cheatsheet-and-faq.md

title	desc	layout	published	indexed	categories
Cheatsheet & FAQ	Tips and Tricks to tame the Dragon.	default	true	true	Tutorials

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By Pineapple

DragonRuby Starters

• Ruby is strongly-typed (not stringly-typed, like JavaScript), but you don't need to explicitly state a type
• You can use .class to find out what type you actually have

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• Integers ("Fixnum") are signed 64-bit; there is a BigNum type available, but i've not tried to use it
• Floats are double-precision
• Strings are their own type (but this isn't important for you unless you're used to a language without them)

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• A ? at the end of a method name indicates that it will return a boolean
• A ! at the end of a method name indicates that the method is "dangerous" (usually available for "modiy in-place" on methods that would return a changed copy)
• Type conversion will use a method named .to_ followed by a letter indicating the type: Integer, Float, String, Array, Hash, probably some others too (note that not all conversions may be sensible)

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• You generally don't need parenthesis around things (like if conditions and function calls)

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• elsif is a five-letter word that will make you say a lot of four-letter words

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• repl.rb is an incredibly useful tool for experimenting with syntax and techniques

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For loops still exist, but there's often better choice

(1..10).each do |i|
  puts i
end

does a thing 10 times

• (1...10) excludes the final number
• if you just want to start at 0, then 10.times do is easier

.each is a thing to iterate over an array

a = [3, 6, 8, 4, 5]

a.each do |n|
  puts n
end

and if you need it, a.each_with_index do |n, i| exists as well

but if you just want to do the same operation on the whole array, .map might be more suitable
and if you just want to get the sum of the array, .reduce(:+) will do that as well

My little dragon: arrays are magic

There are some powerful tricks that can be done with arrays (and with anything that's Enumerable, which includes arrays and hashes)
the trick is trying to understand it all...

• .length exists

• .pop and .push remove and add items from the high-index end ("the end") of the array

• .shift and .unshift remove and add items from the zero end ("the beginning") of the array

a = [3, 6, 8, 4, 5]
puts a.pop      # 5
puts a          # [3, 6, 8, 4]
a.push(7)
puts a          # [3, 6, 8, 4, 7]
puts a.shift    # 3
puts a          # [6, 8, 4, 7]
a.unshift(2)
puts a          # [2, 6, 8, 4, 7]

• .append adds to the end, .prepend adds to the beginning

• << is another way of doing an append (which is often used in adding things to the outputs lists)

• .delete_at() deletes a specific index

• .insert() inserts a thing at a position

• Maybe useful things (that i'm used to having to write entire functions to do): .reverse, .sort, .shuffle, .rotate (can take a rotation distance)

• .permutation !!! this thing is a chunky function that you don't need to write. if you just want to get a list of permutations, apply .to_a as well

and i haven't even started on array maths yet... this example from https://www.ruby-lang.org/en/ still feels incredibly game-changing to me (as primarily a C programmer):

# Ruby knows what you
# mean, even if you
# want to do math on
# an entire Array
cities  = %w[ London
              Oslo
              Paris
              Amsterdam
              Berlin ]
visited = %w[Berlin Oslo]

puts "I still need " +
     "to visit the " +
     "following cities:",
     cities - visited
     
# => I still need to visit the following cities:
# => London
# => Paris
# => Amsterdam

• Nested arrays are considered to be multi-dimensional, and have some tricks as well

• I'm not quite sure how to use .map_2d, but it exists

• Also .transpose (warning: this will error if the array is not rectangular)

Sometimes, hashes are better

If you don't need an order, but need paired data values, a hash is probably more suitable

Comparisons to arrays:

• .map becomes .transform_keys or .transform_values

• .each still exists, but expected to pass out both key and value (or an array of size 2, if you just give one iteration variable); in this way, it's more like how .each_with_index works on arrays (.each_pair is an alias in case this is more useful). .each_key and .each_value are available as well

• '.size' exists (and .length might also exist as an alias)

• .merge is useful. newhash = hash1.merge(hash2) will have hash2's values overrule hash1 where keys match (.merge! does the same thing, but modifies the object it is called on)