how to replace a substring in a string

This is a discussion on how to replace a substring in a string within the C++ Programming forums, part of the General Programming Boards category; How can I replace a substring within a string? For example, if a line consists of different words separated from ...

  1. #1
    Registered User
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    Question how to replace a substring in a string

    How can I replace a substring within a string? For example, if a line consists of different words separated from each other by comma, such as in:
    file,tcp,ftp,SF,5,6,3,normal.
    How can I locate the second word (identified by 1st and 2nd commas) and replace that word with a number of my choice?
    Dmitry Kashlev

  2. #2
    Registered User
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    The easiest way would be to remove the part of the string just after what you want to delete and then make the change. After you make the change, concatenate the part you removed earlier.
    Code:
    #include <iostream.h>
    #include <string.h>
    
    int main ( )
    {
        char *newVal = "12345";
        char string[1024] = "file,tcp,ftp,SF,5,6,3,normal";
        char holder[256];
        char *first_cut, *cut_end;
    
        // Hack it up
        first_cut = strchr (string, ',');
        cut_end = strchr (first_cut, ',');
        strcpy (holder, cut_end);
        strcpy (++first_cut, newVal);
        strcat (first_cut, holder);
    
        cout<<string<<endl;
    
        return 0;
    }

  3. #3
    geek SilentStrike's Avatar
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    Or using std::string, which is a bit clearer (for me, at least)

    Code:
    #include <iostream>
    #include <string>
    
    // good std::string documentation here http://www.cppreference.com/cppstring.html
    
    int main() {
    	typedef std::string::size_type size_type;
    	std::string test("file,tcp,ftp,SF,5,6,3,normal");
    	std::string replacement("udp");
    
    	// find returns position of comma, one more is passed that
    	size_type firstIndex = test.find(',') + 1;
    	size_type secondIndex = test.find(',', firstIndex);
    
    	// see if we found two commas
    	if (firstIndex == std::string::npos || secondIndex == std::string::npos) {
    		std::cout << test << " not valid format" << std::endl;
    		return 0;
    	}
    
    	// second parameter is length, not ending spot
    	test.replace(firstIndex, secondIndex - firstIndex, replacement);
    	
    	std::cout << test << std::endl;
    	
    	return 0;
    }
    Prove you can code in C++ or C# at TopCoder, referrer rrenaud
    Read my livejournal

  4. #4
    Code Goddess Prelude's Avatar
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    Code:
    first_cut = strchr (string, ',');
    cut_end = strchr (first_cut, ',');
    strcpy (holder, cut_end);
    strcpy (++first_cut, newVal);
    strcat (first_cut, holder);
    Be sure to check that both calls to strchr succeeded or you will be dealing with NULL pointers. In such a situation strcpy will choke.

    -Prelude
    My best code is written with the delete key.

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