#include <iostream>
#include <algorithm>
 
 
void insert_blank(char* line, std::size_t line_size, std::size_t pos)
{
    std::size_t index = 0;
    while (line[index++])
        ;
 
    if (index + 1 >= line_size || pos >= index) // sanity check
        return;
 
    while (index != pos)
    {
        line[index] = line[index - 1];
        --index;
    }
 
    line[index] = ' ';
}
 
std::istream& get(std::istream& is, int& val)
{
    // get a number and remove the trailing newline.
    return (is >> val).ignore(100, '\n');
}
 
int main()
{
    const char* prompt = "Enter a string (or just hit <ENTER> to quit)\n> ";
 
    const std::size_t buffer_size = 1024;
    char line_buffer[buffer_size];
 
    while (std::cout << prompt && std::cin.getline(line_buffer, buffer_size-1) && line_buffer[0])
    {
        const char* prompt = "Enter the position to insert blank (or negative to quit)\n> ";
 
        int position;
        while (std::cout << prompt && get(std::cin, position) && position >= 0)
        {
            insert_blank(line_buffer, buffer_size, position);
            std::cout << "modified: \"" << line_buffer << "\"\n";
        }
 
    }
}

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abc
2
1
0
6
-1