I have the code similar to below:
my #array1 = (); #2d array to be used
my $string1 = "blank1";
my $string2 = "blank2";
my $string3 = "blank3";
my #temp = ($string1, $string2, $string3);
push (#array1, \#temp);
The reason I am assigning the strings and then putting them into an array is because they are in a loop and the values get updated in the loop (#array1 is not declared in the loop).
When I run my program, it only gives me a reference to an array rather than an actual 2D array. How can I get it to print out the content as a 2D array and not as a reference or flattened out to a 1D array?
I would like an output like [[blank1, blank2, blank3],....] so i can access it like $array1[i][j]
An array can only have scalars for elements. Thus this includes references, to arrays for example, what enables us to build complex data structures. See perldsc, Tom's Perl Data Structure Cookbook.
Elements of those ("second-level") arrays are accessed by dereferencing, so $array1[0]->[1] is the second element of the array whose reference is the first element of the top-level array (#array1). Or, for convenience, a simpler syntax is allowed as well: $array1[0][1].
If we want a list of all elements of a second-level array then dereference it with #, like:
my #l2 = #{ $array1[0] }; # or, using
my #l2 = $array1[0]->#*; # postfix dereferencing
Or, to get just a few elements of the array, but in one scoop -- a slice
my #l2_slice = #{$array1[0]}[1..2]; # or
my #l2_slice = $array1[0]->#[1..2]; # postfix reference slice
what returns the list with the second and third elements of the same second-level array.
The second lines are of a newer syntax called postfix dereferencing, stable as of v5.24. It avails us with the same logic for getting elements as when we drill for a single one, by working left-to-right all the way. So ->#* to get a list of all elements for an arrayref,->%* for a hashref (etc). See for instance a perl.com article and an Effective Perler article.
There is a thing to warn about when it comes to multidimensional structures built with references. There are two distinct ways to create them: by using references to existing, named, variables
my #a1 = 5 .. 7;
my %h1 = ( a => 1, b => 2 );
my #tla1 = (\#a1, \%h1);
or by using anonymous ones, where arrayrefs are constructed by [] and hashrefs by {}
my #tla2 = ( [ 5..7 ], { a => 1, b => 2 } );
The difference is important to keep in mind. In the first case, the references to variables that the array carries can be used to change those variables -- if we change elements of #tla1 then we really change the referred variables
$tla1[0][1] = 100; # now #a1 == 5, 100, 7
Also, changing variables with references in #tla1 is seen via the top-level array as well.
With anonymous arrays and hashes in #tla this isn't the case as elements (references) of #tla access independent data, which cannot be accessed (and changed) in any other way.
Both of these ways to build complex data structures have their uses.
Related
As seen below, I have a foreach loop inside which, a value inside an array of hashes is being replaced with a value from another array of hashes.
The second foreach loop is just to print and test whether the values got assigned correctly.
foreach my $row (0 .. $#row_buff) {
$row_buff[$row]{'offset'} = $vars[$row]{'expression'};
print $row_buff[$row]{'offset'},"\n";
}
foreach (0 .. $#row_buff) {
print $row_buff[$_]{'offset'},"\n";
}
Here #row_buff and #vars are the two array of hashes. They are prefilled with values for all keys used.
The hashes were pushed into the arrays like so:
push #row_buff, \%hash;
ISSUE:
Let's say the print statement in the first foreach print's like this:
string_a
string_b
string_c
string_d
Then the print statement in the second foreach loop print's like so:
string_d
string_d
string_d
string_d
This is what confuses me. Both print statements are supposed to print the exact same way am I right? But the value that gets printed by the second print statement is just the last value alone in a repeated manner. Could someone please point me to what could be going wrong here? Any hint is greatly appreciated. This is my first time putting up a question so pardon me if I missed anything.
UPDATE
There was a bit of information that I could have added, sorry about that everyone. There was one more line before the foreach, it was like so:
#row_buff = (#row_buff) x $itercnt;
foreach my $row (0 .. $#row_buff) {
$row_buff[$row]{'offset'} = $vars[$row]{'expression'};
print $row_buff[$row]{'offset'},"\n";
}
foreach (0 .. $#row_buff) {
print $row_buff[$_]{'offset'},"\n";
}
$itercnt is an integer. I was using it to replicate the #row_buff that many number of times.
