C ++从字符串中选择N个字符

Question

I have a String. Let it be string a = "abcde"; .

And I want to select only a few characters (Let me say from 1 to 3).

In python I would do it like a[1:3] . But C++ doesn't allow me to do that. It only allows for example: a[n] , not a[n:x].

Is there a way to select n characters from string in C++?

Or do I need to do it with erase() ?

Answer 1

You can use substr() :

std::string a = "abcde";
std::string b = a.substr(0, 3);

Notice that indexing starts at 0 .

If you want to shorten the string itself, you can indeed use erase() :

a.erase(3); // removes all characters starting at position 3 (fourth character)
            // until the end of the string

Answer 2

If you want to reassign the object you can write for example

std::string a = "abcde";

a = a.substr( 0, 3 );

However to select the characters there is no need to change the object itself. Most member functions of class std::string accept two parameters: the initial position in a string and the number of characters to process. Also you can use iterators to process selected characters as for example a.begin() , std::next( a.begin(), 3 ) . You can use iterators that specify a range in a string in many standard algorithms.