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"

try if(it->second != "qa" && it->second != "qb").

Dimitrios Bouzas
– Dimitrios Bouzas

2014-07-29 21:41:51 +00:00
Commented Jul 29, 2014 at 21:41 — Dimitrios Bouzas
– Dimitrios Bouzas, Commented Jul 29, 2014 at 21:41
Possible Duplicate

scohe001
– scohe001

2014-07-29 21:48:50 +00:00
Commented Jul 29, 2014 at 21:48 — scohe001
– scohe001, Commented Jul 29, 2014 at 21:48

shuttle87 · Accepted Answer · 2014-07-29 21:44:14Z

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.

Brad · Accepted Answer · 2014-07-29 21:44:47Z

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 )

You can compare directly on the right hand side with a null-terminated C-string, no need for QA and QB.
If you are going to have a QA and a QB, they should be declared as constant strings.

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