How to insert an integer with leading zeros into a std::string?

Question

In a C++14 program, I am given a string like

std::string  s = "MyFile####.mp4";

and an integer 0 to a few hundred. (It'll never be a thousand or more, but four digits just in case.) I want to replace the "####" with the integer value, with leading zeros as needed to match the number of '#' characters. What is the slick C++11/14 way to modify s or produce a new string like that?

Normally I would use char* strings and snprintf(), strchr() to find the "#", but figure I should get with modern times and use std::string more often, but know only the simplest uses of it.

Answer 1

What is the slick C++11/14 way to modify s or produce a new string like that?

I don't know if it's slick enough but I propose the use of std::transform(), a lambda function and reverse iterators.

Something like

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

int main ()
 {
   std::string str { "MyFile####.mp4" };
   int         num { 742 };

   std::transform(str.rbegin(), str.rend(), str.rbegin(),
                    [&](auto ch) 
                     {
                       if ( '#' == ch )
                        {
                          ch   = "0123456789"[num % 10]; // or '0' + num % 10;
                          num /= 10;
                        }

                       return ch;
                     } // end of lambda function passed in as a parameter
                  ); // end of std::transform() 

   std::cout << str << std::endl;  // print MyFile0742.mp4
 }