Ruby collection of strings as a parameter - ruby

I have a players() method that returns all players names.
I want to use this, as a parameter to twitter streaming, like this:
#client.track(players) do |tweet|
The gem does not allow it, so I have to do this:
#client.track('player1', 'player2', 'player3') do |tweet|
But that's terrible, since I have so many players.
Is that a way to parse my array into that list of strings?


I think what you're looking for is *, the "splat" operator:
players = [ "player1", "player2", "player3" ]
#client.track( *players )
# ...is equivalent to...
#client.track( "player1", "player2", "player3" )
Putting a splat before any object that responds to to_ary (like an Array, Hash, and most other Enumerables) will transform its items into an argument list. It's very handy and you'll see it a lot as you explore more Ruby code. Here's another example:
a = [ 3, 4 ]
[ 1, 2 ].push( a ) # no splat
# => [ 1, 2, [ 3, 4 ] ]
[ 1, 2 ].push( *a ) # splat!
# => [ 1, 2, 3, 4 ]
You can use it directly on objects, too—not just variables:
[ 1, 2, *[ 3, 4 ] ]
# => [ 1, 2, 3, 4 ]

Related

Difficulty when constructing a nested data structure

While trying to create a JSON message for an API, I found myself struggling to do something that I thought would be simple. I needed to create a message like the following:
{ "list": [ { "foo": 1, "bar": 2 } ] }
However, my first attempt did not work:
say to-json { foo => [ { a => 1, b => 2 } ] };
# {"foo":[{"a":1},{"b":2}]}
Trying to simplify things further confused me more:
say { foo => [ { a => 1 } ] };
# {foo => [a => 1]}
# Note that this is not JSON, but I expected to see curly braces
Then I tried to use some temporary variables, and that worked:
my #list = { a => 1 };
say to-json { foo => #list };
# {"foo":[{"a":1}]}
my %hash = ( a => 1 );
say to-json { foo => [ %hash ] };
# {"foo":[{"a":1}]}
What's going on here?
And is there a way I can achieve my desired output without an extra temporary variable?

You've discovered the single argument rule. Numerous constructs in Raku will iterate the argument they are provided with. This includes the [...] array composer. This is why when we say:
say [1..10];
We get an array that contains 10 elements, not 1. However, it also means that:
say [[1,2]];
Iterates the [1,2], and thus results in [1,2] - as if the inner array were not there. A Hash iterates to its pairs, thus:
{ foo => [ { a => 1, b => 2 } ] }
Actually produces:
{ foo => [ a => 1, b => 2 ] }
That is, the array has the pairs. The JSON serializer then serializes each pair as a one-element object.
The solution is to produce a single-element iterable. The infix , operator is what produces lists, so we can use that:
say to-json { foo => [ { a => 1, b => 2 }, ] };
# note the , here ^
Then the single argument to be iterated is a 1-element list with a hash, and you get the result you want.
Easy way to remember it: always use trailing commas when specifying the values of a list, array or hash, even with a single element list, unless you actually are specifying the single iterable from which to populate it.

Translate Ruby hash (key,value) to separate keys

I have a map function in ruby which returns an array of arrays with two values in each, which I want to have in a different format.
What I want to have:
"countries": [
{
"country": "Canada",
"count": 12
},
{and so on... }
]
But map obviously returns my values as array:
"countries": [
[
"Canada",
2
],
[
"Chile",
1
],
[
"China",
1
]
]
When using Array::to_h I am also able to bringt it closer to the format I actually want to have.
"countries": {
"Canada": 2,
"Chile": 1,
"China": 1,
}
I have tried reduce/inject, each_with_object but in both cases I do not understand how to access the incoming parameters. While searching here you find many many similar problems. But haven't found a way to adapt those to my case.
Hope you can help to find a short and elegant solution.

