How to loop over JSON objects in a ruby function

Question

I want to loop over JSON objects and check for a condition in a ruby function. I have 2 classes:

Class A
Class B

Class A has a function that reads the JSON file. This function returns an array of hash objects representing the data:

def self.data
  path = 'data/songs.json'
  file = File.read(path)
  JSON.parse(file)
end

I want to use this method in class B to access the songs.json objects and check if a certain condition is true.

Here is the code for class B:

def mention_count 
  count = 0
  array= []
  array.push(Song.data)
  array.each do |i|
    if(@name === i[:name])
      count +=1;
    end
  end
end

Th JSON file has a key called name, and I am checking if it is identical to the class B variable name. If true the count variable is increased and returned.

The JSON file is an array of JSON object and it looks like this

[
{
"Name": "Bad",
"Artist": "Michael Jackson",
"Record": 825,
},
{
"Name": "Thriller",
"Artist": "Michael Jackson",
"Record": 846,
}]

I am not getting the expected result. The array.push(Song.data) is only adding the first object of the JSON file and not the whole JSON file.

How do I call the method Song.data in class B and loop over the JSON objects?

Could you please post (in your question) some of the sample JSON data you're reading from the file? — SRack
– SRack, Commented Dec 4, 2017 at 17:36

tadman · Accepted Answer · 2017-12-05 19:38:24Z

2

Ruby has a lot of convenience methods that make this sort of thing really simple:

def mention_count
  Song.data.map do |song|
    song['Name']
  end.count(@name)
end

Here the map method is used to extract the song name, then count looks for how many instances of that particular name there are. You can also do it this way:

def mention_count
  Song.data.count do |song|
    @name == song['Name']
  end
end

Where that counts songs with matching names more directly.

Note that in your original code you're doing this:

array= []
array.push(Song.data)

This creates a new array, then adds the array to it as an element. If you wanted to add the elements of Song.data to the end of the array you'd have to do this:

array += Song.data

Which concatenates it. In any case this is irrelevant since you never modify array so you can just use Song.data directly.

edited Dec 5, 2017 at 19:38

answered Dec 4, 2017 at 17:46

tadman

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6 Comments

SRack Over a year ago

I like count do... in there ^^ Very clean and readable. Know of any advantages to using either of your approaches or reduce as in my answer?

tadman Over a year ago

@SRack reduce and it's twin inject are very versatile, but not always as good as their special-purpose counterparts like count.

Shannon Makenna Over a year ago

@tadman I am using the Song.data.count method but the method returns 0 all the time even when there the count should increament. I'm I accessing the name key correctly or is there something else missing?

tadman Over a year ago

You'll want to check you're accessing the right property. What if it's song.name or song['name']? One quick way to find out is to call Song.data.map { |s| s[:name } on the console to see if you're getting the right output. Experiment as necessary.

SRack Over a year ago

Yep, I'd tend to agree tadman - count do slipped my mind when writing the answer. +1 for yours. @ShannonMakenna - I believe it'll be song["Name"] you need - when you parse the JSON it maintains the key structure; if you don't explicitly parse it, it'll be coerced into a hash with symbolised keys. Hope this gets you sorted :)

|

SRack · Accepted Answer · 2017-12-04 17:48:30Z

0

It seems at first glance that you're wrapping an array of data (returned in class A) in another array in class B. Therefore, when you're calling each on the array, the first result is the original array. You get me?

Therefore, you could use:

Song.data.each do ....
  ...
end

Does that help?

Assuming that solves it, there's a purpose built method for this, where you can pass a count while you loop through your objects using reduce(0).

I'd refactor the code in class B a little, as follows:

def mention_count 
  Song.data.reduce(0) do |count, i|
    count += 1 if @name == i[:name]
    count
  end
end

To briefly break this down:

reduce(0) passes the arg (0) along with the objects you're looking to loop through (to count and i respectively)
if @name == i[:name], count is incremented
count is returned, so it's available for the next item we loop through
reduce then returns the value of count when all looping is complete

Hope that helps!

edited Dec 4, 2017 at 17:48

answered Dec 4, 2017 at 17:40

SRack

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3 Comments

tadman Over a year ago

The reduce version here could be count + (@name == i[:name] ? 1 : 0) but that's kind of confusing.

SRack Over a year ago

I did consider that, but went with the above for readability's sake @tadman. I tend towards using tap { ... } on the accumulator when I know it's needed to be returned each timet, but it doesn't work with the integer specified here.

tadman Over a year ago

The tap method is really useful for getting out of a jam, but try not to over-use it. I see people spike it in places it doesn't belong just to save maybe one line of code and introduce a whole bunch of complexity and nuance anyway.

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