Reverse string using pointer arithmetic

Question

I am trying to understand how the below function *reverseString(const char *str) reverses the string with pointer arithmetic. I have been Googling and watched videos which handle similar cases but unfortunately they didn't help me out. Could you please somebody help me what may be missing for me to understand how this function works? I am using Windows 10 Pro (64 bit) with Visual Studio Community 2019. Thank you in advance for your help as always. I appreciate it.

#include <iostream>
#include <cstring>

using namespace std;

char *reverseString(const char *str)
{
    int len = strlen(str); // length = 10
    char *result = new char[len + 1]; // dynamically allocate memory for char[len + 1] = 11
    char *res = result + len; // length of res = 11 + 10 = 21?

    *res-- = '\0';  // subtracting ending null character from *res ?
    while (*str)
        *res-- = *str++;  // swapping character?
    return result;  // why not res?
}

int main()
{
    const char* str = "Constantin";
    cout << reverseString(str) << endl;
}

axby · Accepted Answer · 2021-10-11 01:23:15Z

1

    char *result = new char[len + 1];

This line allocates a new string (length of the other string's characters, plus one for the terminating null), and stores it in result. Note that result points to the beginning of the string, and is not modified.

    char *res = result + len;

This line is making res point to the end of result: res is a pointer that equals the address of result, but len characters after.

    *res-- = '\0';

This line:

writes a terminating NULL to res, which currently points to the end of result, and
decrements res so that it now points to the previous character of result

    while (*str)
        *res-- = *str++;  // swapping character?

These lines:

loop until str points to a NULL
write the character pointed to by str to the destination memory pointed to by res, and
res--: decrement res to point to the previous location in memory, and
str++: increment str to point to the next character of str

    return result;  // why not res?

Result is returned because it points to the (beginning of) the new string.

answered Oct 11, 2021 at 1:23

axby

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1 Comment

yurachika2020 Over a year ago

Thank you so much for your explanation @uuser9596600! It became clear what I am missing to understand this function. I need to understand how the pointer works.

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