Replace a string to another string using C++

Question

The problem is I don't know the length of the input string. My function can only replace if the input string is "yyyy". I think of the solution is that first, we will try to convert the input string back to "yyyy" and using my function to complete the work.

Here's my function:

void findAndReplaceAll(std::string & data, std::string toSearch, std::string replaceStr)
{
    // Get the first occurrence
    size_t pos = data.find(toSearch);

    // Repeat till end is reached
    while( pos != std::string::npos)
    {
        // Replace this occurrence of Sub String
        data.replace(pos, toSearch.size(), replaceStr);
        // Get the next occurrence from the current position
        pos = data.find(toSearch, pos + replaceStr.size());
    }
}

My main function

std::string format = "yyyyyyyyyydddd";
findAndReplaceAll(format, "yyyy", "%Y");
findAndReplaceAll(format, "dd", "%d");

My expected output should be :

%Y%d

Does this answer your question? Replace part of a string with another string — Azeem
– Azeem, Commented Nov 4, 2019 at 5:10
I don't think so, because my string has only 1 type of character but does not know its exact length. — Tam Nguyen
– Tam Nguyen, Commented Nov 4, 2019 at 6:41
The answers in that thread are handling variable sizes as well. Maybe, there's more to your use-case. You need to update your question with a complete working example and the inputs and outputs for your string replacement scenarios. — Azeem
– Azeem, Commented Nov 4, 2019 at 6:50
Sorry for my bad, I have just edited the post. I hope you can help me out. — Tam Nguyen
– Tam Nguyen, Commented Nov 4, 2019 at 7:00
Check this live run: ideone.com/ngZ8Jo. Isn't this correct? The two groups of toSearch string have been replaced with the replaceStr. — Azeem
– Azeem, Commented Nov 4, 2019 at 7:10

J. García · Accepted Answer · 2019-11-04 15:21:28Z

5

Use regular expressions.

Example:

#include <iostream>
#include <string>
#include <regex>
int main(){
    std::string text = "yyyyyy";
    std::string sentence = "This is a yyyyyyyyyyyy.";
    std::cout << "Text: " << text << std::endl;
    std::cout << "Sentence: " << sentence << std::endl;

    // Regex
    std::regex y_re("y+"); // this is the regex that matches y yyy or more yyyy

    // replacing
    std::string r1 = std::regex_replace(text, y_re, "%y"); // using lowercase
    std::string r2 = std::regex_replace(sentence, y_re, "%Y"); // using upercase 

    // showing result
    std::cout << "Text replace: " <<   r1 << std::endl;
    std::cout <<  "Sentence replace: " << r2 << std::endl;
    return 0;
}

Output:

Text: yyyyyy
Sentence: This is a yyyyyyyyyyyy.
Text replace: %y
Sentence replace: This is a %Y.

If you want to make it even better you can use:

// Regex
std::regex y_re("[yY]+");

That will match any mix of lowercase and upper case for any amount of 'Y's . Example output with that Regex:

Sentence: This is a yYyyyYYYYyyy.
Sentence replace: This is a %Y.

This is just a simple example of what you can do with regex, I'd recommend to look at the topic on itself, there is plenty of info her in SO and other sites.

Extra: If you want to match before replacing to alternate the replacing you can do something like:

 // Regex
    std::string text = "yyaaaa";
    std::cout << "Text: " << text << std::endl;
    std::regex y_re("y+"); // this is the regex that matches y yyy or more yyyy


    std::string output = "";
    std::smatch ymatches;
    if (std::regex_search(text, ymatches, y_re)) {
        if (ymatches[0].length() == 2 ) {
            output = std::regex_replace(text, y_re, "%y");
        } else {
            output = std::regex_replace(text, y_re, "%Y");
        }
    }

edited Nov 4, 2019 at 15:21

answered Nov 4, 2019 at 7:24

J. García

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11 Comments

Tam Nguyen Over a year ago

Thanks, your answer works perfectly. I have another question, what about except yy because when it comes to yy I will replace it with %y not %Y. Thanks in advance

J. García Over a year ago

Then you would have to do a test to see if your input string matches to yy first. You can do that with regex_match()

J. García Over a year ago

I will add another snippet to the answer. It didn't work here.

Tam Nguyen Over a year ago

There is a problem, my input string should be like this yyaa so I guess that your yy_re can't do the job. I think just a little fix with the regex. I mean exactly two y character, but the input string will contain some more characters not just only char y.

J. García Over a year ago

No problem. So I edited the snippet with what I came up with only match two yy use %y and if anything else use %Y. Basically, the easiest way I could come up with is just to check the size of the match and do accordingly.

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