This clearly has to do with storing references on the array, instead of independent data. How that comes about isn't clear since details aren't given, but the following discussion should help.
Consider these two basic examples.
First, place a hash (reference) on an array, first changing a value each time
use warnings;
use strict;
use feature 'say';
use Data::Dump qw(dd);
# use Storable qw(dclone);
my %h = ( a => 1, b => 2 );
my #ary_w_refs;
for my $i (1..3) {
$h{a} = $i;
push #ary_w_refs, \%h; # almost certainly WRONG
# push #ary_w_refs, { %h }; # *copy* data
# push #ary_w_refs, dclone \%h; # may be necessary, or just safer
}
dd $_ for #ary_w_refs;
I use Data::Dump for displaying complex data structures, for its simplicity and default compact output. There are other modules for this purpose, Data::Dumper being in the core (installed).
The above prints
{ a => 3, b => 2 }
{ a => 3, b => 2 }
{ a => 3, b => 2 }
See how that value for key a, that we changed in the hash each time, and so supposedly set for each array element, to a different value (1, 2, 3) -- is the same in the end, and equal to the one we assigned last? (This appears to be the case in the question.)
This is because we assigned a reference to the hash %h to each element, so even though every time through the loop we first change the value in the hash for that key in the end it's just the reference there, at each element, to that same hash.∗
So when the array is queried after the loop we can only get what is in the hash (at key a it's the last assigned number, 3). The array doesn't have its own data, only a pointer to hash's data.† (Thus hash's data can be changed by writing to the array as well, as seen in the example below.)
Most of the time, we want a separate, independent copy. Solution? Copy the data.
Naively, instead of
push #ary_w_refs, \%h;
we can do
push #ary_w_refs, { %h };
Here {} is a constructor for an anonymous hash,‡ so %h inside gets copied. So actual data gets into the array and all is well? In this case, yes, where hash values are plain strings/numbers.
But what when the hash values themselves are references? Then those references get copied, and #ary_w_refs again does not have its own data! We'll have the exact same problem. (Try the above with the hash being ( a => [1..10] ))
If we have a complex data structure, carrying references for values, we need a deep copy. One good way to do that is to use a library, and Storable with its dclone is very good
use Storable qw(dclone);
...
push #ary_w_refs, dclone \%h;
Now array elements have their own data, unrelated (but at the time of copy equal) to %h.
This is a good thing to do with a simple hash/array as well, to be safe from future changes, whereby the hash is changed but we forget about the places where it's copied (or the hash and its copies don't even know about each other).
Another example. Let's populate an array with a hashref, and then copy it to another array
use warnings;
use strict;
use feature 'say';
use Data::Dump qw(dd pp);
my %h = ( a => 1, b => 2 );
my #ary_src = \%h;
say "Source array: ", pp \#ary_src;
my #ary_tgt = $ary_src[0];
say "Target array: ", pp \#ary_tgt;
$h{a} = 10;
say "Target array: ", pp(\#ary_tgt), " (after hash change)";
$ary_src[0]{b} = 20;
say "Target array: ", pp(\#ary_tgt), " (after hash change)";
$ary_tgt[0]{a} = 100;
dd \%h;
(For simplicity I use arrays with only one element.)
This prints
Source array: [{ a => 1, b => 2 }]
Target array: [{ a => 1, b => 2 }]
Target array: [{ a => 10, b => 2 }] (after hash change)
Target array: [{ a => 10, b => 20 }] (after hash change)
{ a => 100, b => 20 }
That "target" array, which supposedly was merely copied off of a source array, changes when the distant hash changes! And when its source array changes. Again, it is because a reference to the hash gets copied, first to one array and then to the other.
In order to get independent data copies, again, copy the data, each time. I'd again advise to be on the safe side and use Storable::dclone (or an equivalent library of course), even with simple hashes and arrays.
Finally, note a slightly sinister last case -- writing to that array changes the hash! This (second-copied) array may be far removed from the hash, in a function (in another module) that the hash doesn't even know of. This kind of an error can be a source of really hidden bugs.
Now if you clarify where references get copied, with a more complete (simple) representation of your problem, we can offer a more specific remedy.
