Reversing strings

[texasbeef]

I am trying to reverse a string using pointers and no library functions. I see a couple of videos, but they seem to favor C, not C++. My code fix has to be something simple, anyone have a direction for me to go? I don't want my code fixed and re-posted please.

//Try to reverse a string without library functions

#include <iostream>
#include <cstring>


char* revStr(char* ptrToString)
    {
        if (ptrToString == NULL)
            return NULL;

        char* ptrToBegin = ptrToString;

        int length = 0;

        for (;*ptrToBegin != '\0'; ptrToBegin++)
                length +=1;

        char* ptrToEnd = ptrToBegin + (length-1);

        while (ptrToBegin < ptrToEnd)
            {
                char temp = *ptrToBegin;
                *ptrToBegin = *ptrToEnd;
                *ptrToEnd = temp;
                ptrToBegin++;
                ptrToEnd--;
            }

        return ptrToString;
    }

int main()
    {
        char myStr[20] = "eduD olleH";
        std::cout << myStr << std::endl;
        std::cout << revStr(myStr);
        return 0;
    }

Output:

eduD olleH
eduD olleH

[Peter87]

In the loop on line 16-17 you're incrementing ptrToBegin so that it no longer points to the beginning of the string.

[texasbeef]

Thanks, I reacquainted ptrToBegin and ptrToString and it works fine. I feel like there is a better way to work that loop to get length without messing up where ptrToBegin points, but it works fine now, Again, thanks.

Dale

//Try to reverse a string without library functions

#include <iostream>
#include <cstring>


char* revStr(char* ptrToString)
    {
        if (ptrToString == NULL)
            return NULL;

        char* ptrToBegin = ptrToString;

        int length = 0;

        for (;*ptrToBegin != '\0'; ptrToBegin++)
                length +=1;
                
        ptrToBegin = ptrToString;

        char* ptrToEnd = ptrToBegin + (length-1);

        while (ptrToBegin < ptrToEnd)
            {
                char temp = *ptrToBegin;
                *ptrToBegin = *ptrToEnd;
                *ptrToEnd = temp;
                ptrToBegin++;
                ptrToEnd--;
            }

        return ptrToString;
    }

int main()
    {
        char myStr[20] = "eduD olleH";
        std::cout << myStr << std::endl;
        std::cout << revStr(myStr);
        return 0;
    }

Output:

eduD olleH
Hello Dude

[Chervil]

I feel like there is a better way to work that loop to get length without messing up where ptrToBegin points

Step through the string checking for '\0' using ptrToEnd instead. Then the variable length isn't needed at all.

[texasbeef]

Step through the string checking for '\0' using ptrToEnd instead. Then the variable length isn't needed at all.

Yup, that works like a beast, thanks Chervil!

//Try to reverse a string without library functions

#include <iostream>
#include <cstring>

char* revStr(char* ptrToString)
    {
        if (ptrToString == NULL)
            return NULL;

        char* ptrToBegin = ptrToString;
        char* ptrToEnd = ptrToBegin;
        
        for (;*ptrToEnd != '\0'; ++ptrToEnd);
   
        ptrToEnd -=1;

        while (ptrToBegin < ptrToEnd)
            {
                char temp = *ptrToBegin;
                *ptrToBegin = *ptrToEnd;
                *ptrToEnd = temp;
                ptrToBegin++;
                ptrToEnd--;
            }
        return ptrToString;
    }

int main()
    {
        char myStr[20] = "eduD olleH";
        std::cout << myStr << std::endl;
        std::cout << revStr(myStr);
        return 0;
    }

I had to decrement ptrToEnd to get it to output, but now it is starting to feel like something I could put my handle on. Thanks.

[Chervil]

Looks ok.
A slight variation on your code, does the same thing.

#include <iostream>
#include <cstring>

char* revStr(char* ptrToString)
{
    if (ptrToString == NULL)
        return NULL;
    
    char* ptrToBegin = ptrToString;
    char* ptrToEnd   = ptrToBegin;
    
    while (*ptrToEnd)
        ++ptrToEnd;
    
    --ptrToEnd;
    
    while (ptrToBegin < ptrToEnd)
    {
        char temp = *ptrToBegin;
        *ptrToBegin++ = *ptrToEnd;
        *ptrToEnd--   = temp;
    }
    return ptrToString;
}

int main()
{
    char myStr[20] = "!dlroW olleH";
    std::cout << myStr << std::endl;
    std::cout << revStr(myStr);
}