{ 10091, "Returned by WSAStartup(), indicating that the network subsystem is unusable."},
{ 10092, "Returned by WSAStartup(), indicating that the Windows Sockets DLL cannot support this application."},
{ 10093, "Winsock not initialized."},
{ 10101, "Disconnect."},
{ 11001, "Host not found."},
{ 11002, "Nonauthoritative host not found."},
{ 11003, "Nonrecoverable error."},
{ 11004, "Valid name, no data record of requested type."}
};

void CInit::InitErrors()
{
//find mapped error description
for (int i =0; i<sizeof(response_table)/sizeof(response_code); i++) {
m_mapErrorLookup[response_table[i].nResponse] = response_table[i].sMessage;
}

return;
}




Does this make more since to ya.. Not me, just the only way I know how to use maps.

--
Chizl
[email protected]
http://www.chizl.com/

[Chizl]

I tried this, because I use several maps off the same map, and I see this doesn't work.


//Doesn't work..
typedef std::map< long, std::string> STRING_STORAGE;

STRING_STORAGE m_mapHeaders;

m_mapTo::iterator iter;

//Does work..
typedef std::map< long, std::string> STRING_STORAGE;

STRING_STORAGE::iterator iter;




I use for examle:
STRING_STORAGE m_mapHeaders;
STRING_STORAGE m_mapAttachments;

How would you iterate through this, using it this way? I found .begin() and .end() doesn't work either. .clear() does. Weird..

--
Chizl
[email protected]
http://www.chizl.com/

[Chizl]

I'm coping and pasting so I missed the correct copy, but i'm using the same map var.


// doesn't work.
STRING_STORAGE m_mapHeaders;
m_mapHeaders::iterator iter;





--
Chizl
[email protected]
http://www.chizl.com/

[kjetil7]

Hi,

It's an error in your declaration of the iterator. You should declare it like this:


STRING_STORAGE m_mapHeaders;
STRING_STORAGE::iterator iter;




This should work.

Regards,

Kjetil Haga

[Graham]

Surely map's iterator is a model of Bidirectional Iterator, so only has operator++ and operator-- defined, not operator+/operator- ? You can't do it = it + 2 on a std::map<>::iterator.

He who breaks a thing to find out what it is, has left the path of wisdom - Gandalf

[NMTop40]

yeah, probably wasn't thinking. Why would you want to, anyway?


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