I'm pushing my values into the array...
while ( ... ) {
push #array, { label => "label", value => "value" };
}
This appears to be working.
Then...
use JSON::PP ;
print JSON::PP->new->utf8->encode(#array) ;
only generates...
{"value":"value","label":"label"}
but I need...
[{"value":"value","label":"label"}{"value":"value","label":"label"} etc.. ]
(each item in array outputted, not just the first one...)
Any ideas?
Try passing a reference to the array:
print JSON::PP->new->utf8->encode(\#array) ;
Encode is documented to take a scalar, not an array (so you need the reference, which is a scalar).
Related
How to fill list with JSON data?
Here's my code:
my $groups = get_groups($t);
my #group;
my $i = 0;
do {
push(#group, {
groups => [
{ type => $groups->{groups}->[$i]->{type} , group => $groups->{groups}->[$i]->{group} },
]
});
$i++;
} while ($i < length $groups->{groups});
Here is the json sample:
{
"error":false,
"message":"success",
"group":[
{"type":1,"group":"group1"},
{"type":2,"group":"group2"},
{"type":3,"group":"group3"},
{"type":4,"group":"group4"},
{"type":5,"group":"group5"}
]
}
Function get_groups($t); will return above json. I want to get the array group and put it into list groups. But I got:
Can't use string ("0") as a HASH ref while "strict refs" in use
From the documentation of length:
Returns the length in characters of the value of EXPR. If EXPR is
omitted, returns the length of $_ . If EXPR is undefined, returns
undef.
This function cannot be used on an entire array or hash to find out
how many elements these have. For that, use scalar #array and scalar
keys %hash , respectively.
To get the number of elements in an array reference, you need to dereference it and put it into scalar context.
my $foo = [ qw/a b c/ ];
my $number_of_elements = scalar #{ $foo }; # 3
What you actually want to do is loop over every team in the teams array. No need to get the number of elements.
my #teams;
foreach my $team ( #{ $opsteams->{teams} } ) {
push #teams, {
type => $team->{type},
team => $team->{team},
};
}
There are some extra layers of depth in your code. I'm not sure what they are for. It actually looks like you just want the teams in #teams, which really would be
my #teams = #{ $opsteams->{teams} };
I am trying to parse JSON that is coming to me in the form of an array of arrays (think a table of data). The issue is that this table may contain arrays or maps as elements and these elements may by empty. Here is an example:
json <- '[[1,"foo",[],{}],[1,"bar",[1],{"foo":"bar"}]]'
# Result is a list of 2 where each sublist is of length 4
jsonlite::fromJSON(json)
# Result is a character vector of length 6
> unname(unlist(jsonlite::fromJSON(json)))
[1] "1" "foo" "1" "bar" "1" "bar"
So when I try and cast this to a 2 by 4 matrix I am getting the wrong answer. I would like [] to map to the string "[]" and {} to "{}" so I don't lose elements. It is totally fine for me to return the nested array as "[1]" instead of parsing it as a list. To me this seems like I need to tell the json parser to stop recursing and treat the elements as characters at a certain point but I can't figure out how to do this. I'm not tied to the jsonlite package so basically anything is fair game as long as it is not slow.
You could recursively iterate the parsed json to find the empty lists and replace them with the values you want. For example
renameEmptyLists <- function(x) {
if (is.list(x)) {
if( length(x)==0 ) {
return(if(!is.null(names(x))) { "{}" } else {"[]"} )
} else {
return(lapply(x, renameEmptyLists))
}
} else {
x
}
}
jj <- jsonlite::fromJSON(json)
unname(unlist(renameEmptyLists(jj)))
# [1] "1" "foo" "[]" "{}" "1" "bar" "1" "bar"
And to be clear, you where "loosing" them during the unlist(). If you look at the jj object in my example, you will see that the parse correctly identified the empty list and the empty named list.
I am confused about accessing the contents of some JSON data that I have decoded. Here is an example
I don't understand why this solution works and my own does not. My questions are rephrased below
my $json_raw = getJSON();
my $content = decode_json($json_raw);
print Data::Dumper($content);
At this point my JSON data has been transformed into this
$VAR1 = { 'items' => [ 1, 2, 3, 4 ] };
My guess tells me that, once decoded, the object will be a hash with one element that has the key items and an array reference as the value.
$content{'items'}[0]
where $content{'items'} would obtain the array reference, and the outer $...[0] would access the first element in the array and interpret it as a scalar. However this does not work. I get an error message use of uninitialized value [...]