You are given two arrays:
countries= [['Canada', 2], ['Chile', 1], ['China', 1]]
keys = [:country, :count]
You could write
[keys].product(countries).map { |arr| arr.transpose.to_h }
#=> [{:country=>"Canada", :count=>2},
# {:country=>"Chile", :count=>1},
# {:country=>"China", :count=>1}]
or simply
countries.map { |country, cnt| { country: country, count: cnt } }
#=> [{:country=>"Canada", :count=>2},
# {:country=>"Chile", :count=>1},
# {:country=>"China", :count=>1}]
but the first has the advantage that no code need be changed in the names of the keys change. In fact, there would be no need to change the code if the arrays countries and keys both changed, provided countries[i].size == keys.size for all i = 0..countries.size-1. (See the example at the end.)
The initial step for the first calculation is as follows.
a = [keys].product(countries)
#=> [[[:country, :count], ["Canada", 2]],
# [[:country, :count], ["Chile", 1]],
# [[:country, :count], ["China", 1]]]
See Array#product. We now have
a.map { |arr| arr.transpose.to_h }
map passes the first element of a to the block and sets the block variable arr to that value:
arr = a.first
#=> [[:country, :count], ["Canada", 2]]
The block calculation is then performed:
b = arr.transpose
#=> [[:country, "Canada"], [:count, 2]]
b.to_h
#=> {:country=>"Canada", :count=>2}
So we see that a[0] (arr) is mapped to {:country=>"Canada", :count=>2}. The next two elements of a are then passed to the block and similar calculations are made, after which map returns the desired array of three hashes. See Array#transpose and Array#to_h.
Here is a second example using the same code.
countries= [['Canada', 2, 9.09], ['Chile', 1, 0.74],
['China', 1, 9.33], ['France', 1, 0.55]]
keys = [:country, :count, :area]
[keys].product(countries).map { |arr| arr.transpose.to_h }
#=> [{:country=>"Canada", :count=>2, :area=>9.09},
# {:country=>"Chile", :count=>1, :area=>0.74},
# {:country=>"China", :count=>1, :area=>9.33},
# {:country=>"France", :count=>1, :area=>0.55}]

Just out of curiosity:
countries = [['Canada', 2], ['Chile', 1], ['China', 1]]
countries.map(&%i[country count].method(:zip)).map(&:to_h)
#⇒ [{:country=>"Canada", :count=>2},
# {:country=>"Chile", :count=>1},
# {:country=>"China", :count=>1}]

Ruby inject condition

I have an array filled with hashes. The data structure looks like this:
students = [
{
"first_name" => "James",
"last_name" => "Sullivan",
"age" => 20,
"study_results" => {"CAR" => 1, "PR1" => 1, "MA1" => 1, "BEN" => 2, "SDP" => nil}
}
]
I want to find students with the mark 1 from at least two subjects.
I tried to convert the hash with marks into an array and then use the inject method to count the number of 1 and find out if the number is > 1:
students.select{|student
(
(student["study_results"].to_a)
.inject(0){|sum, x| sum += 1 if x.include?(1)}
) > 1
}
Is there any way to put a condition into the method, or should I find a different way to solve it?

I commend you for attempting to solve this before posting, but you made it unnecessarily complicated. I'd write it like this:
students.select{|student| student['study_results'].values.count(1) >= 2}
That's all, no need for inject. You were misusing it here.
Ruby collections have TONS of useful methods. If you find yourself using inject or each, there's a better method for this, 90% of the time.
Explanation:
student['study_results'] # => {"CAR"=>1, "PR1"=>1, "MA1"=>1, "BEN"=>2, "SDP"=>nil}
student['study_results'].values # => [1, 1, 1, 2, nil]
student['study_results'].values.count(1) # => 3
student['study_results'].values.count(2) # => 1
student['study_results'].values.count(3) # => 0
student['study_results'].values.count(nil) # => 1
Documentation
Hash#values
Array#count

Statistics with Ruby

I'm writing a command line tool to calculate statistical data and I'm using minitest. load_data returns an array of user input. I would like to know why does this test pass when I use the method concat, but if I use the << operator the test passes even when empty. Aren't these two the same thing?
class Test < MiniTest::Unit::TestCase
def setup
#collection = DataSet.new
end
def test_data_is_not_empty
assert ! #collection.load_data.empty?
end
end
class DataSet
def initialize
#collected = []
end
def append
print 'Please input a list of data: '
value = gets.chomp.split(',').map(&:to_f)
#collected.concat(value)
end
end

Use concat:
[ "a", "b" ].concat( ["c", "d"] ) #=> [ "a", "b", "c", "d" ]
a = [ 1, 2, 3 ]
a.concat( [ 4, 5 ] )
a #=> [ 1, 2, 3, 4, 5 ]
In your case, it looks like it would be something like
#collected.concat( load_data )
If load_data is a member of another object, such as #collection, then do
#collected.concat( #collection.load_data )
If you want to append each element my_data at a time from within load_data, then do
#collected << my_input

What is the best way to convert an array to a hash in Ruby

In Ruby, given an array in one of the following forms...
[apple, 1, banana, 2]
[[apple, 1], [banana, 2]]
...what is the best way to convert this into a hash in the form of...
{apple => 1, banana => 2}

Simply use Hash[*array_variable.flatten]
For example:
a1 = ['apple', 1, 'banana', 2]
h1 = Hash[*a1.flatten(1)]
puts "h1: #{h1.inspect}"
a2 = [['apple', 1], ['banana', 2]]
h2 = Hash[*a2.flatten(1)]
puts "h2: #{h2.inspect}"
Using Array#flatten(1) limits the recursion so Array keys and values work as expected.

