EXAMPLES:
undef $foo;
my $bar = undef;
SEE ALSO:
perldoc -f undef
Questions tagged [undef]
57 questions
39
votes
3 answers
When do we need #ifdef before #undef?
In many of the C and C++ files I have seen macros like this:
#ifdef X
#undef X // no statements in between
#endif
I feel that, it's adequate to simply write:
#undef X
If the macro X wasn't defined, then the #undef should have no effect.
Is it ok…
iammilind
- 68,093
- 33
- 169
- 336
20
votes
1 answer
Why does Perl's string multiplication not warn on undef?
Under
use strict;
use warnings;
my $foo = undef;
the expressions
$foo . ''
and
"$foo"
both produce
Use of uninitialized value $foo in ... at ...
but the following expression gives the empty string without warning:
$foo x 1
Anyone knows why? I…
Stefan Majewsky
- 5,427
- 2
- 28
- 51
19
votes
1 answer
How do I remove all undefs from array?
While reading from a configuration file in Perl there might be cases when a line is invalid and it does not need to get added to my array of valid lines. Since I'm using a for loop here, even the invalid lines create an undef entry. How can I remove…
flohei
- 5,248
- 10
- 36
- 61
16
votes
1 answer
How do I remove a Tcl procedure?
How do I remove a tcl procedure?
One can
unset a variable,
override an alias with interp alias {} myproc {} otherproc,
override a proc with one defined inside another namespace with namespace import -force.
But I did not find a way to make a…
cfi
- 10,915
- 8
- 57
- 103
15
votes
6 answers
How can you get Perl to stop when referencing an undef value?
How do you get Perl to stop and give a stack trace when you reference an undef value, rather than merely warning? It seems that use strict; isn't sufficient for this purpose.
Neil
- 24,551
- 15
- 60
- 81
9
votes
1 answer
Why does opening an undef not fail?
This code dies as I expect it to:
use strict;
use warnings;
open my $fh, "<", "" or die $!;
But this does not:
use strict;
use warnings;
open my $fh, "<", undef or die $!;
What is going on here?
Chas. Owens
- 64,182
- 22
- 135
- 226
9
votes
2 answers
What values should a boolean function in Perl return?
SHORT QUESTION
What are the best ways to represent true and false consistently in libraries of Perl code?
1 / 0?
1 / the special empty string that Perl's native boolean operators
return?
undef?
() (i.e. the empty list)?
QUESTION BACKGROUND
We all…
Krazy Glew
- 7,210
- 2
- 49
- 62
8
votes
5 answers
In Perl, is there graceful way to convert undef to 0 manually?
I have a fragment in this form:
my $a = $some_href->{$code}{'A'}; # a number or undef
my $b = $some_href->{$code}{'B'}; # a number or undef
$a = 0 unless defined($a);
$b = 0 unless defined($b);
my $total = $a + $b;
The reality is even more messy,…
Anon Gordon
- 2,469
- 4
- 28
- 34
7
votes
3 answers
Perl: mapping to lists' first element
Task: to build hash using map, where keys are the elements of the given array @a, and values are the first elements of the list returned by some function f($element_of_a):
my @a = (1, 2, 3);
my %h = map {$_ => (f($_))[0]} @a;
All the okay until f()…
indexless
- 389
- 3
- 11
7
votes
9 answers
Check for value definedness in C++
I'm working in C++ and I need to know if a scalar value (for instance a double) is "defined" or not. I also need to be able to "undef" it if needed:
class Foo {
public:
double get_bar();
private:
double bar;
void calculate_bar() {
…
tunnuz
- 23,338
- 31
- 90
- 128
7
votes
1 answer
The poison value and undefined value in LLVM
LLVM introduces the concept of "poison value", which I never feel sure to understand. For example, for the statement
%add = add nsw i32 %x, 1
If %x+1 is strictly larger than the largest i32 integer, an arbitrary value is to be assigned to %add.…
zell
- 9,830
- 10
- 62
- 115
6
votes
3 answers
Warnings in Perl Eval
I need to hide warnings within eval but the rest of the code should continue to throw warning messages. Here is what I have -
eval "\$value = $hash->{key}";
now value of $hash->{key} could be a function call, like:
$hash->{key} =…
user589672
- 61
- 1
- 2
6
votes
1 answer
Perl `defined' and `undef' subroutine scope
Please take a look at the following code:
use strict;
use warnings;
print "subroutine is defined\n" if defined &myf;
myf();
sub myf
{
print "called myf\n";
}
undef &myf;
#myf();
print "now subroutine is defined\n" if defined &myf;
The…
password636
- 981
- 1
- 7
- 18
5
votes
1 answer
Some string values ("0", "", and " ") are defined, but (==undef)
In my perl script, I used if ($a == undef) condition, I thought it is the same as if (not defined $a), where $a is a string as read from a csv file. However, I noticed that several string values ("", " ", "0", "-0", "+0", "@", "a", and many other…
Douglas
- 71
- 1
5
votes
3 answers
scope of #undef C++
I have a question about using #undef to redefine macros.
I have a file global.h which contains a number of #define-d macros. In the code that uses these macros, I find that the values that the macros hold are not generic enough. I want to redefine…
Sriram
- 10,298
- 21
- 83
- 136