while loop's "=" question

[Snaggletooth]

Hello,

I am going through my C book when I ran across something I don't understand about the while loop. The book uses the below option1 and says I can replace it with option2. It does work. I just am clueless on why.

My 3 questions about option2 are:
1) How does the "=" sign work in the condition spot?
2) The "=" works but is it proper? I am use to the true/false conditions...
3) This will probably be answered in question 1 but, how does the "\0" at the end of the string get copied?


option1

while(str2[count2]) /* Copy up to null from str2 */
    str1[count1++] = str2[count2++];
str1[count1] = '\0';    /* Make sure we add terminator  */

option2

while((str1[count1++] = str2[count2++]));

Thanks!

[oogabooga]

The code is interpreted something like this:

whiletop:
  tmp = (str1[count1] = str2[count2]);
  count1++;
  count2++;
  if (tmp)
    goto whiletop;

Remember that the "value" of an assignment statement is the value assigned to the left-hand side. As long as that value is non-zero, it's considered true.

[Malcolm McLean]

What happened was than Denis Ritchie, who invented C, originally thought it would be a good idea to write C full of terse loops like this. It was pretty quickly realised that this is in fact a bad idea, and you need to lay out code to kmake it simple to read.

So while(x = y) sets x to y, and then terminates if the result is zero. So on the last interation, x is zero, y is zero, and the loop body isn't executed.

If we make it

while(str[count1++] = str[count2++]]

the same thing happens, but str[count1] is is to zero. Then count1 is incremented for one last time, so it indexes the value after the zero.

But you don't want to write code like that if you can help it. Keep things as clear as possible.

[manasij7479]

What happened was than Denis Ritchie, who invented C, originally thought it would be a good idea to write C full of terse loops like this.

That terseness is sometimes elegant (...it seems to me...), though.

[QuantumPete]

That terseness is sometimes elegant (...it seems to me...), though.

Readability trumps elegance every time!

[dmh2000]

one motivation was that Ritchie was using a 300baud teletype in the beginning, so keeping it terse mattered.

[Snaggletooth]

Thanks! I am pretty sure I understand. Now I should be able to remember 'the "value" of an assignment statement is the value assigned to the left-hand side.' Kind of feel like I need a C puzzle book with explanations to help burn the rules and tips in.