NOTE: For a concise and efficient solution, please see Marc-André Lafortune's answer below.
This answer was originally offered as an alternative to approaches using flatten, which were the most highly upvoted at the time of writing. I should have clarified that I didn't intend to present this example as a best practice or an efficient approach. Original answer follows.
Warning! Solutions using flatten will not preserve Array keys or values!
Building on #John Topley's popular answer, let's try:
a3 = [ ['apple', 1], ['banana', 2], [['orange','seedless'], 3] ]
h3 = Hash[*a3.flatten]
This throws an error:
ArgumentError: odd number of arguments for Hash
from (irb):10:in `[]'
from (irb):10
The constructor was expecting an Array of even length (e.g. ['k1','v1,'k2','v2']). What's worse is that a different Array which flattened to an even length would just silently give us a Hash with incorrect values.
If you want to use Array keys or values, you can use map:
h3 = Hash[a3.map {|key, value| [key, value]}]
puts "h3: #{h3.inspect}"
This preserves the Array key:
h3: {["orange", "seedless"]=>3, "apple"=>1, "banana"=>2}

The best way is to use Array#to_h:
[ [:apple,1],[:banana,2] ].to_h #=> {apple: 1, banana: 2}
Note that to_h also accepts a block:
[:apple, :banana].to_h { |fruit| [fruit, "I like #{fruit}s"] }
# => {apple: "I like apples", banana: "I like bananas"}
Note: to_h accepts a block in Ruby 2.6.0+; for early rubies you can use my backports gem and require 'backports/2.6.0/enumerable/to_h'
to_h without a block was introduced in Ruby 2.1.0.
Before Ruby 2.1, one could use the less legible Hash[]:
array = [ [:apple,1],[:banana,2] ]
Hash[ array ] #= > {:apple => 1, :banana => 2}
Finally, be wary of any solutions using flatten, this could create problems with values that are arrays themselves.

Update
Ruby 2.1.0 is released today. And I comes with Array#to_h (release notes and ruby-doc), which solves the issue of converting an Array to a Hash.
Ruby docs example:
[[:foo, :bar], [1, 2]].to_h # => {:foo => :bar, 1 => 2}

Edit: Saw the responses posted while I was writing, Hash[a.flatten] seems the way to go.
Must have missed that bit in the documentation when I was thinking through the response. Thought the solutions that I've written can be used as alternatives if required.
The second form is simpler:
a = [[:apple, 1], [:banana, 2]]
h = a.inject({}) { |r, i| r[i.first] = i.last; r }
a = array, h = hash, r = return-value hash (the one we accumulate in), i = item in the array
The neatest way that I can think of doing the first form is something like this:
a = [:apple, 1, :banana, 2]
h = {}
a.each_slice(2) { |i| h[i.first] = i.last }

You can also simply convert a 2D array into hash using:
1.9.3p362 :005 > a= [[1,2],[3,4]]
=> [[1, 2], [3, 4]]
1.9.3p362 :006 > h = Hash[a]
=> {1=>2, 3=>4}