∗ An important way of using a reference that is correct, and which is often used, is when the structure taken the reference of is declared as a lexical variable every time through
for my $elem (#data) {
my %h = ...
...
push #results, \%h; # all good
}
That lexical %h is introduced anew every time so the data for its reference on the array is retained, as the array persists beyond the loop, independently for each element.
It is also more efficient doing it this way since the data in %h isn't copied, like it is with { %h }, but is just "re-purposed," so to say, from the lexical %h that gets destroyed at the end of iteration to the reference in the array.
This of course may not always be suitable, if a structure to be copied naturally lives outside of the loop. Then use a deep copy of it.
The same kind of a mechanism works in a function call
sub some_func {
...
my %h = ...
...
return \%h; # good
}
my $hashref = some_func();
Again, the lexical %h goes out of scope as the function returns and it doesn't exist any more, but the data it carried and a reference to it is preserved, since it is returned and assigned so its refcount is non-zero. (At least returned to the caller, that is; it could've been passed yet elsewhere during the sub's execution so we may still have a mess with multiple actors working with the same reference.) So $hashref has a reference to data that had been created in the sub.
Recall that if a function was passed a reference, when it was called or during its execution (by calling yet other subs which return references), changed and returned it, then again we have data changed in some caller, potentially far removed from this part of program flow.
This is done often of course, with larger pools of data which can't just be copied around all the time, but then one need be careful and organize code (to be as modular as possible, for one) so to minimize chance of errors.
† This is a loose use of the word "pointer," for what a reference does, but if one were to refer to C I'd say that it's a bit of a "dressed" C-pointer
‡ In a different context it can be a block
I'm trying to assign an array to a value in my hash as follows:
$authors->[$x]->{'books'} = #books;
$authors is an array of hashes that contains his/her first name, last name, date of birth, etc. And now I'm creating a books key where I want to assign an array of books. However, when I try to print it afterwords, it's just printing the size of the array, as if I'm doing $value = scalar #books.
What am I doing wrong?
Array elements and hash values are scalars, so when you are nesting arrays and hashes, you must use references. Just as $authors->[$x] is not a hash but a reference to a hash, you must set $authors->[$x]->{'books'} to a reference to the array.
$authors->[$x]->{'books'} = \#books; # reference the original array
$authors->[$x]->{'books'} = [#books]; # reference a copy
You would then access elements of the array using something like
$authors->[$x]->{'books'}->[0]
which can be abbreviated
$authors->[$x]{books}[0]
or access the whole array as
#{$authors->[$x]{books}}
Your original attempt
$authors->[$x]->{'books'} = #books;
is exactly equivalent to
$authors->[$x]->{'books'} = scalar #books;
because the left operand of the = operator is a hash value, which is a scalar, so the right operand is evaluated in scalar context to provide something that can be assigned there.
P.S.
On rereading this answer I realized it may be confusing to say "a hash value is a scalar" because of the possible interpretation of "hash value" as meaning "the value of a hash variable" i.e. "the whole hash". What I mean when I write "hash value" is an item that is stored in a hash as a value (as opposed to a key).
While the first awnser is absolutely right,
as an alternative, you could also fo this:
push #{$authors->[$x]->{'books'}}, #books;
Then $authors->[$x]->{'books'} will be an Array that contains a copy of all
the elements from #books.
This might be more "foolproof" then working with references, as
mentioned above.
How to create multiple dimentional array with 2 arrays?
#param=("param1","param2","param3");
#value=("value1_1, value1_2, value1_3", "value2_1, value2_2, value2_3","value3_1, value3_2, value3_3");
Output:
#out=(["param1"]["value1_1", "value1_2", "value1_3"], ["param2"]["value2_1", "value2_2", "value2_3"], ["param3"]["value3_1", "value3_2", "value3_3"])
I've tried this way:
$j=0;
foreach $i(#param1){
push #{$out[$i]}, split(", ", $value[$j]);
$j++;}
It's not exactly clear to me what data structure you want to create.
However, I assume you are trying to create a hash of arrays (a hash table is also known as dictionary or associative array), not an array.
The difference in Perl is that an array always uses integers as keys, whereas a hash always uses strings.
