Ruby Golf Cheatsheet

A handy cheatsheet to make your way down when you’re in the final stages of a golf challenge looking to shave off those last few characters.

This is a living document and I’ll be updating it as time goes on.

Looping

• Don’t loop over arrays with .each. You can shave a character off by using .map instead.
• n.times { } is almost always shorter than 1.upto(n), except for when you need to call .to_i on n (then upto becomes shorter, because it can take a string). Sometimes .map can be even shorter.
• If you’re looping or recursing in a single direction and have a guard clause returning if a value is equal to some sentinel, you can shave a character off by switching from equality to inequality comparison.

return if i == 0
return if i < 1

• Reuse your loops when you can by bunging everything into a single array and operating based on value or type.

["a", "b", 2, 4].map { |e| e.to_s==e ? e.upcase : e * 2 }
=> ["A", "B", 4, 8]

• If you want to loop over a value that can be a scalar or array, you can use the splat operator to treat both types identically: the scalar becomes an array containing only it:

[*items].each {|item| ... }

Variables and values

• The numbers 100 to 126 can be written as ?d to ?~.
• Use scientific notation for large numbers. 1e4 is shorter than 10000.
• Single-character strings can be prefixed with a ? instead of quoting them, e.g. ?x instead of “x” and ?? instead of “?”. ?\n and ?\t also work.
• Use 1 or !p for true. Use p (which returns nil) or !0 for false.

String manipulation

• Use [] to check if a string contains another.

‘foo’[‘f’] #=> ‘f’

• Use tr instead of gsub for character-wise substitutions.

‘01011’.tr(‘01’,’AB’) #=> ‘ABABB’

• When using gsub, use the \1 escape characters instead of a block. Don’t forget you can re-use them outside of the gsub once they’re set, too.

"golf=great short=awesome".gsub(/(\w+)=(\w+)/,'(\1~>\2)')
"golf=great short=awesome".gsub(/(\w+)=(\w+)/){"(#{$1}~>#{$2})")

• If you need to remove trailing newlines, use chop instead of chomp.
• Replace .to_s with direct string interpolation, e.g. “#{n+10}”.
• When you’re using string interpolation, you can omit the curly braces if your variable name has a sigil ($, @) in front of it. Very useful for special variables.

puts "this program has read #$. lines of input"

Array manipulation

• Use the splat operator to get the head and tail of an array, or vice versa.

tail, *head = [1,2,3]
tail => 1
head => [2,3]
// or
*head, tail = [1,2,3]
head => [1,2]
tail => 3

• Use the * operator on arrays to join them.

[1,2,3] * ?,
=> "1,2,3"

• Replace .compact with -[nil] or -[p]. Replace .reject(&:zero?) with -[0].
• If you’re assigning a variable to the first element in an array, a, = c is shorter than a = c[0].
• If you’re assigning an array with one scalar element in it, *a = 0 is shorter than a = [0].
• To check array length without .length, you can instead check if the array has anything at a given index. If you grab past the last index you will get nil, a falsey value. Note: this won’t work if you have falsey values (false, nil) in the array already.

if !a[4] # equivalent to: if a.length < 5

• When casting to an array, use [*(…)] instead of (…).to_a. This also applies to ranges for easy consecutive-number arrays, e.g. x = [*0..5].
• Sum an array’s contents with .inject :+, e.g. [1, 2, 3].inject :+.

Hash manipulation

• You can turn a one-dimensional array into a hash with Hash.

fruit = ["apple", "red", "banana", "yellow"]
=> ["apple", "red", "banana", "yellow"]

Hash[*fruit]    
=> {"apple"=>"red", "banana"=>"yellow"}

Control flow

• Use abort to terminate the program and print a string to STDERR — shorter than puts followed by exit
• Use procs over functions whenever possible. Call the proc with c[n].

# 28 characters
def c n
/(\d)\1/=~n.to_s
end

# 23 characters, saves 5
c=->n{/(\d)\1/=~n.to_s}

• If you have return if some_condition clauses at the top of your function, you can drop the return and turn the whole method into a ternary. If the rest of the function can be expressed in a single line, you can avoid wrapping it in parentheses.

def method x
  return if x < 5
  y = x * 5
  y * 2
end
def method x
  x < 5 ? nil : (
    y = x * 5
    y * 2
  )
end

I/O

• If you read a line with gets, you can then use ~/$/ to find its length (this doesn’t count a trailing newline if it exists)

Special variables

$!   # The exception object passed to #raise.
$@   # The stack backtrace generated by the last exception raised.
$&   # Depends on $~. The string matched by the last successful 
       match.
$`   # Depends on $~. The string to the left of the last successful 
       match.
$'   # Depends on $~. The string to the right of the last successful 
       match.
$+   # Depends on $~. The highest group matched by the last 
       successful match.
$1   # Depends on $~. The Nth group of the last successful match. 
       May be > 1.
$~   # The MatchData instance of the last match. Thread and scope 
       local. MAGIC
$=   # The flag for case insensitive. Defaults to nil. Deprecated.
$/   # The input record separator (eg #gets). Defaults to newline.
$\   # The output record separator (eg #print and IO#write). Default 
       is nil.
$,   # The output field separator for the print and Array#join. 
       Defaults to nil.
$;   # The default separator for String#split. See -F flag.
$.   # The current line number of the last file from input.
$<   # Same as ARGF.
$>   # The default output for print, printf. Defaults to $stdout.
$_   # The last input line of string by gets or readline. Thread and 
       scope local.
$0   # Contains the name of the script being executed. May be 
       assignable.
$*   # Same as ARGV.
$$   # The process number of the Ruby running this script. Read 
       only.
$?   # The status of the last executed child process. Read only. 
       Thread local.
$:   # Same as $LOAD_PATH.
$"   # The array contains the module names loaded by require.
$-0  # See $/.
$-a  # Autosplit mode. True if option -a is set. Read-only variable.
$-d  # Same as $DEBUG.
$-F  # See $;.
$-i  # In in-place-edit mode, this variable holds the extension, otherwise nil.
$-I  # Same as $LOAD_PATH.
$-l  # True if option -l is set. Read-only.
$-p  # True if option -p is set. Read-only.
$-v  # Same as $VERBOSE.
$-w  # True if option -w is set.

Dates

require 'date'; z = Date::DAYNAMES

is much shorter than

%w(Sun Mon Tues Wednes Thurs Fri Satur).map { |p| “#{p}day” }