Basic tips

A list of short tips for easy small byte saves:

• 'g' vs. ?g for single-length strings
• n.times{} for looping from 0 to n-1, inclusive
• (a..b).map{} vs. a.upto(b) for looping from a to b, inclusive
• a.upto(b-1) vs. (a...b).map{} for looping from a to b-1, inclusive
• p(n) vs. p n, if precedence allows it (applies to most Ruby functions)
• print vs. $><< to output without a newline
• ARGV vs. $* to access arguments
• a if b vs. b&&a for conditionals (b||a for the inverse)
• a.join vs. a*'' for joining arrays ('' can be replaced with any delimiter)
• a.uniq vs. a|[] vs. a&a for removing repeated elements
• s.index(/REGEX/) vs. /REGEX/=~s for getting the first occurrence of a regex
• "#{$_}" vs. "#$_" for string interpolation of dollar variables
• 'foo' vs. :foo for shortening string literals (limited use)
• ['what','is','code','golf'] vs. %w(what is code golf) for hardcoded string arrays
• a=[0] vs. *a=0 for initializing a 1-element array
• a=[0,1,2] vs. a=0,1,2 for initializing a multi-element array
• x.map{_1.abs} vs. x.map(&:abs) (or x.map &:abs) for simple method calls

Numbered parameters

This feature introduced in Ruby 2.7 allows you to drop the block parameter:

[1,2,3].map{|x|p x}
[1,2,3].map{p _1}

[[1,2],[3,4]].map{|x,y|p x+y}
[[1,2],[3,4]].map{p _1+_2}

Beware, however, as they have certain limitations in nested loops:

a.map{|x|b.map{|y|p x+y}}  # OK
a.map{|x|b.map{p x+_1}}    # OK
a.map{b.map{|y|p _1+y}}    # NOT OK
a.map{b.map{p _1+_2}}      # (VERY) NOT OK

Chaining expressions

Ruby has a strange feature that allows you to chain expressions in an obfuscated way. This allows for things like:

b^=3;a-=1|b;a&3
# ->
3&a-=1|b^=3

Magic variables

Certain predefined variables like $. or $* can be abused to omit the initialization of an extra variable:

# `$.` is always initialized to 0
n=0;9.times{p n+=_1}
9.times{p$.+=_1}

# `$*` (a.k.a. ARGV) is an empty list if no arguments are given
a=[];9.times{puts (a<<_1)*' '}
9.times{puts ($*<<_1)*' '}

Two other variables that are less used, but can still be useful are $/, which is "\n" by default, and $0, which is "-" by default (on code.golf).

Subscripting integers

To get the ith bit of an integer, simply use the subscript operator:

n>>i&1
n[i]

Subscripts can be extended to work with ranges as well. For example, n[2..5] gives the binary bits from 2 to 5, as a decimal integer.

Splatting

Splatting is very flexible, and can be used in a multitude of ways. It is particularly good for grabbing the first/last/middle elements of array:

a,*b,c=*1..5  # a=1, b=[2,3,4], c=5
a,*,c=*1..5   # a=1, c=5
a,=*1..5      # a=1
*,c=*1..5     # c=5

You can also use it to print an array of numbers, each on its own line, with

p *a

Note that you would use puts on an array of strings, but it doesn't require splatting:

# Functionally identical
puts *a
puts a

Operator methods

Operators can in fact be used as methods. For example, 1+2*3 may be written as 1.+(2.*(3)). The precedence changes when using it is used this way, which sometimes saves a byte:

a*(b+c)
a.*b+c

String conversion

There are some neat ways to do string conversion, involving array multiplication. For example, say that you wanted to concatenate a number to a string:

"#{n}"+s
n.to_s+s
[n,p]*s

If you simply want to convert a number to a string, there is also a shorter way, but it involves an already-present string variable:

"#{n}"
n.to_s
[n]*''
[n]*s   # Assumes a defined string variable `s`

filter vs grep

By passing a lambda into grep, it can behave like a filter. This allows for a 2-byte save:

(1..n).filter{|n|}
(1..n).grep->n{}

The downside is that the precedence of the grep version is pretty wacky, which makes it less useful.

Looping a constant amount

In general, n.times or eval''*n is the shortest way to loop n times. But with clever use of predefined variables, shorter alternatives exist that loop a predetermined amount:

$:.map{}   # 8
$:.max{}   # 7
$:.sort{}  # 16
$".map{}   # 45
$".max{}   # 44
$".sort{}  # 506

# These two hold a block parameter, which contains the current index, like `.times`
$:.fill{}  # 8 (same length as `9.times`, so what's the point?)
$".fill{}  # 45

Note that the amount of iterations illustrated above are specific to code.golf. In the case of max and sort, the return value within the block must be a number, otherwise it will cause an error.

Obscuring 1.upto

A rather specific (and rare) example in which combining eval and $. can match the length of 1.upto:

1.upto(n){pred(_1)&&p(_1)}
eval'p$.if pred($.+=1);'*n

It should be noted that there are situations where the latter is shorter, such as if the space between if and pred can be reused.

Packers

2:1 packer

puts CODE.force_encoding('utf-16le').encode('utf-8')

eval'...'.encode('utf-16le').b

3:1 packer

r="";"...".chars{|c|97.upto(99){r<<c.ord%_1+32}};eval r