C++ String insert iteration - c++

Question

I have the string:
string str = "1234567890";
//Magic code
cout<<str<<endl;
Which i want to output: 12 34 56 78 90
I assume std has some neat feature/function to help solve this. How do I that in the most convenient way?

Answer 1

The std::string::insert with a for loop could help you to insert spaces into an std::string very easely:
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main() {
string str = "1234567890";
for(auto it = str.begin(); it != str.end(); it += min<int>(str.end() - it, 2))
it = (it != str.begin() ? str.insert(it, ' ') + 1 : it);
cout << str << endl;
}
The std::string::insert returns an iterator pointing to the character inserted, therefore it has to be incremented to step over the inserted character.
Because std::string has a random-access iterator it can be incremented or decremented more than by one. The min<int>(str.end() - it, 2) ensures that the next step won't go out of bounds.

Answer 2

A more general approach. Define a function that inserts a given character char_to_insert into a given string s every interval characters, excluding beginning and end of string:
std::string insert_char(const std::string& s, char char_to_insert, size_t interval)
{
// corner cases
if (!interval || !s.length()) return s;
// compute number of characters to insert
auto number_of_chars_to_insert = (s.length()-1)/interval;
// compute total length
auto output_length = s.length() + number_of_chars_to_insert;
// pre-allocate output string with all characters equal to char_to_insert
std::string retval(output_length, char_to_insert);
// cycle input string, keeping track of position in input and output strings
size_t pos = 0, pos_in_input = 0;
for (const auto& c : s)
{
// copy from input to output
retval[pos++] = c;
// advance in output every interval chars
if ((++pos_in_input) % interval == 0)
++pos;
}
return retval;
}
Then:
int main()
{
std::string s = "1234567890";
for (size_t i = 1; i != 5; ++i)
std::cout << insert_char(s, ' ', i) << std::endl;
return 0;
}
The output:
1 2 3 4 5 6 7 8 9 0
12 34 56 78 90
123 456 789 0
1234 5678 90

Answer 3

There is no built-in to do what you want.
Instead, the most convenient solution is probably to iterate through the string and output pairs of digits as you go:
string str = "1234567890";
for (auto it = str.begin(); it != str.end(); ++it){
std::cout << *(it);
if (++it != str.end()){
std::cout << *it << " ";
}
}
std::cout << std::endl;
Or non-iterator version:
string str = "1234567890";
for (idx = 0; idx < str.length(); idx += 2){
std::cout << str.substr(idx, 2) << " ";
}
std::cout << std::endl;
Both of these examples will have a trailing space on the line, but I've left it in to keep the examples simpler.