Summary & TL;DR:
This answer hopes to be a comprehensive wrap-up of information from other answers.
The very short version, given the data from the question plus a couple extras:
flat_array = [ apple, 1, banana, 2 ] # count=4
nested_array = [ [apple, 1], [banana, 2] ] # count=2 of count=2 k,v arrays
incomplete_f = [ apple, 1, banana ] # count=3 - missing last value
incomplete_n = [ [apple, 1], [banana ] ] # count=2 of either k or k,v arrays
# there's one option for flat_array:
h1 = Hash[*flat_array] # => {apple=>1, banana=>2}
# two options for nested_array:
h2a = nested_array.to_h # since ruby 2.1.0 => {apple=>1, banana=>2}
h2b = Hash[nested_array] # => {apple=>1, banana=>2}
# ok if *only* the last value is missing:
h3 = Hash[incomplete_f.each_slice(2).to_a] # => {apple=>1, banana=>nil}
# always ok for k without v in nested array:
h4 = Hash[incomplete_n] # or .to_h => {apple=>1, banana=>nil}
# as one might expect:
h1 == h2a # => true
h1 == h2b # => true
h1 == h3 # => false
h3 == h4 # => true
Discussion and details follow.
Setup: variables
In order to show the data we'll be using up front, I'll create some variables to represent various possibilities for the data. They fit into the following categories:
Based on what was directly in the question, as a1 and a2:
(Note: I presume that apple and banana were meant to represent variables. As others have done, I'll be using strings from here on so that input and results can match.)
a1 = [ 'apple', 1 , 'banana', 2 ] # flat input
a2 = [ ['apple', 1], ['banana', 2] ] # key/value paired input
Multi-value keys and/or values, as a3:
In some other answers, another possibility was presented (which I expand on here) – keys and/or values may be arrays on their own:
a3 = [ [ 'apple', 1 ],
[ 'banana', 2 ],
[ ['orange','seedless'], 3 ],
[ 'pear', [4, 5] ],
]
Unbalanced array, as a4:
For good measure, I thought I'd add one for a case where we might have an incomplete input:
a4 = [ [ 'apple', 1],
[ 'banana', 2],
[ ['orange','seedless'], 3],
[ 'durian' ], # a spiky fruit pricks us: no value!
]
Now, to work:
Starting with an initially-flat array, a1:
Some have suggested using #to_h (which showed up in Ruby 2.1.0, and can be backported to earlier versions). For an initially-flat array, this doesn't work:
a1.to_h # => TypeError: wrong element type String at 0 (expected array)
Using Hash::[] combined with the splat operator does:
Hash[*a1] # => {"apple"=>1, "banana"=>2}
So that's the solution for the simple case represented by a1.
With an array of key/value pair arrays, a2:
With an array of [key,value] type arrays, there are two ways to go.
First, Hash::[] still works (as it did with *a1):
Hash[a2] # => {"apple"=>1, "banana"=>2}
And then also #to_h works now:
a2.to_h # => {"apple"=>1, "banana"=>2}
So, two easy answers for the simple nested array case.
This remains true even with sub-arrays as keys or values, as with a3:
Hash[a3] # => {"apple"=>1, "banana"=>2, ["orange", "seedless"]=>3, "pear"=>[4, 5]}
a3.to_h # => {"apple"=>1, "banana"=>2, ["orange", "seedless"]=>3, "pear"=>[4, 5]}
But durians have spikes (anomalous structures give problems):
If we've gotten input data that's not balanced, we'll run into problems with #to_h:
a4.to_h # => ArgumentError: wrong array length at 3 (expected 2, was 1)
But Hash::[] still works, just setting nil as the value for durian (and any other array element in a4 that's just a 1-value array):
Hash[a4] # => {"apple"=>1, "banana"=>2, ["orange", "seedless"]=>3, "durian"=>nil}
Flattening - using new variables a5 and a6
A few other answers mentioned flatten, with or without a 1 argument, so let's create some new variables:
a5 = a4.flatten
# => ["apple", 1, "banana", 2, "orange", "seedless" , 3, "durian"]
a6 = a4.flatten(1)
# => ["apple", 1, "banana", 2, ["orange", "seedless"], 3, "durian"]
I chose to use a4 as the base data because of the balance problem we had, which showed up with a4.to_h. I figure calling flatten might be one approach someone might use to try to solve that, which might look like the following.
flatten without arguments (a5):
Hash[*a5] # => {"apple"=>1, "banana"=>2, "orange"=>"seedless", 3=>"durian"}
# (This is the same as calling `Hash[*a4.flatten]`.)
At a naïve glance, this appears to work – but it got us off on the wrong foot with the seedless oranges, thus also making 3 a key and durian a value.
And this, as with a1, just doesn't work:
a5.to_h # => TypeError: wrong element type String at 0 (expected array)
So a4.flatten isn't useful to us, we'd just want to use Hash[a4]
The flatten(1) case (a6):
But what about only partially flattening? It's worth noting that calling Hash::[] using splat on the partially-flattened array (a6) is not the same as calling Hash[a4]:
Hash[*a6] # => ArgumentError: odd number of arguments for Hash
Pre-flattened array, still nested (alternate way of getting a6):
But what if this was how we'd gotten the array in the first place?
(That is, comparably to a1, it was our input data - just this time some of the data can be arrays or other objects.) We've seen that Hash[*a6] doesn't work, but what if we still wanted to get the behavior where the last element (important! see below) acted as a key for a nil value?
In such a situation, there's still a way to do this, using Enumerable#each_slice to get ourselves back to key/value pairs as elements in the outer array:
a7 = a6.each_slice(2).to_a
# => [["apple", 1], ["banana", 2], [["orange", "seedless"], 3], ["durian"]]
Note that this ends up getting us a new array that isn't "identical" to a4, but does have the same values:
a4.equal?(a7) # => false
a4 == a7 # => true
And thus we can again use Hash::[]:
Hash[a7] # => {"apple"=>1, "banana"=>2, ["orange", "seedless"]=>3, "durian"=>nil}
# or Hash[a6.each_slice(2).to_a]
But there's a problem!
It's important to note that the each_slice(2) solution only gets things back to sanity if the last key was the one missing a value. If we later added an extra key/value pair:
a4_plus = a4.dup # just to have a new-but-related variable name
a4_plus.push(['lychee', 4])
# => [["apple", 1],
# ["banana", 2],
# [["orange", "seedless"], 3], # multi-value key
# ["durian"], # missing value
# ["lychee", 4]] # new well-formed item
a6_plus = a4_plus.flatten(1)
# => ["apple", 1, "banana", 2, ["orange", "seedless"], 3, "durian", "lychee", 4]
a7_plus = a6_plus.each_slice(2).to_a
# => [["apple", 1],
# ["banana", 2],
# [["orange", "seedless"], 3], # so far so good
# ["durian", "lychee"], # oops! key became value!
# [4]] # and we still have a key without a value
a4_plus == a7_plus # => false, unlike a4 == a7
And the two hashes we'd get from this are different in important ways:
ap Hash[a4_plus] # prints:
{
"apple" => 1,
"banana" => 2,
[ "orange", "seedless" ] => 3,
"durian" => nil, # correct
"lychee" => 4 # correct
}
ap Hash[a7_plus] # prints:
{
"apple" => 1,
"banana" => 2,
[ "orange", "seedless" ] => 3,
"durian" => "lychee", # incorrect
4 => nil # incorrect
}
(Note: I'm using awesome_print's ap just to make it easier to show the structure here; there's no conceptual requirement for this.)
So the each_slice solution to an unbalanced flat input only works if the unbalanced bit is at the very end.
Take-aways:
Whenever possible, set up input to these things as [key, value] pairs (a sub-array for each item in the outer array).
When you can indeed do that, either #to_h or Hash::[] will both work.
If you're unable to, Hash::[] combined with the splat (*) will work, so long as inputs are balanced.
With an unbalanced and flat array as input, the only way this will work at all reasonably is if the last value item is the only one that's missing.
Side-note: I'm posting this answer because I feel there's value to be added – some of the existing answers have incorrect information, and none (that I read) gave as complete an answer as I'm endeavoring to do here. I hope that it's helpful. I nevertheless give thanks to those who came before me, several of whom provided inspiration for portions of this answer.

