[Ruby] How to use the map method. How to process the value of an object and get it by hash or symbol.

This is a personal memo.

By using the map method, you can get an array that processes each element of the original object.

・object.map{|variable|processing} ┗ Variable contains each element of the object ┗ Processing result is returned as an array ┗ Non-destructive processing (as is of the original object)

table of contents

1. For range objects (#rangeオブジェクトの場合)
2. For arrays
3. One-dimensional array
4. Two-dimensional array
5. Multidimensional Array
6. For objects (#オブジェクトの場合)
7. When there are two arguments
8. Extract keys as an array (#キーを配列として取り出す)
9. Extract values ​​as an array (#値を配列として取り出す)
10. Process the value of the object and retrieve it as an object (#オブジェクトの値を処理してオブジェクトとして取り出す)
11. Convert from array to symbol (#配列からシンボルに変換)
12. Convert from array to hash (#配列からハッシュに変換)
13. When the object has one argument

python

range = 1..3
range2 = range.map{|x| x*2 }

p range2   
#output
[2, 4, 6]

1D

arr = [1, 2, 3]
arr2 = arr.map{|x| x*2 }

p arr2   
#output
[2, 4, 6]

[1, 2] * 2 = [1, 2, 1, 2]

2D

arr = [1, [2, 3]]
arr2 = arr.map{|x| x*2 }

p arr2   
#output
[2, [2, 3, 2, 3]]

[[1, 2]] * 2 = [[1, 2], [1, 2]]

multidimensional

arr =  [1, [2,3], [4, 5, [6, 7]],[[8, 9]]]
arr2 = arr.map{|x| x*2 }

p arr2   
#output
[2, [2, 3, 2, 3], [4, 5, [6, 7], 4, 5, [6, 7]], [[8, 9], [8, 9]]]

In addition, the processing result is ** returned as an array **.

As a general usage, it is used when you want to retrieve only the key or value of an object as an array. (Specify two variables)

Extract keys as an array

Extract only keys as an array

obj = {:a=>1, :b=>2, :c=>3}
obj2 = obj.map{|x, y| x }

p obj2
#output
[:a, :b, :c]

error

obj = {:a=>1, :b=>2, :c=>3}
obj2 = obj.map{|x, y| x*2 }

NoMethodError (undefined method `*' for :a:Symbol)

Take out as it is

obj = {:a=>1, :b=>2, :c=>3}
obj2 = obj.map{|x, y| y }

p obj2
#output
[1, 2, 3]

Example of four arithmetic operations

obj = {:a=>1, :b=>2, :c=>3}
obj2 = obj.map{|x, y| y*2 }

p obj2
#output
[2, 4, 6]

▼ Convert from array to symbol

Create [key, process] in the process result, Convert to a symbol with the to_h method.

python

obj = {:a=>1, :b=>2, :c=>3}
obj2 = obj.map{|x, y| [x, y*2]}

p obj2
#output
[[:a, 2], [:b, 4], [:c, 6]]

p obj2.to_h
#output
{:a=>2, :b=>4, :c=>6}

python

obj = {:a=>1, :b=>2, :c=>3}
obj2 = obj.map{|x, y| [x, y*2]}.to_h

p obj2
#output
{:a=>2, :b=>4, :c=>6}

Create [key, process] in the process result, to_h{|Variable 1,Variable 2| [Variable 1.to_s,processing]}Convert to hash with.

python

obj = {:a=>1, :b=>2, :c=>3}
 obj2 = obj.map{|x, y| [x, y*2]}.to_h{|v, w| [v.to_s, w]}

p obj2
#output
{"a"=>2, "b"=>4, "c"=>6}

When processed, KV pairs are processed as a set.

python

obj = {:a=>1, :b=>2, :c=>3}
obj2 = obj.map{|x| x*2 }

p obj2   
#output
[[:a, 1, :a, 1], [:b, 2, :b, 2], [:c, 3, :c, 3]]