I've got a data structure that is a hash that contains an array of hashes. I'd like to reach in there and pull out the first hash that matches a value I'm looking for. I tried this:
my $result = shift grep {$_->{name} eq 'foo'} #{$hash_ref->{list}};
But that gives me this error: Type of arg 1 to shift must be array (not grep iterator). I've re-read the perldoc for grep and I think what I'm doing makes sense. grep returns a list, right? Is it in the wrong context?
I'll use a temporary variable for now, but I'd like to figure out why this doesn't work.
A list isn't an array.
my ($result) = grep {$_->{name} eq 'foo'} #{$hash_ref->{list}};
… should do the job though. Take the return from grep in list context, but don't assign any of the values other than the first.
I think a better way to write this would be this:
use List::Util qw/first/;
my $result = first { $_->{name} eq 'foo' } #{ $hash_ref->{list} };
Not only will it be more clear what you're trying to do, it will also be faster because it will stop grepping your array once it has found the matching element.
Another way to do it:
my $result = (grep {$_->{name} eq 'foo'} #{$hash_ref->{list}})[0];
Note that the curlies around the first argument to grep are redundant in this case, so you can avoid block setup and teardown costs with
my $result = (grep $_->{name} eq 'foo', #{$hash_ref->{list}})[0];
“List value constructors” in perldata documents subscripting of lists:
A list value may also be subscripted like a normal array. You must put the list in parentheses to avoid ambiguity. For example:
# Stat returns list value.
$time = (stat($file))[8];
# SYNTAX ERROR HERE.
$time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
# Find a hex digit.
$hexdigit = ('a','b','c','d','e','f')[$digit-10];
# A "reverse comma operator".
return (pop(#foo),pop(#foo))[0];
As I recall, we got this feature when Randal Schwartz jokingly suggested it, and Chip Salzenberg—who was a patching machine in those days—implemented it that evening.
Update: A bit of searching shows the feature I had in mind was $coderef->(#args). The commit message even logs the conversation!
Related
I am seeking explanation of the syntax of Perl's uniq and fidrstidx function from module MoreUtils.pm.
Having sought that, I already know other ways to get uniq array elements from an array having duplicate elements and finding the first index from an array by below ways :
## remove duplicate elements ##
my #arr = qw (2 4 2 8 3 4 6);
my #uniq = ();
my %hash = ();
#uniq = grep {!$hash{$_}++ } #arr;
### first index ###
#arr = qw (Java ooperl Ruby cgiperl Python);
my ($index) = grep {$arr[$_] =~ /perl/} 0..$#arr;
Can anybody please explain me second line of this below sub uniq function comprising map and ternary operator from MoreUtils.pm:
map {$h{$_}++ == 0 ? $_ : () } #;
and also
the &# passed to firstidx function and the below line in the body of the function :
local *_ = \$_[$i];
What I understand that sub routine ref is passed to firstidx. But a bit more detailed explanation will be much appreciated.
Thanks.
Your second question was answered in the comments.
Your first question asks about map {$h{$_}++ == 0 ? $_ : () } #; from List::MoreUtils. In recent versions, it's actually in List::MoreUtils::PP (for Pure Perl) since many of the subroutines are also implemented in C and XS. Here's the current version of the Pure Perl uniq:
sub uniq (#)
{
my %seen = ();
my $k;
my $seen_undef;
grep { defined $_ ? not $seen{ $k = $_ }++ : not $seen_undef++ } #_;
}
This has the same map technique although it's using grep instead. The grep goes through all of the elements in #_ and has to return either true or false for each of them. The elements which evaluate to true end up in the output list. The code then wants to make an element evaluate to true the first time it sees it and false the rest of the times.
In this code it handles undef separately. If the current element is not undef, it does the first branch of the conditional operator and the second branch otherwise. Now let's look at the branches.
The defined case adds an element to a hash. No one left code comments about the use of $k but it probably has something to do with not disturbing $_. That $k becomes the key for the hash:
not $seen{ $k = $_ }++
If that is the first time that key has been encountered the value of the hash is undef. That post-increment does its work after the value is used so hold off on thinking about that for a moment. The low-precendence not sees the value of $seen{$k}, which is undef. The not turns the false value of undef into true. That true indicates that the grep has seen $_ for the first time. It becomes part of the output list. Then the ++ does its work and increments the undef value to 1. On all subsequent encounters with the same value the hash value will be true. The not will turn the true value into false and that element won't be in the output list.