However, the following does work:
$content->{items}[0]
where $content->{items} yields the array reference and [0] accesses the first element of that array.
Questions
Why does $content{'items'} not return an array reference? I even tried #{content{'items'}}, thinking that, once I got the value from content{'items'}, it would need to be interpreted as an array. But still, I receive the uninitialized array reference.
How can I access the array reference without using the arrow operator?
Beginner's answer to beginner :) Sure not as profesional as should be, but maybe helps you.
use strict; #use this all times
use warnings; #this too - helps a lot!
use JSON;
my $json_str = ' { "items" : [ 1, 2, 3, 4 ] } ';
my $content = decode_json($json_str);
You wrote:
My guess tells me that, once decoded, the object will be a hash with
one element that has the key items and an array reference as the value.
Yes, it is a hash, but the the decode_json returns a reference, in this case, the reference to hash. (from the docs)
expects an UTF-8 (binary) string and tries to parse that
as an UTF-8 encoded JSON text,
returning the resulting reference.
In the line
my $content = decode_json($json_str);
you assigning to an SCALAR variable (not to hash).
Because you know: it is a reference, you can do the next:
printf "reftype:%s\n", ref($content);
#print: reftype:HASH ^
#therefore the +------- is a SCALAR value containing a reference to hash
It is a hashref - you can dump all keys
print "key: $_\n" for keys %{$content}; #or in short %$content
#prints: key: items
also you can assing the value of the "items" (arrayref) to an scalar variable
my $aref = $content->{items}; #$hashref->{key}
#or
#my $aref = ${$content}{items}; #$hash{key}
but NOT
#my $aref = $content{items}; #throws error if "use strict;"
#Global symbol "%content" requires explicit package name at script.pl line 20.
The $content{item} is requesting a value from the hash %content and you never defined/assigned such variable. the $content is an scalar variable not hash variable %content.
{
#in perl 5.20 you can also
use 5.020;
use experimental 'postderef';
print "key-postderef: $_\n" for keys $content->%*;
}
Now step deeper - to the arrayref - again you can print out the reference type
printf "reftype:%s\n", ref($aref);
#reftype:ARRAY
print all elements of array
print "arr-item: $_\n" for #{$aref};
but again NOT
#print "$_\n" for #aref;
#dies: Global symbol "#aref" requires explicit package name at script.pl line 37.
{
#in perl 5.20 you can also
use 5.020;
use experimental 'postderef';
print "aref-postderef: $_\n" for $aref->#*;
}
Here is an simple rule:
my #arr; #array variable
my $arr_ref = \#arr; #scalar - containing a reference to #arr
#{$arr_ref} is the same as #arr
^^^^^^^^^^ - array reference in curly brackets
If you have an $arrayref - use the #{$array_ref} everywhere you want use the array.
my %hash; #hash variable
my $hash_ref = \%hash; #scalar - containing a reference to %hash
%{$hash_ref} is the same as %hash
^^^^^^^^^^^ - hash reference in curly brackets
If you have an $hash_ref - use the %{$hash_ref} everywhere you want use the hash.
For the whole structure, the following
say $content->{items}->[0];
say $content->{items}[0];
say ${$content}{items}->[0];
say ${$content}{items}[0];
say ${$content->{items}}[0];
say ${${$content}{items}}[0];
prints the same value 1.
$content is a hash reference, so you always need to use an arrow to access its contents. $content{items} would refer to a %content hash, which you don't have. That's where you're getting that "use of uninitialized value" error from.
I actually asked a similar question here
The answer:
In Perl, a function can only really return a scalar or a list.
Since hashes can be initialized or assigned from lists (e.g. %foo = (a => 1, b => 2)), I guess you're asking why json_decode returns something like { a => 1, b => 2 } (a reference to an anonymous hash) rather than (a => 1, b => 2) (a list that can be copied into a hash).
I can think of a few good reasons for this:
in Perl, an array or hash always contains scalars. So in something like { "a": { "b": 3 } }, the { "b": 3 } part has to be a scalar; and for consistency, it makes sense for the whole thing to be a scalar in the same way.
if the hash is quite large (many keys at top-level), it's pointless and expensive to iterate over all the elements to convert it into a list, and then build a new hash from that list.
in JSON, the top-level element can be either an object (= Perl hash) or an array (= Perl array). If json_decode returned a list in the former case, it's not clear what it would return in the latter case. After decoding the JSON string, how could you examine the result to know what to do with it? (And it wouldn't be safe to write %foo = json_decode(...) unless you already knew that you had a hash.) So json_decode's behavior works better for any general-purpose library code that has to use it without already knowing very much about the data it's working with.
