Ruby Arrays: select(), collect(), and map()

As a former Java developer and new to the concept of blocks, these API options seem perplexing at first.   They're not.  All three take a block.  How they use the block is what distinguishes them.  You should read the RDoc for yourself but, if you want a quick a dirty summary, continue reading this post.

collect {|item|block} and map{|item|block} do the same thing - return an array of things returned by the block.  This is different from returning specific items in the collection being iterated over. 

Which leads me to select. 

select{|item|block} will return actual collection items being iterated over if, for each item, the block condition evaluates to true. Not the same as returning what the block, itself, may return.  In the case of select, the block would always return an instance of class TrueClass or FalseClass.  Typically, [true, false, ..., true] is not what you're looking for in your resulting array.

Slightly modifying the core RDoc example:

a = ["a", "b", "c", "d"]      #=> ["a", "b", "c", "d"]
a.map {|item|"a" == item}     #=> [true, false, false, false]
a.select {|item|"a" == item}  #=> ["a"]

Happy coding!

Comments

[m2]

Thanks for the Array#select example! _m2

[Ammar Yousuf]

Hey Ben,

I searched for "ruby .collect" and your site was the first result, pretty cool :-)

ttyl, Ammar

[PICO]

Ive been to your site like 20 times in the past year or so. Very straightforward simple concise with example. GOOD JOB.

[amol]

a=["a","b","c","d","e"]
=> ["a", "b", "c", "d", "e"]
irb(main):002:0> a.map{|i| "a"==i}
=> [true, false, false, false, false]
irb(main):003:0> a.select{|i|"a"==i}
=> ["a"]
irb(main):004:0> a.collect{|i|"a"==i}
=> [true, false, false, false, false]
irb(main):005:0> b=a.map{|i| "a"==i}
=> [true, false, false, false, false]
irb(main):006:0> print b
truefalsefalsefalsefalse=> nil
irb(main):007:0> b=a.select{|i|"a"==i}
=> ["a"]
irb(main):008:0> print b
a=> nil
irb(main):009:0> b=a.collect{|i|"a"==i}
=> [true, false, false, false, false]
irb(main):010:0> print b
truefalsefalsefalsefalse=> nil
irb(main):011:0> b=a.select{|i|"a"+i}
=> ["a", "b", "c", "d", "e"]
irb(main):012:0> b=a.collect{|i|"a"+i}
=> ["aa", "ab", "ac", "ad", "ae"]
irb(main):013:0> b=a.map{|i| "a"+i}
=> ["aa", "ab", "ac", "ad", "ae"]
irb(main):014:0>