The resulting data structure would then look like
%out = (
'param1' => ['value1_1', 'value1_2', 'value1_3'],
'param2' => ['value2_1', 'value2_2', 'value2_3'],
'param3' => ['value3_1', 'value3_2', 'value3_3'],
);
We can create this data structure like so:
my %out;
for my $i (0 .. $#param) {
$out{$param[$i]} = [split /,\s*/, $value[$i]];
}
Note that $#foo is the highest index in the #foo array. Therefore, 0 .. $#foo would be the range of all indices in #foo. Also note that entries in hashes are accessed with a curly brace subscript $hash{$key}, unlike arrays which use square brackets $array[$index].
You can access multiple elements in a hash or array at the same time by using a slice – #foo{'a', 'b', 'c'} is equivalent to ($foo{a}, $foo{b}, $foo{c}). We can also transform a list of elements by using the map {BLOCK} LIST function. Together, this allows for the following solution:
my %out;
#out{#param} = map { [split /,\s*/, $_] } #value;
Inside the map block, the $_ variable is set to each item in the input list in turn.
To learn more about complex data structures, read (in this order):
perldoc perreftut
perldoc perllol
perldoc perldsc
You can also read the documentation for the map function and for the foreach loop.
I'm a total Perl newb, but still cannot believe I cannot figure this out with all the info I've read through online, but, I've burned too much time and am suffering from block at this point. Hoping to learn something based on my real life example...
Ok, I think I have an array of arrays, created like this:
my #array1 = ();
my #array2 = ();
my $ctr1 = 0;
my $col;
[sql query]
while(($col)=$sth->fetchrow_array() ) {
$array1[$ctr1]=$col;
$ctr1++;
}
print STDERR "#array1";
##results in 10 rows, a mac address in each
##00:00:00:00:00:00 00:11:11:11:11:11 22:22:22:22:22:22 33:33:33:33:33:33 ...
Now I do another query. While looping through results, I am looking for those 10 mac addresses. When I find one, I add a row to array2 with the mac and the sequential number accumulated to the point, like this:
[sql query]
while(($col)=$sth->fetchrow_array() ) {
$ctr2++;
if( my ($matched) = grep $_ eq $col, #array1 ) {
push( #array2, ($col,$ctr2) );
}
}
print STDERR "#array2";
##results in 10 rows, a mac address and an integer in each
##00:00:00:00:00:00 2 00:11:11:11:11:11 24 22:22:22:22:22:22 69 33:33:33:33:33:33 82 ...
Now the easy part. I want to loop through array2, grabbing the mac address to use as part of a sql query. Therein lies the problem. I am so ignorant as to exactly what I am doing that even though I had it almost working, I can't get back to that point. Ignorance is definitely not bliss.
When I loop through array2, I am getting a host of errors, based on the different forms of the statement. The one I think is right is listed below along with the error message...
my $ctr3 = 0;
foreach $ctr3 (#array2) {
my $chkmac = $array2[$ctr3][0]; <--- gacks here with the error below - line 607
[SQL query]
[Thu May 30 14:05:09 2013] [error] Can't use string ("00:66:55:77:99:88") as an ARRAY ref while "strict refs" in use at /path/to/test.cgi line 607.\n
I believe the issue is that my array of arrays is not an array of arrays. If it were, it would work as coded, or so I think from the reading... That said, I cannot fathom what I am dealing with otherwise. This will be a head slapper I'm all but sure, but I am stumped.... Little help, please?
TIA
O
For an array of arrays you want to use an array reference, e.g.
push #array2, [$col, $ctr2];
When accessing an element within an array refernce, you'll want to use the -> operator. Also, when looping through an array, it's not necessary to index back into that same array. So the last part would look more like:
foreach $ctr3 (#array2) {
my $chkmac = $ctr3->[0];
....
When you do the foreach there, $ctrl3 won't have the index in it, it'll have the value. So you should just need to do $ctrl3->[0]. Note the -> which dereferences the array reference (#array2 is actually an array of array references).
EDIT: As AKHolland pointed out, #array2 actually isn't an array of array references, although that's what it should be. You also need to change:
push( #array2, ($col, $ctr2) );
To
push( #array2, [$col, $ctr2] );
This makes an array reference, rather than a list. A list in this context just collapses down into regular arguments to push, meaning you're pushing two separate strings into #array2.
