intro

• key-value pairs

merge method

• merge is used for merging hashes together.
• it's for hash only, not for array and ranges.
  • basic merge: h1.merge(h2) # default with new value.
  • h1.merge(h2) {|k, o, n| o} # use origin hash value.
  • h1.merge(h2) {|k, o, n| n} # use new hash value.
  • h1.merge(h2) {|k, o, n| o + n} # use sum of two.
  • h1.merge(h2) {|k, o, n| o > n ? o : n} # use the max one.

map, collect

• when to use it? when you feel lazy, manipulate all elements.
• %w(apple banana orange).map {|fruit| fruit.capitalize if fruit.start_with?('b') }
  • return => [nil, "Banana", nil]
• tip1: need a return value for each iteration, otherwise it will be nil
• tip2: the number of items in equals the number of items out.
• tip3: even though it works with array, hash, and range, it always return array.

reduce, inject

• an accumulator that accumulates values in memo.
• (1..10).reduce {|memo, n| memo + n} same as (1..10).reduce(&:+)
• tip: need a return value as memo in each iteration.

sort

• provide a comparison operator (space ship operator) <=> to compare.
• comparison operator is going to do more than just check true or false.
• a <=> b, return -1 if a less than b, 1 if a greater than b, 0 if a equal to b.
  • e.g. normal sort: (1..10).sort
  • e.g. reverse sort: (1..10).sort {|a, b| b <=> a}
  • e.g. %w(apple banana pear).sort_by(&:length)
• sort is support for array, range and hashes.
• when sort hash, ruby convert it to array first:
  • hash = {c: 3, a: 5, b: 4, d: 2, e: 1}
  • hash.sort {|a, b| a[0] <=> b[0]}
  • hash.sort {|a, b| a[1] <=> b[1]}