what difference between while(*p){p++;} ,while (*++p){;} and while(*p++){;}?

Question

It\'s about strcat function.

while (*p)
    p++;

and

while (*++p)
    ;

both works, but

Answer 1

Let's assume that p is a string.

while (*p) p++; /* (1) */
while (*++p) ;  /* (2) */
while (*p++) ;  /* (3) */

• (1) is different from (2) if p is an empty string.
• (1) is different from (3) because with (3), even if the current value of *p is a '\0' character, p is incremented.

Answer 2

Both

while (*p) p++;

and

while (*++p) ;

will advance p to point to the 0 terminator in a string, whereas

while (*p++) ;

will advance p to point one character past the 0 terminator.

To see why, let's assume the following characters in memory:

Address         0x00 0x01 0x02 0x03
-------         ---- ---- ---- ----
0x8000           'a'  'b'  'c'   0
0x8004           ...

Assume that p starts at address 0x8000. Here's how the first loop plays out:

1.  *p = 'a'
2.  p = 0x8001
3.  *p = 'b'
4.  p = 0x8002
5.  *p = 'c'
6.  p = 0x8003
7.  *p = 0
8.  end loop

Here's how the second loop plays out:

1.  p = 0x8001
2.  *p = 'b'
3.  p = 0x8002
4.  *p = 'c'
5.  p = 0x8003
6.  *p = 0
7.  end loop

And here's the last one:

1.  *p = 'a'
2.  p = 0x8001
3.  *p = 'b'
4.  p = 0x8002
5.  *p = 'c'
6.  p = 0x8003
7.  *p = 0;
8.  p = 0x8004
9.  end loop

In the last version, evaluating *p++ advances the pointer even if the value of *p is 0.

Answer 3

*++p means increment this variable, then use it
*p++ means use the variable, then increment it

Answer 4

// move p forward as long as it points to a non-0 character,
// leave it pointing to a 0 character.
while (*p) p++;  

// move p forward until it points to a 0 character, skipping the first 
// character before you start
while (*++p);

// move p forward until it points one past a 0 character
while (*p++);

So given that (1) "works": (2) also works if p initially points to a non-empty string. (3) doesn't work at all because p ends up pointing to a different place.

*++p increments p and then evaluates to whatever p now points to. *p++ evaluates to whatever p initially points to, but also increments p. Hence (2) and (3) are different.

(1) and (3) are different because (3) executes the p++, then it decides whether to stop. (1) first looks at *p to decide whether to stop, and if not stopping it executes the p++.