I can't double characters inside string with function

Question

I am trying to create a simple function that double the characters inside of a string and outputs the new string. Ex. "hello world" would become "hheelloo wwoorrl

Answer 1

Altough the solution by songyuanyao is nice, I think more C++ functions can be used...

#include <string>
#include <string_view>

// use string view, so a character array is also accepted
std::string DoubleChar(std::string_view const &str) noexcept {
    std::string newString;
    newString.reserve(str.size() * 2);
    for (auto character : str) { // use range based loop
        newString.append(2, character); // append the character twice
    }
    return newString;
}

#include <iostream>

int main() {
    std::string const str = "Hello world";

    std::cout << DoubleChar(str) << '\n';
}

Answer 2

The problem is, newString is always empty, it doesn't contain any elements. Access on it like newString[i] and newString[i+1] leads to UB.

You need to add elements to it, e.g.

string doubleChar(const string& str) { // apply pass-by-reference to avoid copy
    string newString;
    newString.reserve(str.size() * 2); // reserve the capacity to avoid reallocation
    for(int i =0;i<str.size();++i){
        newString += str[i];
        newString += str[i];
        // or use push_back or append
    }
    return newString;
}

Answer 3

using namespace std;
string doubleChar(string str) {
    string newString(str.size() * 2, '\0');
    for(int i =0;i<str.size();++i){
        newString[2*i] = str[i];
        newString[2*i+1] = str[i];
    }
    return newString;
}

The length of result should be set twice of the input. And index should be multiplied by 2.