while loop - Why does this index work? c++ -

all, professor gave following loop use in weeks assignment:

char *ptr1, *ptr2;   char c; ptr1 = &arr[0];   ptr2 = &arr[idx-1];  while(ptr1 < ptr2){   c     = *ptr1;     *ptr1 = *ptr2;   *ptr2 = c;    ptr1++;   ptr2--; }   

is position of ptr1 vs. ptr2? program reverses c_str , works fine, don't understand why.

do have problem understanding of individual statements? if not, seeing how works question of going through steps.

the first 3 statements of loop swap character pointed ptr1 , 1 pointed ptr2. initially, ptr1 , ptr2 point first , last characters of string. second pass, pointing second , second-last characters of string, etc.

initially:

        +-----+-----+-----+-----+-----+-----+      +-----+ arr --> |   |  b  |  c  |  d  |  e  | nul |    c |  ?  |         +-----+-----+-----+-----+-----+-----+      +-----+            ^                       ^            |                       |           ptr1                    ptr2 

after c = *ptr1;:

        +-----+-----+-----+-----+-----+-----+      +-----+ arr --> |   |  b  |  c  |  d  |  e  | nul |    c |   |         +-----+-----+-----+-----+-----+-----+      +-----+            ^                       ^            |                       |           ptr1                    ptr2 

after *ptr1 = *ptr2;:

        +-----+-----+-----+-----+-----+-----+      +-----+ arr --> |  e  |  b  |  c  |  d  |  e  | nul |    c |   |         +-----+-----+-----+-----+-----+-----+      +-----+            ^                       ^            |                       |           ptr1                    ptr2 

after *ptr2 = c;:

        +-----+-----+-----+-----+-----+-----+      +-----+ arr --> |  e  |  b  |  c  |  d  |   | nul |    c |   |         +-----+-----+-----+-----+-----+-----+      +-----+            ^                       ^            |                       |           ptr1                    ptr2 

after ptr1++; ptr2--;:

        +-----+-----+-----+-----+-----+-----+      +-----+ arr --> |  e  |  b  |  c  |  d  |   | nul |    c |   |         +-----+-----+-----+-----+-----+-----+      +-----+                  ^           ^                  |           |                 ptr1        ptr2 

after pass:

        +-----+-----+-----+-----+-----+-----+      +-----+ arr --> |  e  |  d  |  c  |  b  |   | nul |    c |  b  |         +-----+-----+-----+-----+-----+-----+      +-----+                       ^ ^                       | |                    ptr1 ptr2 

loop ends.