The map you show implements the grep. It returns an element when the condition is true and returns no elements when it is false:
map {$h{$_}++ == 0 ? $_ : () } #_;
For each element it adds it as the key in the hash and compares the value to 0. The first time an element is seen that value is undef. In numeric context an undef is 0. So, the == returns true and the first branch of the conditional operator fires, returning $_ to the output list. The ++ then increments the hash value from undef to 1. The next time it encounters the same value the hash value is not 0 and the second branch of the conditional operator returns the empty list. That adds no elements to the output list.
Newer version of List::MoreUtils don't use the construct any more, but as Сухой27 explained,
map { CONDITION ? $_ : () } LIST
is just a fancy alternative to
grep { CONDITION } LIST
I don't think there's any overarching reason the author chose map for this implementation, and in fact it was simplified to grep in later versions of List::MoreUtils.
The firstidx syntax is firstidx BLOCK LIST. Like the builtin map and grep, it is specified that the code in BLOCK will operate on the variable $_, and that the code is allowed to make changes to $_. So in the firstidx implementation, it is not sufficient to set $_ to each value in LIST. Rather, $_ must be aliased to each element of LIST so that a change in $_ inside BLOCK also results to a change in the element in the LIST. This is accomplished by manipulating the symbol table
local *_ = \$scalar # make $_ an alias of $scalar
And you use local so that when firstidx is done, we haven't clobbered any useful information that was previously in the $_ variable.
I have the following code snippet in Perl:
my $argsize = #args;
if ($argsize >1){
foreach my $a ($args[1..$argsize-1]) {
$a =~ s/(.*[-+*].*)/\($1\)/; # if there's a math operator, put in parens
}
}
On execution I'm getting "Use of unitialized value $. in range (or flip) , followed by Argument "" isn't numeric in array element at... both pointing to the foreach line.
Can someone help me decipher the error message (and fix the problem(s))? I have an array #args of strings. The code should loop through the second to n't elements (if any exist), and surround individual args with () if they contain a +,-, or *.
I don't think the error stems from the values in args, I think I'm screwing up the range somehow... but I'm failing when args has > 1 element. an example might be:
<"bla bla bla"> <x-1> <foo>
The long and short of it is - your foreach line is broken:
foreach my $a (#args[1..$argsize-1]) {
Works fine. It's because you're using a $ which says 'scalar value' rather than an # which says array (or list).
If you use diagnostics you get;
Use of uninitialized value $. in range (or flip) at
(W uninitialized) An undefined value was used as if it were already
defined. It was interpreted as a "" or a 0, but maybe it was a mistake.
To suppress this warning assign a defined value to your variables.
To help you figure out what was undefined, perl will try to tell you
the name of the variable (if any) that was undefined. In some cases
it cannot do this, so it also tells you what operation you used the
undefined value in. Note, however, that perl optimizes your program
and the operation displayed in the warning may not necessarily appear
literally in your program. For example, "that $foo" is usually
optimized into "that " . $foo, and the warning will refer to the
concatenation (.) operator, even though there is no . in
your program.
You can reproduce this error by:
my $x = 1..3;
Which is actually pretty much what you're doing here - you're trying to assign an array value into a scalar.
There's a load more detail in this question:
What is the Perl context with range operator?
But basically: It's treating it as a range operator, as if you were working your way through a file. You would be able to 'act on' particular lines in the file via this operator.
e.g.:
use Data::Dumper;
while (<DATA>) {
my $x = 2 .. 3;
print Dumper $x;
print if $x;
}
__DATA__
line one
another line
third line
fourth line
That range operator is testing line numbers - and because you have no line numbers (because you're not iterating a file) it errors. (But otherwise - it might work, but you'd get some really strange results ;))
But I'd suggest you're doing this quite a convoluted way, and making (potentially?) an error, in that you're starting your array at 1, not zero.
You could instead:
s/(.*[-+*].*)/\($1\)/ for #args;
Which'll have the same result.
(If you need to skip the first argument:
my ( $first_arg, #rest ) = #args;
s/(.*[-+*].*)/\($1\)/ for #rest;
But that error at runtime is the result of some of the data you're feeding in. What you've got here though:
use strict;
use warnings;
my #args = ( '<"bla bla bla">', '<x-1>', '<foo>' );
print "Before #args\n";
s/(.*[-+*].*)/\($1\)/ for #args;
print "After: #args\n";
I've just taken over maintenance of a piece of Perl system. The machine it used to run on is dead so I do not know which version of Perl it was using, but it was working. It included the following line to count the lines in a page of ASCII text
my $lcnt = $#{#{$page{'lines'}}};
In Perl 5.10.1 ( we are now running this on CentOS 6.3 ) the above code no longer works. I instead use the following, which works fine.
my #arr = #{$page{'lines'}};
my $lcnt = $#arr;
I'll admit my Perl isn't great but from what I can see the first version should never have worked as it is trying to deference an array rather than an array ref
First question - is my guess at why this first line of code doesn't now work correct, and secondly did it work earlier due to a now fixed bug in a prior Perl version?
