while( ) and test condition
nembo kid
Ok, but also if i<0 the while loop is executed.
So, which are the rules? Which values must assume the "test condition"
to be assumed like true o false? Thanks in advance
#include <stdio.h>
int main( void )
{
int i = 0; /* while loop isn't executed */
while (i)
{
printf ("i'm while-looping...Ctrl-C to exit");
}
}
pete
while (i)
means the exact same thing as
while ((i) != 0)
Therefore:
while (i>0)
means the exact same thing as
while ((i>0) != 0)
and:
while (i<0)
means the exact same thing as
while ((i<0) != 0)
--
pete
Richard Bos
> So, which are the rules? Which values must assume the "test condition"
> to be assumed like true o false? Thanks in advance
_All_ logical conditions in C, regardless of whether they're in a while
loop, a for loop, an if statement, or wherever, are false if they
compare equal to 0, and true in all other cases.
(Note: compare equal to 0 need not mean "is an integer and has the value
zero". For example, you can put a pointer expression in an if statement,
and it will trigger if the pointer is not a null pointer.)
Richard
August Karlstrom
As others have already said, zero is false and all other values are
true. Anyway, for clarity it is probably best to be explicit and do a
comparison (unless a variable or function has a boolean interpretation
already) -- we want to concentrate on real problems, not on "tricks"
within the language.
August
Sjoerd
> Anyway, for clarity it is probably best to be explicit and do a
> comparison (unless a variable or function has a boolean interpretation
> already) -- we want to concentrate on real problems, not on "tricks"
> within the language.
Actually, some people don't use an explicit comparison and on top of that
use a non-trivial expression, such as this:
while (line = fgets(buf, 10, fp)) ...
This does not compare line to the result of fgets() because there is only
one = sign. Instead, this first assigns the result of fgets() to line and
then compares it to 0 to see if the loop should be entered.
August Karlstrom
> Actually, some people don't use an explicit comparison and on top of that
> use a non-trivial expression, such as this:
>
> while (line = fgets(buf, 10, fp)) ...
>
> This does not compare line to the result of fgets() because there is only
> one = sign. Instead, this first assigns the result of fgets() to line and
> then compares it to 0 to see if the loop should be entered.
Exactly, expressions with side-effects are error-prone and IMHO ugly. I
avoid them whenever I can. See also
http://en.wikipedia.org/wiki/Command-Query_Separation
August
Andrew Poelstra
> Sjoerd wrote:
> > Actually, some people don't use an explicit comparison and on top of that
> > use a non-trivial expression, such as this:
> >
> > while (line = fgets(buf, 10, fp)) ...
> >
> > This does not compare line to the result of fgets() because there is only
> > one = sign. Instead, this first assigns the result of fgets() to line and
> > then compares it to 0 to see if the loop should be entered.
>
> Exactly, expressions with side-effects are error-prone and IMHO ugly. I
> avoid them whenever I can. See also
>
I disagree.
while(line = fgets(buf, sizeof buf, fp))
is much clearer than the alternative
line = fgets(buf, sizeof buf, fp);
while(line != NULL) /* What does line mean? */
{
...
line = fgets(buf, sizeof buf, fp);
}
Fred
> On Thu, 2008-09-04 at 16:01 +0200, August Karlstrom wrote:
> > Sjoerd wrote:
> > > Actually, some people don't use an explicit comparison and on top of that
> > > use a non-trivial expression, such as this:
>
> > > while (line = fgets(buf, 10, fp)) ...
>
> > > This does not compare line to the result of fgets() because there is only
> > > one = sign. Instead, this first assigns the result of fgets() to line and
> > > then compares it to 0 to see if the loop should be entered.
>
> > Exactly, expressions with side-effects are error-prone and IMHO ugly. I
> > avoid them whenever I can. See also
>
> I disagree.
>
> while(line = fgets(buf, sizeof buf, fp))
>
> is much clearer than the alternative
I disagree - it is not at all clear what the original
author really intended. Did she mean to set line to the
result of fgets, or did she mean to compare line
to the result of fgets (and made a mistake)?
A much clearer way is to explicitly demonstrate
what was intended:
while( (line = fgets(buf, sizeof buf, fp)) != NULL )
--
Fred Kleinschmidt
fjb...@yahoo.com
Indeed. Some compilers will warn about `while(line = fgets(...))' on
the grounds that you may have meant `while(line == fgets(...))'.
Adding the explicit test, as in `while((line = fgets(...)) != NULL)',
suppresses this warning.
Richard Tobin
Fred <fred.l.kl...@boeing.com> wrote:
>> while(line = fgets(buf, sizeof buf, fp))
>I disagree - it is not at all clear what the original
>author really intended. Did she mean to set line to the
>result of fgets, or did she mean to compare line
>to the result of fgets (and made a mistake)?
>
>A much clearer way is to explicitly demonstrate
>what was intended:
>
>while( (line = fgets(buf, sizeof buf, fp)) != NULL )
Gcc - quite reasonably I think - shuts up provided you put parentheses
around the expression:
while( (line = fgets(buf, sizeof buf, fp)) )
I don't see any advantage to making the test against NULL explicit.
-- Richard
--
Please remember to mention me / in tapes you leave behind.
Richard
The line above *is* C. If you think that is error prone I suggest you
consider another programming language.
August Karlstrom
Command Query Separation is a general principle applicable to imperative
languages, C for instance. Nothing, except for the libraries I use,
stops me from using this principle in my programs.
August