I have a JSON string strcutred like so:
[{"ip":"", "comment":""}, {"ip":"", "comment":""}]
I'm trying to figure out how to remove one of these objects by identification from the IP and/or comment keys.
The closest i've got so far is this:
my $jsn = decode_json('[{"ip":"1.2.3.4", "comment":"one"}, {"ip":"10.10.10.10","comment":"two"}]');
foreach(#{$jsn}){
if($_->{ip} eq '1.2.3.4'){
print "Found!";
splice #{$jsn}, $_, 1;
}
}
I know splice doesn't work in this example. If i could get the index of the object (ideally without a counter), i think i could then remove the correct object.
grep is your friend here. It creates a new list of elements in an existing list that match an expression.
my #filtered = grep { $_->{ip} ne '1.2.3.4' } #$jsn;
Related
I kind of feel stupid for asking this, but haven't been able to find a way to get the key of a JSON value. I know how to retrieve the key if I have an iterator of the object. I also know of operator[].
In my case the key is not a known value, so can't use get(const char *key) or operator[]. Also can't find a getKey() method.
My JSON looks like this:
{Obj_Array: [{"122":{"Member_Array":["241", "642"]}}]}
For the piece of code to parse {"122":{"Member_Array":["241", "642"]}} I want to use get_key()-like function just to retrieve "122" but seems like I have to use an iterator which to me seems to be overkill.
I might have a fundamental lack of understanding of how jsoncpp is representing a JSON file.
First, what you have won't parse in JsonCPP. Keys must always be enclosed in double quotes:
{"Obj_Array": [{"122":{"Member_Array":["241", "642"]}}]}
Assuming that was just an oversight, if we add whitespace and tag the elements:
{
root-> "Obj_Array" : [
elem0-> {
key0-> "122":
val0-> {
key0.1-> "Member_Array" :
val0.1-> [
elem0.1.0-> "241",
elem0.1.1-> "642" ]
}
}
]
}
Assuming you have managed to read your data into a Json::Value (let's call it root), each of the tagged values can be accessed like this:
elem0 = root[0];
val0 = elem0["122"]
val0_1 = val0["Member_Array"];
elem0_1_0 = val0_1[0];
elem0_1_1 = val0_1[1];
You notice that this only retrieves values; the keys were known a priori. This is not unusual; the keys define the schema of the data; you have to know them to directly access the values.
In your question, you state that this is not an option, because the keys are not known. Applying semantic meaning to unknown keys could be challenging, but you already came to the answer. If you want to get the key values, then you do have to iterate over the elements of the enclosing Json::Value.
So, to get to key0, you need something like this (untested):
elem0_members = elem0.getMemberNames();
key0 = elem0_members[0];
This isn't production quality, by any means, but I hope it points in the right direction.
How to create a json file in laravel to store data in json format so that I can use it in ajax coz I'm having problem in autocomplete search. I have done this but it worked in Category not in item.
$catagories=Catagory::where('status',1)->get(['name'])->toArray();
$items=Item::where('status',1)->get()->toArray();
$cats=array();
$item=array();
foreach($catagories as $cat){
array_push($cats,$cat['name'])
}
file_put_contents('category.json',json_encode($cats));
file_put_contents('items.json',json_encode($item));
Your variable $item is never used, so it won't print anything on the file. You need to use the $items variable
Try
file_put_contents('items.json',json_encode($items));
I think that it is because your item array is empty
$catagories=Catagory::where('status',1)->get(['name'])->toArray();
$items=Item::where('status',1)->get()->toArray();
$cats=array();
$item=array();
foreach($catagories as $cat){
array_push($cats,$cat['name'])
}
file_put_contents('category.json',json_encode($cats));
file_put_contents('items.json',json_encode($item));
You are using $item array on last line which is empty, coz you didn't fill it from $items array
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 document like this:
{
Name : val
AnArray : [
{
Time : SomeTime
},
{
Time : AnotherTime
}
...arbitrary more elements
}
I need to update "Time" to a Date type (right now it is string)
I would like to do something psudo like:
foreach record in document.AnArray { record.Time = new Date(record.Time) }
I've read the documentation on $ and "dot" notation as well as a several similar questions here, I tried this code:
db.collection.update({_id:doc._id},{$set : {AnArray.$.Time : new Date(AnArray.$.Time)}});
And hoping that $ would iterate the indexes of the "AnArray" property as I don't know for each record the length of it. But am getting the error:
SyntaxError: missing : after property id (shell):1
How can I perform an update on each member of the arrays nested values with a dynamic value?
There's no direct way to do that, because MongoDB doesn't support an update-expression that references the document. Moreover, the $ operator only applies to the first match, so you'd have to perform this as long as there are still fields where AnArray.Time is of $type string.
You can, however, perform that update client side, in your favorite language or in the mongo console using JavaScript:
db.collection.find({}).forEach(function (doc) {
for(var i in doc.AnArray)
{
doc.AnArray[i].Time = new Date(doc.AnArray[i].Time);
}
db.outcollection.save(doc);
})
Note that this will store the migrated data in a different collection. You can also update the collection in-place by replacing outcollection with collection.
I am trying to output some data from Perl to JSON. I can do a simple output, but would like to structure it better.
I have an array with an id, a start time and an end time. This is the code I am using to output:
print header('application/json');
my $json->{$entry} = \#array;
my $json_text = to_json($json);
print $json_text;
Which returns:
{"Season":[["1","1330065300","1344038401"],["7","1298505601","1312416001"]]}
But I would like to output something more like:
{"Season":0[{"id":1,"DateStart":1330065300,"DateEnd":1344038401},{"id":7,"DateStart":1298505601,"DateEnd":1312416001}]}
Can anyone help on how to better structure my output?
---UPDATE------
Thanks Michael. I have tried to implement your example.
This is the code at the moment:
foreach my $key (keys %$seasons)
{
$seasons->{$key} =
[
map
{
{ id=>$_[0], DateStart=>$_[1], DateEnd=>$_[2] }
} #{$seasons->{$key}}
];
}
But it returns the error (referring to the foreach line):
Not a HASH reference at line 148
$seasons is an arrayref return from a SQL fetchall_arrayref
Any clues?
You basically want to convert an array of arrays to an array of hashes, and you can do this using map. Assuming $data is your structure, this should do it:
for my $key (keys %$data) {
$data->{$key} = [
map {
{ id => $_->[0], DateStart => $_->[1], DateEnd => $_->[2] }
} #{$data->{$key}}
];
}
If you want to output an array of objects with key/value pairs instead of an array of arrays, then put appropriate data into to_json in the first place.
i.e. an array of hashrefs and not an array of arrayrefs.
You can use map to transform the data.
Whenever you're trying something like this, always check CPAN to see if someone has done it before and not try to reinvent the wheel. I found a module called JSON that seems to do exactly what you want.
There's an example on that page that does exactly what you want. Here's a quick paraphrase:
use JSON; # imports encode_json, decode_json, to_json and from_json.
# simple and fast interfaces (expect/generate UTF-8)
my $utf8_encoded_json_text = encode_json \#array;
Can't get easier than that. The best part is that this will work no matter how complex your array structure gets.