I have to wonder exactly what you passed as an array to json_decode, because my results differ from yours.
#!/usr/bin/perl
use JSON qw (decode_json);
use Data::Dumper;
my $json = '["1", "2", "3", "4"]';
my $fromJSON = decode_json($json);
print Dumper($fromJSON);
The result is $VAR1 = [ '1', '2', '3', '4' ];
Which is an array ref, where your result is a hash ref
So did you pass in a hash with element items which was a reference to an array?
In my example you would get the array by doing
my #array = #{ $fromJSON };
In yours
my #array = #{ $content->{'items'} }
I don't understand why you dislike the arrow operator so much!
The decode_json function from the JSON module will always return a data reference.
Suppose you have a Perl program like this
use strict;
use warnings;
use JSON;
my $json_data = '{ "items": [ 1, 2, 3, 4 ] }';
my $content = decode_json($json_data);
use Data::Dump;
dd $content;
which outputs this text
{ items => [1 .. 4] }
showing that $content is a hash reference. Then you can access the array reference, as you found, with
dd $content->{items};
which shows
[1 .. 4]
and you can print the first element of the array by writing
print $content->{items}[0], "\n";
which, again as you have found, shows just
1
which is the first element of the array.
As #cjm mentions in a comment, it is imperative that you use strict and use warnings at the start of every Perl program. If you had those in place in the program where you tried to access $content{items}, your program would have failed to compile, and you would have seen the message
Global symbol "%content" requires explicit package name
which is a (poorly-phrased) way of telling you that there is no %content so there can be no items element.
The scalar variable $content is completely independent from the hash variable %content, which you are trying to access when you write $content{items}. %content has never been mentioned before and it is empty, so there is no items element. If you had tried #{$content->{items}} then it would have worked, as would #{${$content}{items}}
If you really have a problem with the arrow operator, then you could write
print ${$content}{items}[0], "\n";
which produces the same output; but I don't understand what is wrong with the original version.
I have a classic JSON problem, and i know that many post are asking about that...
But i doubt that the JSON i try to grab has a correct structure.
The files Begin like that :
[{
"time":"0-12h",
"articles":[
{
"id":1,
"domain_id":22,
"title":"Hi Guys"
}
{
"id":2,
"domain_id":17,
"title":"Hi everyone"
}
]
}]
I have try a lot of combinaison to echo the title :
$data = json_decode($json, true);
echo $data->articles;
Or
echo $data->articles->title;
Or
echo $data->articles[0]->title;
Nothing works... :(
Can you help me ?
Thanks !
The second argument true to json_decode() means it should create associative arrays rather than objects for {} in the JSON. So in addition to dealing with the indexed arrays as Explosion Pills points out, you also need to use array syntax to access the keyed elements:
$data[0]['articles'][0]['title']
If you want to be able to use -> syntax, leave out the second argument or set it to false.
I'm hoping the missing comma in the JSON is an error when transcribing to the question. If not, you also need to fix the code that creates the JSON in the first place.
$data itself is an array. Try
$data[0]->articles[0]->title;
Also the JSON is not valid (missing a comma before the second articles array element).
there is a comma , missing
}
,
{
json_decode with the second parameter true returns an array
print_r($data['articles']);
echo $data['articles'] would output Array
I have some JSON data collected via boost and I can not work out how to access some of the data that is in an array:
JSON data : {"dvm_gnd": {"num" : 4, "value": [1,2,3,4]}, "xx_gn" : {"num : 1, "value": 5}}
I can easily get the "num" and single "value" (5) out using:
BOOST_FOREACH(ptree::value_type &v, pt) {
float value = v.second.get<float>("value")
}
However I have no idea how to access the elements of the array out? What does the ptree.get() return?
Thanks
Ross
Try this:
BOOST_FOREACH(ptree::value_type &v, pt.get_child("dvm_gnd.value")) {
float value = v.second.data();
}
I'm sure you've moved on by now, but in case someone else comes across this, ptree puts those array values as children with a blank name, so the code you want looks something like:
BOOST_FOREACH(const ptree::value_type &v, pt.get_child("dvm_gnd.value")) {
float value = v.second.get<float>("");
}
Or you can use the optional or default value version of get