Select named array from a hash of array of arrays

Question

I have a hash of an array of arrays. The array is indexed by the hash (that is the way I am reading this):

[
 {"name":"G18","data": [["X301",141],["x7901",57],["x2100",142],["x90",58]]},
 {"name":"G19","data": [["M16",141],["M203",57],["M29S",142]]},
 {"name":"G20","data": [["X301",141],["x7901",57],["x2100",142],["x90",58]]}
]

I want to select the hashes that contain the array G18, and return only the data.

I tried searching for answer, but I haven't found anything like this yet.

Please edit to state whether the number of hashes h for which h.key("name") #=> true and h["name"] === "Gi8" #=> true can be zero or more than one. — Cary Swoveland
– Cary Swoveland, Commented Feb 14, 2018 at 22:00
Note "G18" is not an array and you presumably mean that you want to return the value of :data. If you assign a variable to your array (arr = [{...) you could say that you "wish to return h[:data] for the element of h of arr for which h[:name] == "G18"is true. That assumes there is exactly one such hash h having that property. If zero or more than one could have that property, the statement would have to be adjusted according. — Cary Swoveland
– Cary Swoveland, Commented Feb 14, 2018 at 22:39

Graham · Accepted Answer · 2018-02-15 01:14:50Z

3

This will work for you if you have only one item with name "G18":

a.find {|e| e[:name] == "G18" }[:data]

See: Enumerable#find in the official docs.

edited Feb 15, 2018 at 1:14

Graham

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answered Feb 14, 2018 at 21:14

marmeladze

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3 Comments

Jeremiah S. Over a year ago

Thank you, that solved my problem, greatly appreciated.

Cary Swoveland Over a year ago

a.find {|e| e[:name] == "G17" }[:data] raises an exception because a.find {|e| e[:name] == "G17" } #=> nil and there is no method NilClass#[]. You could use Ruby's Safe Navigation operator to deal with this case: a.find {|e| e[:name] == "G17" }&.[](:data) #=> nil. Note that you cannot write a.find {|e| e[:name] == "G17" }&.[:data].

marmeladze Over a year ago

@CarySwoveland , blame me not -)) just tell those mad pythonistas that they need basic arithmetics, not decimal handling libraries, string representations, logarithms etc -))

jvillian · Accepted Answer · 2018-02-14 21:14:28Z

3

Given:

ary = [
  {"name":"G18","data": [["X301",141],["x7901",57],["x2100",142],["x90",58]]},
  {"name":"G19","data": [["M16",141],["M203",57],["M29S",142]]},
  {"name":"G20","data": [["X301",141],["x7901",57],["x2100",142],["x90",58]]}
]

Try:

ary.select{|hsh| hsh[:name] == 'G18'}.first[:data]
 => [["X301", 141], ["x7901", 57], ["x2100", 142], ["x90", 58]]

In fact, marmeladze's answer is the correct one:

ary.find{|hsh| hsh[:name] == 'G18'}[:data]

Using select was a misfire.

answered Feb 14, 2018 at 21:14

jvillian

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Comments

Dylan Richards · Accepted Answer · 2018-02-14 21:26:07Z

2

collection = [
  {"name":"G18","data": [["X301",141],["x7901",57],["x2100",142],["x90",58]]},
  {"name":"G19","data": [["M16",141],["M203",57],["M29S",142]]},
  {"name":"G20","data": [["X301",141],["x7901",57],["x2100",142],["x90",58]]}
]

def data_for_name(name, collection)
  collection.find { |item| item[:name] == name }[:data]
end

p data_for_name("G18", collection)

answered Feb 14, 2018 at 21:26

Dylan Richards

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4 Comments

Cary Swoveland Over a year ago

How does this differ from @marmeladze's earlier answer?

Dylan Richards Over a year ago

It's always a good idea in Ruby to extract logic such as this into methods. In this case, doing so allows this code to work with any collection of hashes that have name and data keys. Also lets us defer until runtime the decision as to what item we're looking for.

Cary Swoveland Over a year ago

I think what you said is obvious and could be applied equally to any SO answer that has not been wrapped in the obvious method.

Dylan Richards Over a year ago

@CarySwoveland -- As someone who teachers beginners how to code, this is certainly not obvious.

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