How can I iterate through a string and also know the index (current position)?

Question

Often when iterating through a string (or any enumerable object), we are not only interested in the current value, but also the position (index). To accomplish this by using

Answer 1

I would use it-str.begin() In this particular case std::distance and operator- are the same. But if container will change to something without random access, std::distance will increment first argument until it reach second, giving thus linear time and operator- will not compile. Personally I prefer the second behaviour - it's better to be notified when you algorithm from O(n) became O(n^2)...

Answer 2

I've never heard of a best practice for this specific question. However, one best practice in general is to use the simplest solution that solves the problem. In this case the array-style access (or c-style if you want to call it that) is the simplest way to iterate while having the index value available. So I would certainly recommend that way.

Answer 3

A good practice would be based on readability, e.g.:

string str ("Test string");
for (int index = 0, auto it = str.begin(); it < str.end(); ++it)
   cout << index++ << *it;

Or:

string str ("Test string");
for (int index = 0, auto it = str.begin(); it < str.end(); ++it, ++index)
   cout << index << *it;

Or your original:

string str ("Test string");
int index = 0;
for (auto it = str.begin() ; it < str.end(); ++it, ++index)
   cout << index << *it;

Etc. Whatever is easiest and cleanest to you.

It's not clear there is any one best practice as you'll need a counter variable somewhere. The question seems to be whether where you define it and how it is incremented works well for you.

Answer 4

For strings, you can use string.c_str() which will return you a const char*, which can be treated as an array, example:

const char* strdata = str.c_str();

for (int i = 0; i < str.length(); ++i)
    cout << i << strdata[i];

Answer 5

Like this:

    std::string s("Test string");
    std::string::iterator it = s.begin();

    //Use the iterator...
    ++it;
    //...

    std::cout << "index is: " << std::distance(s.begin(), it) << std::endl;

Answer 6

Since std::distance is only constant time for random-access iterators, I would probably prefer explicit iterator arithmetic. Also, since we're writing C++ code here, I do believe a more C++ idiomatic solution is preferable over a C-style approach.

string str{"Test string"};
auto begin = str.begin();

for (auto it = str.begin(), end = str.end(); it != end; ++it)
{
    cout << it - begin << *it;
}