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php

switch() and default:

php.internals

Frank M. Kromann

21 years ago

Hello Everyone,

I just discovered a small thing in the switch() statement. The position of
the default: clause has to be at the end of the code:

$a = 1;
switch ($a) {
default :
case 0 :
$b = 1;
break;
case 1 :
$b = 2;
break;
}
echo $b; // should print 2 but it prints 1

$a = 1;
switch ($a) {
case 1 :
$b = 2;
break;
default :
case 0 :
$b = 1;
break;
}
echo $b; // prints 2 as expected.

This is tested on Linux with PHP5 CVS-HEAD

What changed ?

- Frank

Andi Gutmans

21 years ago

It's always been like that and has been documented for ages in the manual.

Andi

At 08:24 PM 10/7/2004 -0700, Frank M. Kromann wrote:

Benj Carson

21 years ago

Hi,

Sorry, I forgot to CC my last reply to the list.  I noticed similar
behaviour and filed a bug report: 30285.  The case described in the bug
report is as follows:

$x = "a";
switch ($x) {

default:
echo "default";
break;

case "a":
echo "a";
break;
}

// Prints "a"

Even though the documentation mentions that default should always be last,
this is a BC break from 5.0.x and at least 4.3.2.

Thanks,

Benj

On October 7, 2004 10:59 pm, Andi Gutmans wrote:

Unnamed Person

21 years ago

prints "a" on 4.2.3 Novell, 4.3.3 Linux, 5.0.0 Windows.

On Fri, October 8, 2004 12:21 am, Benj Carson said:

Ron Korving

21 years ago

No matter what behaviour PHP shows, I would find it bad coding if you place
default anywhere but at the bottom, simply because you might run into
unexpected behaviour in other versions of the PHP engine, which I assume
you're experiencing now. You could've seen this coming. When you write a
switch, think of it as the input being matched case by case, with "default"
being the case that matches anything. This means you don't wanna place the
default case anywhere but at the bottom.

Ron


"Benj Carson" <benjcarson@digitaljunkies.ca> wrote in message
news:200410072321.19072.benjcarson@digitaljunkies.ca...

Benj Carson

21 years ago

Coming from C (or Java), I find the new behaviour a little strange.  If you
use default to match invalid conditions, putting it at the beginning of a
switch doesn't seem to be poor practice to me (putting it in the middle
would be pretty ugly though).  IMO, stating what happens to bogus values at
the beginning of the switch block is just as clear as doing the same thing
with if and continue statements at the beginning of a foreach loop.


Benj


On October 8, 2004 12:10 am, Ron Korving wrote:

Christian Schneider

21 years ago

Benj Carson wrote:

> Coming from C (or Java), I find the new behaviour a little strange.  If you

I fully agree. Although I wouldn't have used the word 'little' here :-)

> use default to match invalid conditions, putting it at the beginning of a
> switch doesn't seem to be poor practice to me (putting it in the middle

Even putting in the middle makes sense IMHO:
Looking at some of my code I found constructs like
switch ($sort)
{
case 'firstname': ...
default: case 'lastname': ...
case 'email': ...
}
which seems completely legal and understandable to me: I can sort by
different fields (which have a natural order in my data structure so
that's how I list them) and one of them happens to be the default if
$sort is not set. I don't think it's up to the programming language to
judge if such a construct makes sense or not.

> On October 8, 2004 12:10 am, Ron Korving wrote:
>>assume you're experiencing now. You could've seen this coming. When you
>>write a switch, think of it as the input being matched case by case, with
>>"default" being the case that matches anything. This means you don't
>>wanna place the default case anywhere but at the bottom.

But the important thing is that PHP4 DOES NOT IMPLEMENT IT THIS WAY!

php -r '$a = 1; switch ($a) { default: print "0\n"; break; case 1: print
"1\n"; }'
prints 1 for PHP 4.3.9 (but prints 0 for 5.1.0-dev)

So a) the documentation does not match the PHP behaviour people got used
to, b) it is different to every other language having a switch statement
I know, c) it would be a nasty BC break to change it now.

I strongly suggest that the behaviour is defined a la C (the position of
default: does not matter) and the documentation is changed accordingly!

There is 1000 times more PHP 4 code around (which may rely on the C-like
behaviour) than PHP 5 code so we still have time. But not much. We have
to resort this as quickly as possible to avoid a big mess...

- Chris

Ron Korving

21 years ago

Well, you convinced me (no need to get all physical with me tho ;)). If this
is how C acts (which I did not know), then I prefer it to work like this
too. I always assumed (which is bad, I know) the default case worked like a
"catch all", which catches anything that crosses its path, and therefor
needed to be defined as the last case in the switch. Apparently, this is not
the case throughout the history of the switch statement (which I didn't
know), so I agree with you, especially since this has worked in PHP4.