Appending to the answer but using anonymous arrays and annotating:
Hash[*("a,b,c,d".split(',').zip([1,2,3,4]).flatten)]
Taking that answer apart, starting from the inside:
"a,b,c,d" is actually a string.
split on commas into an array.
zip that together with the following array.
[1,2,3,4] is an actual array.
The intermediate result is:
[[a,1],[b,2],[c,3],[d,4]]
flatten then transforms that to:
["a",1,"b",2,"c",3,"d",4]
and then:
*["a",1,"b",2,"c",3,"d",4] unrolls that into
"a",1,"b",2,"c",3,"d",4
which we can use as the arguments to the Hash[] method:
Hash[*("a,b,c,d".split(',').zip([1,2,3,4]).flatten)]
which yields:
{"a"=>1, "b"=>2, "c"=>3, "d"=>4}

if you have array that looks like this -
data = [["foo",1,2,3,4],["bar",1,2],["foobar",1,"*",3,5,:foo]]
and you want the first elements of each array to become the keys for the hash and the rest of the elements becoming value arrays, then you can do something like this -
data_hash = Hash[data.map { |key| [key.shift, key] }]
#=>{"foo"=>[1, 2, 3, 4], "bar"=>[1, 2], "foobar"=>[1, "*", 3, 5, :foo]}

Not sure if it's the best way, but this works:
a = ["apple", 1, "banana", 2]
m1 = {}
for x in (a.length / 2).times
m1[a[x*2]] = a[x*2 + 1]
end
b = [["apple", 1], ["banana", 2]]
m2 = {}
for x,y in b
m2[x] = y
end

For performance and memory allocation concerns please check my answer to Rails mapping array of hashes onto single hash where I bench-marked several solutions.
reduce / inject can be the fastest or the slowest solution depending on which method you use it which.

If the numeric values are seq indexes, then we could have simpler ways...
Here's my code submission, My Ruby is a bit rusty
input = ["cat", 1, "dog", 2, "wombat", 3]
hash = Hash.new
input.each_with_index {|item, index|
if (index%2 == 0) hash[item] = input[index+1]
}
hash #=> {"cat"=>1, "wombat"=>3, "dog"=>2}

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