Need help reversing a string

Dev Shed Forums Sponsor:

Get inside! Sample the range of functionality easily built with JMSL Library for Time Series Data Analysis, Heat Maps, Portfolio Optimization, Monte Carlo Simulation, Stock Price Charting and more. Download Now!

February 3rd, 2003, 02:22 PM

o0 IceFreeze 0o

Junior Member

Join Date: Jan 2003

Posts: 4

Time spent in forums: < 1 sec
Reputation Power: 0

Need help reversing a string

My problem is that when I run this program my output is not right, it should reverse the string, but instead it prints out random characters. Any help would be appreciated

PHP Code:


		
			
#include <iostream.h>



char string[11];



void reverse()

{

    int character = 9;

    int character_two = 0;

    char string_two[11];

    while(character != -1)

    {        

        string[character] = string_two[character_two];

        character--;

        character_two++;

    }

    cout << string << endl;

}

     

main()

{

    char string[11];

    cout << "Enter a string of characters to reverse, no longer than 10 characters" << endl;

    cin.get(string, 11);

    cin.ignore(50, '\n');

    reverse();

    return 0;

}

February 3rd, 2003, 02:35 PM

Analyser

*bounce*

Join Date: Jan 2002

Location: Delft, The Netherlands

Posts: 510

Time spent in forums: 2 Days 20 h 37 m 7 sec
Reputation Power: 9

Ok, since you allocated 11 bytes and print out a message asking for no more than 10 chars, you know that strings in C are NUL-terminated. So far, so good

But in reverse(), you assume that the user will enter exactly 10 chars. If I enter "hey", you'll be copying way past the end of the string, since you start at 9.

What you need to do, is look for the terminating NUL-byte, and then work your way back. Below is some code I slapped together for the occasion; don't look at it if you want to work it out yourself first.

Code:

void reverse (char *s)
{
        int i, j;
        char c;

        for (i = 0; s[i]; i++)
                ;       /* do nothing */

        i--; /* step back from the NUL-byte */

        for (j = 0; j < i; i--, j++) {
                c = s[j];
                s[j] = s[i];
                s[i] = c;
        }
}

I hope this makes sense to you.

__________________
"A poor programmer is he who blames his tools."
http://analyser.oli.tudelft.nl/

February 3rd, 2003, 04:42 PM

vpopper

Contributing User

Join Date: Jun 2000

Location: Southern California

Posts: 73

Time spent in forums: 2 m 24 sec
Reputation Power: 9

Quote:

What you need to do, is look for the terminating NUL-byte, and then work your way back

Here's another example:

Code:

void reverse(char* s)
{
    char temp, *end;

    // point to last non-null char
    end = s + strlen(s) - 1;

    while (s < end) {
        temp   = *s;
        *s++   = *end;
        *end-- = temp;
    }
}

OR, in C++:

Code:

#include <string>
using namespace std;

int main(int argc, char** argv)
{
    string s = "raboof";
    string::reverse_iterator ri;
    string s2;

    for (ri = s.rbegin(); ri != s.rend(); ri++)
        s2 += *(ri.base()-1);

    cout << s2 << endl;
    return 0;
}

February 3rd, 2003, 05:10 PM

o0 IceFreeze 0o

Junior Member

Join Date: Jan 2003

Posts: 4

Time spent in forums: < 1 sec
Reputation Power: 0

thanks to both of you

February 4th, 2003, 08:32 AM

3dfxMM

Contributing User

Join Date: Aug 2002

Posts: 266

Time spent in forums: 3 Days 19 h 1 m 34 sec
Reputation Power: 12

Another issue is that you input the string into one instance of "string" and then you reverse another instance that is uninitialized.

February 4th, 2003, 10:45 PM

TechNoFear

Offensive Member

Join Date: Oct 2002

Location: in the perfect world

Posts: 594

Time spent in forums: 2 Days 14 h 6 m 15 sec
Reputation Power: 21

Unless we are doing homework there is

_strrev()

from string.h

__________________
The essence of Christianity is told us in the Garden of Eden history. The fruit that was forbidden was on the Tree of Knowledge. The subtext is, All the suffering you have is because you wanted to find out what was going on. You could be in the Garden of Eden if you had just kept your f***ing mouth shut and hadn't asked any questions.

Frank Zappa

February 5th, 2003, 12:55 AM

wild_pointer

Junior Member

Join Date: Feb 2003

Posts: 3

Time spent in forums: < 1 sec
Reputation Power: 0

If you're not doing homework, you might as well save yourself a headache and use std::string and std::reverse (found in <algorithm>).

February 5th, 2003, 12:38 PM

o0 IceFreeze 0o

Junior Member

Join Date: Jan 2003

Posts: 4

Time spent in forums: < 1 sec
Reputation Power: 0

hm...looking back over my problem I fixed this quite easially...
I changed:

PHP Code:


		
			
 string[character] = string_two[character_two];

into:

PHP Code:


		
			
 string_two[character_two] = string[character];

February 5th, 2003, 12:40 PM

o0 IceFreeze 0o

Junior Member

Join Date: Jan 2003

Posts: 4

Time spent in forums: < 1 sec
Reputation Power: 0

I also needed to change the while loop, and added

PHP Code:


		
			
|| string[character] != '\0')

edit: should be != '\0' , but it didn't show up.

Thread Tools	Search this Thread
Email this Page	Search this Thread: Advanced Search
Display Modes	Rate This Thread
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode	Rate This Thread: