A while problem

[Salahuddin]

This code copies the contents of an array of characters (from) to another array(to)
my code was

void cpy_string(char *to,char *from)
{
printf("Copying ...\n");
while(*from!='\0')
     *to++=*from++;
}

But the solution of the book i'm studying from was

void cpy_string(char *to,char *from)
{
	printf("Copying...\n");
	while(*to++ = *from++);
}

now i don't get the next part of code

while(*to++ = *from++);

can any one explain please ?!

[grumpy]

   while (*to++ = *from++);

expands to (by moving the post-increments out of the while statement into the body)

   while (*to = *from)
   {
       ++to;
       ++from;
   }

and since (a = b) sets a to be equal to b, and gives a result of b, this is equivalent to

    while (*from)
    {
         *to = *from;
         ++to;
         ++from;
    }

which, if you want to make the three statements into one, can be done as;

    while (*from)
    {
         *to++ = *from++;
    }

and the braces can be removed .... to get

    while (*from) *to++ = *from++;

Now, since "while (x)" is equivalent to "while (x != 0)", this is equivalent to your code.

So all you've done is show there is more than one way to solve this problem.

[Salahuddin]

when the codition

while(*from)

become false ??
i think when *from is equal to 0 .. right ?
so the loop will keep working cause we don't know if there is zero in the array or not

and how
[code]
while (*from) *to++=*from++;
[/code/
is equal to

while(*to++ = *from++);

please explain and be patient with me.. i'm still a beginner ..
Thanks .

[_Mike]

when the codition

while(*from)

become false ??
i think when *from is equal to 0 .. right ?

Yes, these are all equivalent:
while(*from)
while(*from!=0)
while(*from!='\0')

so the loop will keep working cause we don't know if there is zero in the array or not

If you can't guarantee that there will be a terminating zero then you have to think of some other way to stop the loop, like passing a length argument to the function.
But since the function is named cpy_string I think it's safe to assume that the user should understand to only pass in valid strings.

and how

while (*from) *to++=*from++;

is equal to

while(*to++ = *from++);

please explain and be patient with me.. i'm still a beginner ..
Thanks .

They are not equivalent. The first one (and your function) does not copy the string terminator, but the second one does.

As for how it works:

while(*to++ = *from++);

1) *to = *from
2) test if *to is true
3) to++; from++;
4) if (2) was true then execute the loop body, which is empty. Just a single ;. And then go back to (1)
5) if (2) was not true then exit the loop

[Salahuddin]

Thanks _mike ... really you are great .. it's so clear for me now .