Insert a character into a string - c++

Question

I need to insert a character into a string at every instance of that character. For example if my string was, "This is a test" and my character was 's' then my output would need to look like this: "Thiss iss a tesst"
any idea why this isn't working? Here's what I have so far. I am not supposed to add any extra preprocessor instructions or anything, just using what's here I need to figure this out.
#include <iostream>
#include <string>
using namespace std;
int main(){
string userString;
char userChar;
cin >> userString;
cin >> userChar;
for (int i = 0; i < userString.size(); i++){
if(userString.at(i) == userChar){
userString.insert(userString.begin() + i, userChar);
}
}
cout << userString;
return 0;
Update:
Here's the solution I worked out.
#include <iostream>
#include <string>
using namespace std;
int main(){
string userString;
char userChar;
cout << "enter a string" << endl;
getline(cin, userString);
cout << "enter a character" << endl;
cin >> userChar;
for (int i = userString.size()-1; i >= 0; i--){
if(userString.at(i) == userChar){
userString.insert(userString.begin() + i, userChar);
}
}
cout << userString;
return 0;
}

Answer 1

I don't know why you want to go through the string backwards. Anyway. Your problem is that once you insert a character at some position, your loop will encounter the inserted character again in the next iteration and insert another. Ad infinitum.
#include <cstddef> // std::size_t, the correct type for indexes and sizes of objects in mem
#include <string>
#include <iostream>
int main()
{
std::cout << "Enter a string: ";
std::string userString; // define variables as close
std::getline(std::cin, userString);
std::cout << "Enter a character: ";
char userChar; // to where they're used as possible
std::cin >> userChar;
for (std::size_t i{}; i < userString.size(); ++i) {
if (userString[i] == userChar) { // no need to use std::string::at() 1)
userString.insert(userString.begin() + i, userChar);
++i; // advance the index to not read the same character again.
}
}
std::cout << userString << '\n';
}
1) since it is allready sure that the index will be in a valid range.

Answer 2

Your first solution probably ends up looping infinitely if you ever find one of the chosen character because you always insert one more copy ahead and keeps finding the same char ever after.
std::basic_string has a find function. It's always better to use code offered by a library than self made code. Here's my proposed solution:
std::string& duplicate_char(std::string& str, char val)
{
using std::string;
auto pos = str.find(val); // finds first index of character val or npos if unsuccessful
while (pos != string::npos)
{
str.insert(pos, 1, val); // insert at pos one character val
pos = str.find(val, pos + 2); // find the next occurence of val starting after the newly inserted character
}
return str;
}
You may use this function like this:
int main()
{
std::string testStr{"Thiss iss a tesst"};
duplicate_char(testStr, 's');
std::cout << testStr << std::endl;
}