Collections

Enumerable Map Select

Enumerable methods such as map and select transform and filter collections.

enumerable Many Ruby collections include Enumerable methods for common traversal patterns.

Enumerable Map Select

minimum
enumerable_map_select.rb
Replay: real traced execution (multi-file project)
numbers = [2, 4, 6, 8]
minimum = 6

doubled = numbers.map do |number|
  number * 2
end

selected = doubled.select do |number|
  number >= minimum
end

puts "doubled=#{doubled.join(",")}"
puts "minimum=#{minimum}"
puts "selected=#{selected.join(",")}"

numbers = [2, 4, 6, 8]
minimum = 4

doubled = numbers.map do |number|
  number * 2
end

selected = doubled.select do |number|
  number >= minimum
end

puts "doubled=#{doubled.join(",")}"
puts "minimum=#{minimum}"
puts "selected=#{selected.join(",")}"

numbers = [2, 4, 6, 8]
minimum = 10

doubled = numbers.map do |number|
  number * 2
end

selected = doubled.select do |number|
  number >= minimum
end

puts "doubled=#{doubled.join(",")}"
puts "minimum=#{minimum}"
puts "selected=#{selected.join(",")}"

  1. numbers ← [2, 4, 6, 8], minimum ← 6

    1numbers→ [2, 4, 6, 8] = [2, 4, 6, 8]2minimum→ 6 = 6  #@minimum=4, 1034doubled = numbers[2, 4, 6, 8].map do |number|5  number * 26end

  2. do |number|

    pass 1 of 4
    4doubled = numbers.map do |number2|5  number * 26end
    All 4 passes — pass 1 is the card above
    passnumbernumbersdoubled
    12——
    24——
    36——
    48[2, 4, 6, 8][4, 8, 12, 16]

  3. selected = doubled.select do |number|

    8selected = doubled[4, 8, 12, 16].select do |number|9  number >= minimum10end

  4. do |number|

    pass 1 of 4
    8selected = doubled.select do |number4|9  number >= minimum10end
    All 4 passes — pass 1 is the card above
    passnumberdoubledselected
    14——
    28——
    312——
    416[4, 8, 12, 16][8, 12, 16]

  5. puts "doubled=#{doubled.join(",")}"

    12puts "doubled=#{doubled.join(",")4,8,12,16}"13puts "minimum=#{minimum6}"14puts "selected=#{selected.join(",")8,12,16}"
    outputdoubled=4,8,12,16
minimum=6
selected=8,12,16

  1. numbers ← [2, 4, 6, 8], minimum ← 4

    1numbers→ [2, 4, 6, 8] = [2, 4, 6, 8]2minimum→ 4 = 434doubled = numbers[2, 4, 6, 8].map do |number|5  number * 26end

  2. do |number|

    pass 1 of 4
    4doubled = numbers.map do |number2|5  number * 26end
    All 4 passes — pass 1 is the card above
    passnumbernumbersdoubled
    12——
    24——
    36——
    48[2, 4, 6, 8][4, 8, 12, 16]

  3. selected = doubled.select do |number|

    8selected = doubled[4, 8, 12, 16].select do |number|9  number >= minimum10end

  4. do |number|

    pass 1 of 4
    8selected = doubled.select do |number4|9  number >= minimum10end
    All 4 passes — pass 1 is the card above
    passnumberdoubledselected
    14——
    28——
    312——
    416[4, 8, 12, 16][4, 8, 12, 16]

  5. puts "doubled=#{doubled.join(",")}"

    12puts "doubled=#{doubled.join(",")4,8,12,16}"13puts "minimum=#{minimum4}"14puts "selected=#{selected.join(",")4,8,12,16}"
    outputdoubled=4,8,12,16
minimum=4
selected=4,8,12,16

  1. numbers ← [2, 4, 6, 8], minimum ← 10

    1numbers→ [2, 4, 6, 8] = [2, 4, 6, 8]2minimum→ 10 = 1034doubled = numbers[2, 4, 6, 8].map do |number|5  number * 26end

  2. do |number|

    pass 1 of 4
    4doubled = numbers.map do |number2|5  number * 26end
    All 4 passes — pass 1 is the card above
    passnumbernumbersdoubled
    12——
    24——
    36——
    48[2, 4, 6, 8][4, 8, 12, 16]

  3. selected = doubled.select do |number|

    8selected = doubled[4, 8, 12, 16].select do |number|9  number >= minimum10end

  4. do |number|

    pass 1 of 4
    8selected = doubled.select do |number4|9  number >= minimum10end
    All 4 passes — pass 1 is the card above
    passnumberdoubledselected
    14——
    28——
    312——
    416[4, 8, 12, 16][12, 16]

  5. puts "doubled=#{doubled.join(",")}"

    12puts "doubled=#{doubled.join(",")4,8,12,16}"13puts "minimum=#{minimum10}"14puts "selected=#{selected.join(",")12,16}"
    outputdoubled=4,8,12,16
minimum=10
selected=12,16

Map, Then Select

  1. Start with numbers: [2, 4, 6, 8].
  2. map doubles each number into a new array.
  3. select checks each doubled number.
  4. Only values at least minimum stay in selected. | Original | After map | >= minimum when minimum is 6 | | --- | --- | --- | | 2 | 4 | drop | | 4 | 8 | keep | | 6 | 12 | keep | | 8 | 16 | keep |

Exercise: enumerable_map_select.rb

Double each number with map, then use select to keep values at or above a minimum