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

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