Thanks!
The first version never worked. Assuming $page{'lines'} is an arrayref, this is what you want:
my $lcnt = $#{$page{'lines'}};
Note that this is going to give you one less than the number of items in your arraref. The $# operator is the INDEX of the last item, not the number of items. If you want the number of items in $page{'lines'}, you probably want this:
my $lcnt = scalar(#{$page{'lines'}});
Some things about your code. This:
my $lcnt = $#{#{$page{'lines'}}};
Was never correct. Take a look at the three things going on here
$page{'lines'} # presumably an array ref
#{ ... } # dereference into an array
$#{ ... } # get last index of an array ref
This is equivalent to (continuing on your own code):
my #arr = #{$page{'lines'}};
my $foo = #arr; # foo is now the size of the array, e.g. 3
my $lcnt = $#$foo;
If you use
use strict;
use warnings;
Which you should always do, without question (!), you will get the informative fatal error message:
Can't use string ("3") as an ARRAY ref while "strict refs" in use
(Where 3 will be the size of your array)
The correct way to get the size (number of elements) of an array is to put the array in scalar context:
my $size = #{ $page{'lines'} };
The way to get the index of the last element is using the $# sigil:
my $last_index = $#{ $page{'lines'} };
As you'll note, the syntax is the same, it is just a matter of using # or $# to get what you want, just the same as when using a regular array
my $size = #array;
my $last = $#array;
So, to refer back to the beginning: Using both # and $# is not and was never correct.
Every time I input something the code always tells me that it exists. But I know some of the inputs do not exist. What is wrong?
#!/usr/bin/perl
#array = <>;
print "Enter the word you what to match\n";
chomp($match = <STDIN>);
if (grep($match, #array)) {
print "found it\n";
}
The first arg that you give to grep needs to evaluate as true or false to indicate whether there was a match. So it should be:
# note that grep returns a list, so $matched needs to be in brackets to get the
# actual value, otherwise $matched will just contain the number of matches
if (my ($matched) = grep $_ eq $match, #array) {
print "found it: $matched\n";
}
If you need to match on a lot of different values, it might also be worth for you to consider putting the array data into a hash, since hashes allow you to do this efficiently without having to iterate through the list.
# convert array to a hash with the array elements as the hash keys and the values are simply 1
my %hash = map {$_ => 1} #array;
# check if the hash contains $match
if (defined $hash{$match}) {
print "found it\n";
}
You seem to be using grep() like the Unix grep utility, which is wrong.
Perl's grep() in scalar context evaluates the expression for each element of a list and returns the number of times the expression was true.
So when $match contains any "true" value, grep($match, #array) in scalar context will always return the number of elements in #array.
Instead, try using the pattern matching operator:
if (grep /$match/, #array) {
print "found it\n";
}
This could be done using List::Util's first function:
use List::Util qw/first/;
my #array = qw/foo bar baz/;
print first { $_ eq 'bar' } #array;
Other functions from List::Util like max, min, sum also may be useful for you
In addition to what eugene and stevenl posted, you might encounter problems with using both <> and <STDIN> in one script: <> iterates through (=concatenating) all files given as command line arguments.
However, should a user ever forget to specify a file on the command line, it will read from STDIN, and your code will wait forever on input
I could happen that if your array contains the string "hello", and if you are searching for "he", grep returns true, although, "he" may not be an array element.
Perhaps,
if (grep(/^$match$/, #array)) more apt.
You can also check single value in multiple arrays like,
if (grep /$match/, #array, #array_one, #array_two, #array_Three)
{
print "found it\n";
}
Please explain this apparently inconsistent behaviour:
$a = b, c;
print $a; # this prints: b
$a = (b, c);
print $a; # this prints: c
The = operator has higher precedence than ,.
And the comma operator throws away its left argument and returns the right one.
Note that the comma operator behaves differently depending on context. From perldoc perlop:
Binary "," is the comma operator. In
scalar context it evaluates its left
argument, throws that value away, then
evaluates its right argument and
returns that value. This is just like
C's comma operator.
