Insert characters between a string

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;
}

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@edtheprogrammerguy Before creating this post I referred to [this post](https://stackoverflow.com/questions/3223302/c-insert-char-to-a-string), but I am unsure of how to implement this into my code. — Koi, Apr 10 '18 at 16:33

score 2 · Answer 1 · answered Apr 10 '18 at 16:45

2

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

answered Apr 10 '18 at 16:45

WhozCraig

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`insert()` scales pretty badly - you'll end up copying _O(n²)_ characters, for a _n_-character string. – Toby Speight Apr 10 '18 at 16:48
@Toby Speight For most common string sizes that's probably irrelevant. Much more important is the fact that it's a simple and readable solution. – Jesper Juhl Apr 10 '18 at 16:58

score 1 · Answer 2 · answered Apr 10 '18 at 16:36

1

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]);
}

answered Apr 10 '18 at 16:36

Cameron

96,106
25
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as index starts from 0 so first append will happen at 4 the place – Roushan Apr 10 '18 at 16:43
@Roushan45: Nope. The hyphen is inserted just before the 4th character, not after. This is done so that if the string length is a multiple of three a trailing hyphen won't be inserted. – Cameron Apr 10 '18 at 16:51
mean to say when tha value of i wil be 3 then it will go inside the if but its already at index 4 of string – Roushan Apr 10 '18 at 16:53
When `i` is 3, the `if` will indeed be entered, but the 4th character (at `s[i]`) *has not been inserted yet*. – Cameron Apr 10 '18 at 16:58
got it, i debug it how actually flow works, and works :), sorry for any trouble – Roushan Apr 10 '18 at 17:01
@Roushan45: No worries :) – Cameron Apr 10 '18 at 21:17

score 1 · Accepted Answer · answered Apr 10 '18 at 16:41

1

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

answered Apr 10 '18 at 16:41

Roushan

4,074
3
21
38

This code does work the way I wish it to! Thank you very much. – Koi Apr 10 '18 at 16:46

score 0 · Answer 4 · answered Apr 10 '18 at 16:35

0

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.

answered Apr 10 '18 at 16:35

Jesper Juhl

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70

score 0 · Answer 5 · answered Apr 10 '18 at 16:43

0

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.

answered Apr 10 '18 at 16:43

flapenguin

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3
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Although I actually think [regular expressions](https://blog.codinghorror.com/regular-expressions-now-you-have-two-problems/) are fabulous, I find this Jamie Zawinski quip funny: Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems. – Eljay Apr 10 '18 at 17:40

score 0 · Answer 6 · answered Apr 10 '18 at 16:58

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).

Insert characters between a string

6 Answers6