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Looping items in 'std::map' objects?

Started by TFS_Waldo 11 replies 1.1k views
Original Post
TFS_Waldo
TFS_Waldo
Well, I've been working on a simple project of mine, that I started yesterday. I have a class that uses 'std::map' to store some items, but I can't find out how to loop through the map. The map is 'std::map<std::string, std::string> m_mapItems;'. It's that simple. I thought that the standards were to use iterators, but I can't figure out how. I've done it with an 'std::vector' object. I just can't figure out how to iterate through a 'std::map'. Can someone help me out, please? =) I tried something like this: 

std::map<std::string,std::string>::iterator it;

for (it = m_mapItems.begin(); it != m_mapItems.end(); it++)
{
    // Do stuff here
    // ...
}
But it gives me a load of errors. I can't remember what right now, because I'm on a diff. machine. Thanks in advance, Matt U.
:==- Why don't the voices just leave me alone?! -==:
RDragon1
RDragon1
Looks fine. But you might consider std::for_each
paic
paic
The code should work fine.
When you can compile, post your errors, it will help a lot to find the source of the problem.

Just a quick note, though : the std::map requires its key to have a less operator implemented. Such an operator is not implemented by the default std::string, and as such you can't use std::map<std::string, [put_another_type_here]> as is. You have to first create the less operator for std::string.

Check if you still have the errors with std::map<int, int> or std::map<int, std::string>. If it compile, then you simply have to create a global less operator which takes 2 std::string's as parameters.
TFS_Waldo
TFS_Waldo
Well, it's in VS .NET 2003 Pro. And I get A LOT of compiler errors. They are complaining about me using '!=' and things like that.
:==- Why don't the voices just leave me alone?! -==:
NotAYakk
NotAYakk
Because you have no details, all I can do is guess what it is your problem is.

Here is my guess.

When you evalutate 
*it
, you get a 
std::pair<key, value>&
.

In this case, a 
std::pair<std::string,std::string>&
. (in some STLs, you get a 
std::pair<const std::string,std::string>&
.)

To access the key for your element, call 
it->first
. To access the value, call 
it->second
.

In general, to solve a problem of this nature, write a minimal chunk of code in a minimal source file with minimal #includes that has your problem in it. Often this will let you figure out what is going wrong. If that fails, you now have a very concise file that demonstrates your problem to other people. =)
paic
paic
The != operator ? I thought std::map used < operator :/
Well, anyway. The std::map organise its data by sorting by its key (the first value of a pair) So if you use a key type that doesn't have any comparison operator, the std::map can't sort, and there is a lot of errors.

As I said, your problem probably comes from the fact you're using the std::string which doesn't define <, !=, etc. operators.
TFS_Waldo
TFS_Waldo
Oh yeah, I almost forgot. It points some of the errors to "/vc7/include/xtree". I don't know for sure what. I'll go compile again, and I'll be back.
:==- Why don't the voices just leave me alone?! -==:
Enigma
Enigma
Quote:
Original post by paic
As I said, your problem probably comes from the fact you're using the std::string which doesn't define <, !=, etc. operators.


I don't know what messed up standard library implementation you're using. The SC++L std::string class implements all of the standard relational operators.

Σnigma
bkt
bkt
Just remember with some implementations of std::map if you do the following:
if( keyMap["TheKey"] )

It is going to add the key if it is not already there. So the best way to search the map is the following:
keyMap::iterator it = keyMap.find( ... );if( it != keyMap.end() )

I've ran into that problem a few times. You might want to invest in setting up STLport so that you can have a uniform version of STL running depending on your compiler (or if you're sticking with one, that really shouldn't matter). Good luck!
-John "bKT" Bellone [homepage] [[email=j.bellone@flipsidesoftware.com]email[/email]]
Enigma
Enigma
Quote:
Original post by bkt
Just remember with some implementations of std::map if you do the following:
if( keyMap["TheKey"] )

It is going to add the key if it is not already there.


Again, there is no "some" about it. Under any standards conforming map implementation the expression map[key] will insert a default-constructed element into the map with the key key if no such key already exists.

Σnigma
Agony
Agony
I'd go with NotAYakk's suggestion as the most likely problem. That issue confused me the first time I used std::map, and I've seen it throw other people off before too. Very easy to do.
"We should have a great fewer disputes in the world if words were taken for what they are, the signs of our ideas nly, and not for things themselves." - John Locke o
Bregma
Bregma
Do ignore most of the above posts. std::string does have comparison operators, addressing elements of maps is not relevant to the OP's question, and all conforming C++ standard library implementations are conforming (this includes Microsoft's since about 6 years after the standard was published in November 1997, and GCC since version 3.0), so don;t blame your vendor.

It really really looks like you're missing a header file, that's all.

--smw
Stephen M. Webb
Professional Free Software Developer

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