In list context, it's just the list
argument separator, and inserts both
its arguments into the list. These
arguments are also evaluated from left
to right.
As eugene's answer seems to leave some questions by OP i try to explain based on that:
$a = "b", "c";
print $a;
Here the left argument is $a = "b" because = has a higher precedence than , it will be evaluated first. After that $a contains "b".
The right argument is "c" and will be returned as i show soon.
At that point when you print $a it is obviously printing b to your screen.
$a = ("b", "c");
print $a;
Here the term ("b","c") will be evaluated first because of the higher precedence of parentheses. It returns "c" and this will be assigned to $a.
So here you print "c".
$var = ($a = "b","c");
print $var;
print $a;
Here $a contains "b" and $var contains "c".
Once you get the precedence rules this is perfectly consistent
Since eugene and mugen have answered this question nicely with good examples already, I am going to setup some concepts then ask some conceptual questions of the OP to see if it helps to illuminate some Perl concepts.
The first concept is what the sigils $ and # mean (we wont descuss % here). # means multiple items (said "these things"). $ means one item (said "this thing"). To get first element of an array #a you can do $first = $a[0], get the last element: $last = $a[-1]. N.B. not #a[0] or #a[-1]. You can slice by doing #shorter = #longer[1,2].
The second concept is the difference between void, scalar and list context. Perl has the concept of the context in which your containers (scalars, arrays etc.) are used. An easy way to see this is that if you store a list (we will get to this) as an array #array = ("cow", "sheep", "llama") then we store the array as a scalar $size = #array we get the length of the array. We can also force this behavior by using the scalar operator such as print scalar #array. I will say it one more time for clarity: An array (not a list) in scalar context will return, not an element (as a list does) but rather the length of the array.
Remember from before you use the $ sigil when you only expect one item, i.e. $first = $a[0]. In this way you know you are in scalar context. Now when you call $length = #array you can see clearly that you are calling the array in scalar context, and thus you trigger the special property of an array in list context, you get its length.
This has another nice feature for testing if there are element in the array. print '#array contains items' if #array; print '#array is empty' unless #array. The if/unless tests force scalar context on the array, thus the if sees the length of the array not elements of it. Since all numerical values are 'truthy' except zero, if the array has non-zero length, the statement if #array evaluates to true and you get the print statement.
Void context means that the return value of some operation is ignored. A useful operation in void context could be something like incrementing. $n = 1; $n++; print $n; In this example $n++ (increment after returning) was in void context in that its return value "1" wasn't used (stored, printed etc).
The third concept is the difference between a list and an array. A list is an ordered set of values, an array is a container that holds an ordered set of values. You can see the difference for example in the gymnastics one must do to get particular element after using sort without storing the result first (try pop sort { $a cmp $b } #array for example, which doesn't work because pop does not act on a list, only an array).
Now we can ask, when you attempt your examples, what would you want Perl to do in these cases? As others have said, this depends on precedence.
In your first example, since the = operator has higher precedence than the ,, you haven't actually assigned a list to the variable, you have done something more like ($a = "b"), ("c") which effectively does nothing with the string "c". In fact it was called in void context. With warnings enabled, since this operation does not accomplish anything, Perl attempts to warn you that you probably didn't mean to do that with the message: Useless use of a constant in void context.
Now, what would you want Perl to do when you attempt to store a list to a scalar (or use a list in a scalar context)? It will not store the length of the list, this is only a behavior of an array. Therefore it must store one of the values in the list. While I know it is not canonically true, this example is very close to what happens.
my #animals = ("cow", "sheep", "llama");
my $return;
foreach my $animal (#animals) {
$return = $animal;
}
print $return;
And therefore you get the last element of the list (the canonical difference is that the preceding values were never stored then overwritten, however the logic is similar).
There are ways to store a something that looks like a list in a scalar, but this involves references. Read more about that in perldoc perlreftut.
Hopefully this makes things a little more clear. Finally I will say, until you get the hang of Perl's precedence rules, it never hurts to put in explicit parentheses for lists and function's arguments.
There is an easy way to see how Perl handles both of the examples, just run them through with:
perl -MO=Deparse,-p -e'...'
As you can see, the difference is because the order of operations is slightly different than you might suspect.
perl -MO=Deparse,-p -e'$a = a, b;print $a'
(($a = 'a'), '???');
print($a);
perl -MO=Deparse,-p -e'$a = (a, b);print $a'
($a = ('???', 'b'));
print($a);
Note: you see '???', because the original value got optimized away.