1. Computing

Quick Tip: Last Result in IRB

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There's a really quick way to get the result of the last expression evaluated in IRB. IRB will assign the result of the last expression evaluated to the _ (underscore) variable. This is very Perl-ish, where the $_ special variable is used to get the result of the previous expression.

I can think of a number of places this might be useful, but narrowing down a large Array seems like an obvious example. In this example, I want to find all the numbers from 0 to 100 that are multiples of both 2, 3 and 5. And, as is the way of IRB, I open up IRB and just start typing. It could be done with a single find_all, but that would require more of a plan than one usually has when using IRB.


irb(main):001:0> array = (0..100).to_a
=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]
irb(main):002:0> array.find_all{|i| i % 2 == 0}
=> [0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100]
irb(main):003:0> _.find_all{|i| i % 3 == 0}
=> [0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90, 96]
irb(main):004:0> _.find_all{|i| i % 5 == 0}
=> [0, 30, 60, 90]
irb(main):005:0> 

Note that this is not a special Ruby variable, but rather something implemented by IRB. So you won't be able to use this _ (underscore) special variable anywhere other than IRB. And I'm pretty sure that's a good thing, I think the Perl $_ special variable is a terrible idiom, but useful in the context of IRB.

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