Insert characters between a string - c++

Question

I am faced with a simple yet complex challenge today.
In my program, I wish to insert a - character every three characters of a string. How would this be accomplished? Thank you for your help.
#include <iostream>
int main()
{
std::string s = "thisisateststring";
// Desired output: thi-sis-ate-sts-tri-ng
std::cout << s << std::endl;
return 0;
}

Answer 1

There is no need to "build a new string".
Loop a position iteration, starting at 3, incrementing by 4 with each pass, inserting a - at the position indicated. Stop when the next insertion point would breach the string (which has been growing by one with each pass, thus the need for the 4 slot skip):
#include <iostream>
#include <string>
int main()
{
std::string s = "thisisateststring";
for (std::string::size_type i=3; i<s.size(); i+=4)
s.insert(i, 1, '-');
// Desired output: thi-sis-ate-sts-tri-ng
std::cout << s << std::endl;
return 0;
}
Output
thi-sis-ate-sts-tri-ng

Answer 2

just take an empty string and append "-" at every count divisible by 3
#include <iostream>
int main()
{
std::string s = "thisisateststring";
std::string res="";
int count=0;
for(int i=0;i<s.length();i++){
count++;
res+=s[i];
if(count%3==0){
res+="-";
}
}
std::cout << res << std::endl;
return 0;
}
output
thi-sis-ate-sts-tri-ng

Answer 3

A general (and efficient) approach is to build a new string by iterating character-by-character over the existing one, making any desired changes as you go. In this case, every third character you can insert a hyphen:
std::string result;
result.reserve(s.size() + s.size() / 3);
for (size_t i = 0; i != s.size(); ++i) {
if (i != 0 && i % 3 == 0)
result.push_back('-');
result.push_back(s[i]);
}

Answer 4

Simple. Iterate the string and build a new one
Copy each character from the old string to the new one and every time you've copied 3 characters add an extra '-' to the end of the new string and restart your count of copied characters.

Answer 5

Like 99% problems with text, this one can be solved with a regular expression one-liner:
std::regex_replace(input, std::regex{".{3}"}, "$&-")
However, it brings not one, but two new problems:
it is not a very performant solution
regex library is huge and bloats resulting binary
So think twice.

Answer 6

You could write a simple functor to add the hyphens, like this:
#include <iostream>
struct inserter
{
unsigned n = 0u;
void operator()(char c)
{
std::cout << c;
if (++n%3 == 0) std::cout << '-';
}
};
This can be passed to the standard for_each() algorithm:
#include <algorithm>
int main()
{
const std::string s = "thisisateststring";
std::for_each(s.begin(), s.end(), inserter());
std::cout << std::endl;
}
Exercise: extend this class to work with different intervals, output streams, replacement characters and string types (narrow or wide).