Iterating over a std map and comparing text

Question

I am currently something something like this

std::map<std::string,std::string>::iterator it ;
for(it=mmap.begin();it!=mmap.end();it++)
{
    if(it->second.c_str() != "qa" && it->second.c_str() != "qb")
    {
        //Entered
    }
}

Now the problem with this code is that it goes into the entered section even when the iterator is

it("Testing this","qb")

Thee above means that it->second = "qb"

Now my question is why is the code going ino the if statement if it->second = "qb" My thought is that it should not have because the conditional statment part is it->second.c_str() != "qb"

[Possible Duplicate](http://stackoverflow.com/questions/4843640/why-is-a-a-in-c/) — scohe001, Jul 29 '14 at 21:48

score 3 · Accepted Answer · answered Jul 29 '14 at 21:44

The problem is because it->second.c_str() != "qa" is not the correct way to compare C strings (it compares pointers and not the string contents). You however do not need to convert to a C string first before comparing as you can compare the strings directly with: it->second != "qa". If you for some reason need to use the c string with c_str() then you will need to use strcmp.

score 3 · Answer 2 · answered Jul 29 '14 at 21:44

3

c_str() is a character array (pointer), so you are comparing two pointers which are different.

Use string objects instead:

std::string QA("qa");
std::string QB("qb");
std::map<std::string,std::string>::iterator it ;
for(it=mmap.begin();it!=mmap.end();it++)
{
    if(it->second != QA && it->second  != QB)
    {
        //Entered
    }
}

or actually do C style string compare:

    if ( strcmp(it->second.c_str(), "qa") != 0 && strcmp(it->second.c_str(), "qb") != 0 )

answered Jul 29 '14 at 21:44

Brad

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2

You can compare directly on the right hand side with a null-terminated `C`-string, no need for `QA` and `QB`. – vsoftco Jul 29 '14 at 21:58
If you are going to have a `QA` and a `QB`, they should be declared as constant strings. – Thomas Matthews Jul 29 '14 at 22:10

Iterating over a std map and comparing text

2 Answers2