Ron

"Christian Schneider" <cschneid@cschneid.com> schreef in bericht
news:416667C3.5090506@cschneid.com...

> Benj Carson wrote:
> > Coming from C (or Java), I find the new behaviour a little strange.  If

you

>
> I fully agree. Although I wouldn't have used the word 'little' here :-)
>
> > use default to match invalid conditions, putting it at the beginning of

a

> > switch doesn't seem to be poor practice to me (putting it in the middle
>
> Even putting in the middle makes sense IMHO:
> Looking at some of my code I found constructs like
> switch ($sort)
> {
> case 'firstname': ...
> default: case 'lastname': ...
> case 'email': ...
> }
> which seems completely legal and understandable to me: I can sort by
> different fields (which have a natural order in my data structure so
> that's how I list them) and one of them happens to be the default if
> $sort is not set. I don't think it's up to the programming language to
> judge if such a construct makes sense or not.
>
> > On October 8, 2004 12:10 am, Ron Korving wrote:
> >>assume you're experiencing now. You could've seen this coming. When you
> >>write a switch, think of it as the input being matched case by case,

with

Sascha Schumann

21 years ago

Andi,

from the feedback it is obvious that the engine supported
defaults at places other than the bottom.

In all switch-supporting languages I know, it is possible to
do this:

switch ($expr) {
default:

/* handle everything EXCEPT "foo" and "bar" */

/* fall-through */

case 'foo':
case 'bar':

/* handle everything */
break;
}

Why should the support of this be discontinued?

- Sascha

On Thu, 7 Oct 2004, Andi Gutmans wrote:

Andi Gutmans

21 years ago

I will look into the reason this seems to have changed.
However,  I can assure you that from day 1 this was not supposed to work
and was documented as such for years already (since the days of PHP 3). If
it worked at some point then it was by chance!

Andi

At 05:31 PM 10/8/2004 +0200, Sascha Schumann wrote:

Jason Garber

21 years ago

Hello,

Ergh.  I also hope that it can easily be restored to work the way
it did, even if that was undocumented.  The thought of looking
through ~ 5,000 php scripts before our upgrade is a bit overwhelming
:)

--
Best regards,
Jason                            mailto:jason@ionzoft.com

Friday, October 8, 2004, 11:52:54 AM, you wrote:

AG> I will look into the reason this seems to have changed.
AG> However,  I can assure you that from day 1 this was not supposed to work
AG> and was documented as such for years already (since the days of PHP 3). If
AG> it worked at some point then it was by chance!

AG> Andi

Christian Schneider

21 years ago

Andi Gutmans wrote:

> However,  I can assure you that from day 1 this was not supposed to work
> and was documented as such for years already (since the days of PHP 3).
> If it worked at some point then it was by chance!

This would be a good time to accept the defacto standard, redefine
switch() in the documentation to what it actually does (up to and
including 5.0.2, even if this is by chance), add test cases and make
sure we have a consistent behaviour from now on.

Or is anyone in favour of the documented-but-never-working behavour?

Over and out,
- Chris

Wez Furlong

21 years ago

Slightly OT, but while we are on the subject of switch not being
C-ish, how about making it so that continue inside a switch block
behaves the way it does in C?

This C code snippet:

while (1) {
printf("top\n");
switch (1) {
case 1:
continue;
}
printf("bork\n");
}

will print out "top\n" repeatedly.
The same code in PHP will print out "top\nbork\n" repeatedly.
The get the same behaviour as the C code you need to use continue 2 instead.

IMO, this is awful, and at first I thought it was a BC break, but it
seems that the stock RHEL 4.3.2 version of PHP behaves the same way.

What, if anything, should we do about this?

--Wez.

Hartmut Holzgraefe

21 years ago

Wez Furlong wrote:

> What, if anything, should we do about this?

two letters: 'BC' :(

--
Hartmut Holzgraefe  <hartmut@php.net>

Andi Gutmans

21 years ago

I don't think we should do anything about it. From day 1 we treated
switch() like a loop as far as break/continue is concerned, mainly because
we wanted break/continue to be consistent. You might find this odd but you
actually have more power in PHP than in C as you can break out or continue
from multiple loops.
In my opinion this is definitely not something which should be changed at
this point.
BTW, I think we have a fix for the switch() thingy. I still have to look at
it more closely before commiting though.

Andi

At 05:37 PM 10/8/2004 +0100, Wez Furlong wrote:

Andi Gutmans

21 years ago

Fixed.

At 12:00 PM 10/8/2004 -0400, Jason Garber wrote: