How do I Iterate over a vector of C++ strings?

Question

How do I iterate over this C++ vector?

vector<string> features = {"X1", "X2", "X3", "X4"};

Solution 1

Try this:

for(vector<string>::const_iterator i = features.begin(); i != features.end(); ++i) {
    // process i
    cout << *i << " "; // this will print all the contents of *features*
}

If you are using C++11, then this is legal too:

for(auto i : features) {
    // process i
    cout << i << " "; // this will print all the contents of *features*
} 

Solution 2

C++11, which you are using if this compiles, allows the following:

for (string& feature : features) {
    // do something with `feature`
}

This is the range-based for loop.

If you don’t want to mutate the feature, you can also declare it as string const& (or just string, but that will cause an unnecessary copy).

Comments

[Niklas B.]

Yeah, that one's pretty trivial to find.

[Alok Save]

Perhaps you means ++i and not i++.

[Rontogiannis Aristofanis]

Actually it's the same thing.

[Alok Save]

No, it is not! and you should be using a const_iterator not just a iterator.This is boiler plate code, You should learn it well and well enough to get it right even if asked when asleep.

[mfontanini]

Also, the i variable in the range-based for loop should be a const reference. Otherwise you are creating unnecessary copies of the vector's strings.

[Bo Persson]

@Als - That's old info. Current compilers have no problems inlining either operator++ for std::vector, and optimize away any unused copies.

[Alok Save]

@BoPersson: You never know which compiler one might get to put up with,Besides it never hurts to write more correct and better code, especially in case of boiler plate codes.

[FanaticD]

I believe that in this context, it doesn't really matter if it is ++i or i++ and therefore there is no "correct" or "incorrect" way or even "better" way of coding. Given this context, there is no difference between post and pre incrementation.

[Rontogiannis Aristofanis]

@FanaticD I agree :)