Thread: Reversing Strings

  1. #1
    Registered User
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    Feb 2002

    Reversing Strings

    Hello Again,

    Does anyone have a piece of code that can flip a string around or know how an easy way to do this? I am working on a special calculator that has will compute a variation of two's compliment. I am not good at all with string manipulation.

    An example of this would be:

    INPUT: halo
    OUTPUT: olah

    Thank You!

  2. #2
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    Aug 2002
    Hermosa Beach, CA
    Try the reverse standard algorithm:

    # include <string>
    # include <algorithm>
    # include <iostream>
    using namespace std;
    int main()
    	string s = "halo";
    	cout << "String is: " << s << endl;
    	std::reverse<string::pointer>(s.begin(), s.end());
    	cout << "Reverse string is: " << s << endl;
    	return 0;
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  3. #3
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    Apr 2003
    You can do it this way too:
    string str = "halo";
    string reversed;
    for(int i = str.length() - 1; i >= 0; i--)
    	reversed += str[i];
    Last edited by 7stud; 03-22-2005 at 11:15 PM.

  4. #4
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    Mar 2006
    I know this is an old topic, but why create a new topic when this is exactly what I need.

    Ok, can you comment (//) on the piece of coding 7stud wrote^^. Im trying to understand this but how does the computer know to place character X into the Y position.


  5. #5
    Frequently Quite Prolix dwks's Avatar
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    Apr 2005
    You're not supposed to post in threads over 2 weeks old.

    7stud's code just takes the last character of str and adds it to reversed, making it the first character. Then it takes the next-to-last character and adds it to the end of reversed, which makes it the second character. And so on until the beginning of the string.

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  6. #6
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    Nov 2005
    if you are using char[] arrays, you can simply swap the elements

  7. #7
    Registered User major_small's Avatar
    Join Date
    May 2003
    a slightly more efficient method is to also use a reverse-iterator, which starts at the end of the new string and the beginning of the old string.

    In each iteration, the following happens:
    • The first character of the source string is written to the last character of the target string
    • The last character of the source string is written to the first character of the target string
    • The forward iterator is increased by one
    • The reverse iterator is decreased by one

    I can see how this could be confusing to read, so here's a bit of code to show you more:
    #include <iostream>	//for standard console I/O
    #include <cstring>	//for strlen()
    int main()
    	char source[]="dlroW olleH";	//the source string
    	char target[]="dlroW olleH";	//the target string
    	register short int f;		//the forward iterator
    	register short int r;		//the reverse iterator
    	 * this line gets the length of the source string.  In this case, I
    	 * made sure both strings were the same length.  You could, after this
    	 * point, allocate a target string just for the purposes of copying.
    	register const short int LEN=strlen(source);
    	 * This is the "magic loop".  What it does is set sthe forward iterator
    	 * to zero, the reverse iterator to the last index to be copied, and
    	 * loops until they are of equal size (or somehow they pass eachother).
    	 * In each iteration, it increases the forward iterator and decreases
    	 * the reverse iterator.  The iterators work their ways from the ends
    	 * of the string to the center.
    	for(f=0,r=LEN-1; f<r; f++,r--)
    		 * The first line copies the first character in source to be
    		 * copied into the last character in target to be written.
    		 * The second line does the same, except it uses the last char
    		 * in source to be read to write into the first position of 
    		 * target to be written.
    	std::cout<<target<<std::endl;	//output the target string
    	return 0;			//be nice to your creator.
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