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Hao Tan
Hao Tan

Reputation: 31

How to understand the '...' operator used with Boolean comparisons

select = []
0.upto 5 do |value|
  select << value if (value==2)...(value==2)
end
p select # [2,3,4,5]

Can anyone tell me how to understand this code?

Upvotes: 3

Views: 65

Answers (1)

Derek Wright
Derek Wright

Reputation: 1492

I learned something researching this because I've never seen the range operator used on boolean values. Apparently in this context its called a "flip flop" operator. Basically, the condition evaluates False until the first part of the conditional is True. Then it "flips" and evaluates True until the 2nd part evaluates True. In your example, the 2nd part will never evaluate to True as its already passed the valid condition of value == 2 therefore it will continue on with the range provided. You can see this flip-flopping in action if you change the 2nd condition to value == 4:

select = []
0.upto 5 do |value|
  select << value if (value==2)...(value==4)
end
p select # [2,3,4]

Reference: http://nithinbekal.com/posts/ruby-flip-flop/

Upvotes: 5

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