Reversing a string using pointers

Question

I am currently studying different algorithms to practice my coding strengths and came across an alogrithm to reverse a string. It uses pointers and I am alittle confused on what is going on. The code is below:

void reverse(char *str) {
   char * end = str;
   char tmp;
   if (str) {
      while (*end) {
         ++end;
      }
  --end;
  while (str < end) {
     tmp = *str;
     *str++ = *end;
     *end-- = tmp;
  }
 }
}

So in this code, the end is put to the end of the strings memory location in the first while loop. Then the second while is where I am getting confused. I do not understand what is going with the *str++ = *end and *end-- = *temp.

Why is this happening like this? When the line tmp = *str occurs, does the first letter of the string go into temp or am I thinking of this incorrectly?

It may help to realize, that the first half of the string is swapped with the second half. So the first char is swapped with the last character, then second character with next to last character, and so on, stopping at the middle of the string. — rcgldr
– rcgldr, Commented Mar 5, 2015 at 3:57

Srinath Mandava · Accepted Answer · 2015-03-05 03:40:29Z

3

As you already know end points to the last character. Now in the second for loop he is just swapping the first character with the last character and then moving the str pointer towards its rights and end to its left.

while (str < end) {
     tmp = *str;  // storing the char at str in temp
     *str++ = *end;  // storing the char at end at str position and then incrementing str .. i.e. moving str right
     *end-- = tmp;  // storing the char in tmp at end position and then decrementing tmp .. i.e. moving tmp left 
  }

answered Mar 5, 2015 at 3:40

Srinath Mandava

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2 Comments

user081608 Over a year ago

So if str was "Hello", then tmp would be the location of the H?

Srinath Mandava Over a year ago

@user081608 Nope... temp would be 'H' itself. temp is a char... while str,end are pointers.. initially str points to location oh 'H' and temp would be storing the char pointed be str i.e. 'H'

Hunter McMillen · Accepted Answer · 2015-03-05 03:51:08Z

1

It is always best to try an example, below is what the second while loop would look like for the word "hello". The value of *str is always stored in tmp, then *end is copied into *str, the overall effect is that after each iteration the values of *str and *end switch. Both the post-increment on *str and the post-decrement on *end do not take effect until after the assignment statements.

    *end
    v
hello        tmp='h'
^
*str
------------------
   *end
   v
oellh        tmp='e'
 ^
 *str
------------------
  *end
  v
olleh        tmp='e'
  ^
  *str

execution stops here because str < end produces a false value since they now both point to the same character.

edited Mar 5, 2015 at 3:51

answered Mar 5, 2015 at 3:45

Hunter McMillen

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1 Comment

user081608 Over a year ago

In the line *str++ = *end the str value moves after the switch because it is str++ and not ++str correct?

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