You are correct that your array of arrays is not an array of arrays, since in Perl there is no such thing. So what do you have instead? There's two ways to see.
First, when you print #array2, you come up with a string composed of alternating MACs and counts, separated by spaces. Since the spaces sort-of-signify the division between array elements, we might surmise that what we've got is a single array of heterogeneous elements, such that element 0 is a MAC, element 1 is a count, element 2 is another MAC, and so on.
The other perspective is to look at how #array2 is constructed:
push( #array2, ($col,$ctr2) );
From the documentation for push, we find that push ARRAY LIST works by appending the elements of LIST to the end of ARRAY. This has the effect of flattening the list into the array such that its original identity as a list is lost. You can add all the parentheses you want, when Perl expects a list it flattens all of them away.
So how do you achieve the effect you want? The List-of-Lists documentation has a detailed treatment, but the short answer is that you make a list of array references. Array references are scalars and are therefore legal elements in an array. But they retain their identify as array references!
The anonymous array reference constructor is the square bracket []. In order to push an array reference containing the elements $col and $ctr2 onto the end of #array2, you simply do this:
push( #array2, [$col, $ctr2] );
The code you wrote for accessing a particular element of the array-reference-in-an-array now works. But since I've already written a bunch of paragraphs on the subject, let me finish by explaining what was wrong originally and how changing the push statements suddenly makes it work.
The expression $array2[$ctr3][0] is sometimes written as $array2[$ctr3]->[0] to clarify what it's actually doing. What it does is to take the value of $array2[$ctr3] and treat it as an array reference, taking its 0 element. If we take $ctr3 to be 0 (as it would be at the top of the loop) the value of $array2[$ctr3] is the first element, 00:00:00:00:00:00. When you then subsequently ask Perl to treat 00:00:00:00:00:00 as an array reference, Perl dies because it doesn't know how to treat a string as an array reference.
When instead the value of $array2[$ctr3] is an array reference because that is what you pushed onto #array2 when constructing it, Perl is able to do as you ask, dereferencing the array reference and looking at element 0 of the resulting array, whose value happens to be 00:00:00:00:00:00.
I want to create a reference to an array obtained by a split in Perl.
I'm thinking something like:
my $test = \split( /,/, 'a,b,c,d,e');
foreach $k (#$test) {
print "k is $k\n";
}
But that complains with Not an ARRAY reference at c:\temp\test.pl line 3.
I tried a few other alternatives, all without success.
Background explanation:
split, like other functions, returns a list. You cannot take a reference to a list. However, if you apply the reference operator to a list, it gets applied to all its members. For example:
use Data::Dumper;
my #x = \('a' .. 'c');
print Dumper \#x
Output:
$VAR1 = [
\'a',
\'b',
\'c'
];
Therefore, when you write my $test = \split( /,/, 'a,b,c,d,e');, you get a reference to the last element of the returned list (see, for example, What’s the difference between a list and an array?). Your situation is similar to:
Although it looks like you have a list on the righthand side, Perl actually sees a bunch of scalars separated by a comma:
my $scalar = ( 'dog', 'cat', 'bird' ); # $scalar gets bird
Since you’re assigning to a scalar, the righthand side is in scalar context. The comma operator (yes, it’s an operator!) in scalar context evaluates its lefthand side, throws away the result, and evaluates it’s righthand side and returns the result. In effect, that list-lookalike assigns to $scalar it’s rightmost value. Many people mess this up becuase they choose a list-lookalike whose last element is also the count they expect:
my $scalar = ( 1, 2, 3 ); # $scalar gets 3, accidentally
In your case, what you get on the RHS is a list of references to the elements of the list returned by split, and the last element of that list ends up in $test. You first need to construct an array from those return values and take a reference to that. You can make that a single statement by forming an anonymous array and storing the reference to that in $test:
my $test = [ split( /,/, 'a,b,c,d,e') ];
Surround split command between square brackets to make an anonymous reference.
my $test = [ split( /,/, 'a,b,c,d,e') ];
Giving it a name has different semantics in that changes to the named variable then change what was referenced while each anonymous array is unique. I discovered this the hard way by doing this in a loop.