if statement oddity

[Jens Luedicke]

Hiya..

I encountered a strange problem with an if () statement:

if ((my $r = &{$self->{onFile}}($path) != SUCCESS)) {
return $r;
}

The above code seems to ignore the return value of the function
and somehow overrides it. the return value of the called function
returns FAILED, but $r doesn't reflect that!?

The following code, works as expected:

my $r = &{$self->{onFile}}($path);

if ($r != SUCCESS)) {
return $r;
}

fyi, SUCCESS is a constant. use constant SUCCESS => 1;

both code examples should work the same. no matter what!?

do I miss something? I'm a little disturbed right now.


jens

 

[Gunnar Hjalmarsson]

I encountered a strange problem with an if () statement:

if ((my $r = &{$self->{onFile}}($path) != SUCCESS)) {
return $r;
}

The above code seems to ignore the return value of the function
and somehow overrides it. the return value of the called function
returns FAILED, but $r doesn't reflect that!?

How about enabling warnings? Using the string 'FAILED' in a numerical
comparison isn't very wise.

But the behaviour is about precedence. The return value of the function
is compared with the SUCCESS constant, and $r is assigned the bolean
value resulting from that comparison. This should do what you want:

if ( ( my $r = &{$self->{onFile}}($path) ) ne SUCCESS ) {
return $r;
}

 

[Felix Geerinckx]

if ((my $r = &{$self->{onFile}}($path) != SUCCESS)) {
return $r;
}
[...]

my $r = &{$self->{onFile}}($path);

if ($r != SUCCESS)) {
return $r;
}
[...]

both code examples should work the same. no matter what!?
do I miss something? I'm a little disturbed right now.

Look up the operator precedence rules at the top of

perldoc perlop

(!= is evaluated before =)

 

[Jens Luedicke]

How about enabling warnings? Using the string 'FAILED' in a numerical
comparison isn't very wise.

I use warnings.

FAILED and SUCCESS are numerical constants.

the specific code is on the Filer/DirWalk.pm module of my Filer program.

But the behaviour is about precedence. The return value of the function
is compared with the SUCCESS constant, and $r is assigned the bolean
value resulting from that comparison. This should do what you want:

if ( ( my $r = &{$self->{onFile}}($path) ) ne SUCCESS ) {
return $r;
}

using 'ne' doesn't make a difference and wouldn't be practical in a
numerical comparison.

to avoid problems with precedence the assignment is enclosed in
parentheses. so the assignment should hbe evaluated first and its return
value compared afterwards. At least that's what I expect.

 

[Paul Lalli]

Hiya..

I encountered a strange problem with an if () statement:

if ((my $r = &{$self->{onFile}}($path) != SUCCESS)) {
return $r;
}

to avoid problems with precedence the assignment is enclosed in
parentheses. so the assignment should hbe evaluated first and its return
value compared afterwards. At least that's what I expect.

Exactly where in your code are you using parentheses around the
assignment? You're enclosing the entire if statement in parentheses,
not any particular part of it.

Paul Lalli

 

[Felix Geerinckx]

to avoid problems with precedence the assignment is enclosed in
parentheses. so the assignment should hbe evaluated first and its
return value compared afterwards. At least that's what I expect.

Look at your code (copied from your original post for reference):

if ((my $r = &{$self->{onFile}}($path) != SUCCESS)) {
return $r;
}

The assignment isn't enclosed in parentheses. The full statement is
enclosed in double parentheses.

 

[Jens Luedicke]

The assignment isn't enclosed in parentheses. The full statement is
enclosed in double parentheses.

shit. little typo. big confusion.

mea culpa! shame on me...

/me hides

 

[Brian McCauley]

Hiya..

I encountered a strange problem with an if () statement:

if ((my $r = &{$self->{onFile}}($path) != SUCCESS)) {
return $r;
}

The above code seems to ignore the return value of the function
and somehow overrides it. the return value of the called function
returns FAILED, but $r doesn't reflect that!?

Er? Assuming FAILED != SUCCESS then $r will be set to false. I can't
recell off the top of my head which false but since it immediately goes
out of scope this doesn't matter.

The following code, works as expected:

my $r = &{$self->{onFile}}($path);

if ($r != SUCCESS)) {
return $r;
}

No, that contains a syntax error and will not compile.

fyi, SUCCESS is a constant. use constant SUCCESS => 1;

both code examples should work the same. no matter what!?

do I miss something? I'm a little disturbed right now.

I'm betting you meant

if ((my $r = &{$self->{onFile}}($path)) != SUCCESS) {
return $r;
}

Can I suggest that in future you produce a _minimal_ but _complete_
example that you have actually run and found to behave as you don't
expect. Post it together with a precices description of what it did and
what you expected.

This advice and much other useful advice can be found in the posting
guidelines.

 

[Tad McClellan]

I encountered a strange problem with an if () statement:
[snip]

do I miss something?

Yes.

You missed posting a short and complete program that we can run
that illustrates the problem you are having.

 

[Gunnar Hjalmarsson]

I use warnings.
Good.

FAILED and SUCCESS are numerical constants.

I see. But you failed to explain that as regards FAILED.

 

[Gunnar Hjalmarsson]

Er? Assuming FAILED != SUCCESS then $r will be set to false.

Suppose you mean true, i.e. 1.

 

[Brian McCauley]

Suppose you mean true, i.e. 1.

Yes I mean true.