C++: recursively replace all instances of a letter in a string with another letter

Question

I'm just going through some textbook c++ questions and one of them is to write a function which recursively replaces all instances of a certain letter in a string with another letter. I know that there are pre-existing functions for this, but because this chapter focuses on recursion, this question insists that the solution must be recursive. So I wrote all this out in c++ and it was fine but then I read the footnote to the question and what it says is: "for the manipulation of string objects, only the methods at and length (i.e. size) are allowed as well as the operator +". whaaa? I just don't see how you could make this happen without str.substr(pos,len) but I would be thrilled if somebody could find a way. Thanks a bunch to that special someone yo.

Here's the best code my hamster brain could come up with (also a small iterative alternative commented out at the beginning).

#include <iostream>
#include <string>
using namespace std;

// iterative solution
/* string replace (string in, char from, char to) {
  string res;
  for (int i{0}; i < in.length(); i++) {
    if (in.at(i) == from)
      res += to;
    else
      res += in.at(i);
    }
  return res;
} */

// recursive solution
string replace (string in, char from, char to) {
  if (in.empty())
    return "";
  char first{in.at(0)};
  if (first == from)
    return to + replace (in.substr(1), from, to);
  else
    return in.at(0) + replace (in.substr(1), from, to);
}

int main () {
  string in;
  char from, to;
  cout << "Word: ";
  cin >> in;
  cout << "from: ";
  cin >> from;
  cout << "to: ";
  cin >> to;
  cout << in << " --> " << replace (in, from, to) << '\n';
  return 0;
}

Answer 1

Just provide a default argument that keeps track of the index:

string replace(string in, char from, char to, int i = 0) 
{
  if (i == in.length()) 
    return in;
  if (in.at(i) == from) 
    in.at(i) = to;
  return replace(in, from, to, i + 1);
}

Here's a demo.

This only uses at(), and length(), and doesn't even use +.

Also, avoid using namespace std;, it's bad practice.