How to subtract char out from string in c++?

byMR

May 6, 2022

Hello I want to know how to subract string from string

For example

If string s = "124ab"
I can easily extract integer by using sstream but I don’t know how to extract string

I want to extract ab from s;
I have tons of string and they don’t have any rule.
s can be "3456fdhgab" or "34a678"

>Solution :

You can use std::isdigit to check if a character is a digit. You can use the erase-remove idiom to remove characters that are digits.

Because std::isdigit has an overload it has to be wrapped in a lambda to be used in the algorithm:

#include <string>
#include <iostream>
#include <algorithm>
#include <cctype>

int main() {
    std::string inp{"124ab"};
    inp.erase(std::remove_if(inp.begin(),inp.end(),[](char c){return std::isdigit(c);}),inp.end());
    std::cout << inp;
}

Output:

ab

And because you asked for using a stringstream, here is how you can extract non-digits with a custom operator<<:

#include <string>
#include <iostream>
#include <cctype>
#include <sstream>

struct only_non_digits {
    std::string& data;    
};

std::ostream& operator<<(std::ostream& os, const only_non_digits& x) {
    for (const auto& c : x.data){
        if (std::isdigit(c)) continue;
        os << c;
    }
    return os;
}    

int main() {
    std::string inp{"124ab"};
    std::cout << only_non_digits{inp} << "\n";
    std::stringstream ss;
    ss << only_non_digits{inp};
    std::cout << ss.str() << "\n";
}

Output: