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https://github.com/O-I/Facets

A community-curated list of one-liners in Ruby
https://github.com/O-I/Facets

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A community-curated list of one-liners in Ruby

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Facets

======



A community-curated list of one-liners (or several-liners if elegance demands) in Ruby



### Files, Folders, and Paths



##### Convert the contents of `filename` to a string

```ruby

File.open(filename, 'rb') { |file| file.read }

```



##### Get the current user's home directory

```ruby

Dir.home

```



This only works if `ENV['HOME']` is set, though. RubyTapas #10 gives us an alternative using `Etc` to find the current user's login name and passing that to `Dir.home`:

```ruby

Dir.home(Etc.getlogin)

```



### Strings



##### Calculate the Hamming distance between two strings `str1` and `str2` (returns `nil` if `str1` and `str2` are of unequal length):

```ruby

str1.chars.zip(str2.chars).reduce(0) { |sum, (x, y)| sum + (x == y ? 0 : 1) } if str1.size == str2.size

```



##### Determine whether one string `str1` is a rotation of another string `str2`:

```ruby

(str1 + str1)[str2] == str2

```



##### Get the `n`th bit of an `Integer` `j`:



My first inclination would be to convert to binary and use `[]`, i.e., `j.to_s(2)[n].to_i`, but you can call `[]` directly on an `Integer`:

```ruby

j[n]

```



### Arrays



##### Remove all instances of value `l` from an array `a` and return the result

```ruby

a - [l]

```



Or `a -= [l]` if you would like to reasign the result to `a`. This is the most elegant way I've seen of doing this. Doing something like `a.delete(l)` returns `l` rather than the updated value of `a` which disrupts method chaining. You could always do `a.tap { |x| x.delete(l) }`, but I think the above line is superior and just as chainable if enclosed in parentheses.



##### Convert an array `a` with even index whose elements are alternating key value pairs (e.g., `[k1, v1, k2, v2,..., kn, vn]` into the respective hash `h`

```ruby

h = Hash[*a]

```

This is likely not a good idea if `a` happens to be a large dataset as the splat operator will expand all contents of the array to the stack. In that case, you can use `h = Hash[a.each_slice(2).to_a]`.



In the event `a` is already a 2D array of the form `[[k1, v1], [k2, v2],..., [kn, vn]]`, you can simply do `h = Hash[a]`. And from Ruby 2.1 onwards, you can simply use `Array#to_h`: `h = a.to_h`.



##### Create an `n`x`n` multiplication table represented by a 2-dimensional array:

```ruby

[*1..n].product([*1..n]).map { |arr| arr.reduce(:*) }.each_slice(n).to_a

```

Or, if you're using ActiveSupport:

```ruby

[*1..n].product([*1..n]).map { |arr| arr.reduce(:*) }.in_groups(n)

```



And in terms of clarity, this one's about as good as it gets:

```ruby

Array.new(n) { |x| Array.new(n) { |y| (x+1)*(y+1) } }

```



##### Calculate the frequency distribution of an array `a`:



There are many ways to do this, but using `Enumerable#each_with_object` seems to be the most idiomatic:

```ruby

a.each_with_object(Hash.new(0)) { |element, frequency| frequency[element] += 1 }

```



`Enumerable#reduce` also works, but feels clumsier. Notice that the block parameters are reversed compared to the above and the need to explicitly return the accumulator on each pass:

```ruby

a.reduce(Hash.new(0)) { |frequency, element| frequency[element] += 1; frequency }

```



And yet another way using Ruby 2.2+ that seems truest to a functional style:

```ruby

a.group_by(&:itself).map { |k, v| [k, v.size] }.to_h

```



##### Get all but the first element of an array `a`:



You can use parallel assignment in conjunction with the splat operator if you're planning on using the head:

```ruby

head, *rest = a

```



But if you just want the rest, using `drop` is better:

```ruby

a.drop(1)

```



Both are preferable to `a[1..-1]` in that they return `[]` rather than `nil` if `a` is empty.



##### Find all duplicate values in an array `a` (Ruby 2.2+):

```ruby

a.group_by(&:itself).select { |_, v| v.size > 1 }.keys

```



You can replace `group_by(&:itself)` with `group_by { |n| n }` for lesser versions of Ruby.



##### Calculate rolling averages of an array `a` over interval length `n`:

```ruby

a.each_cons(n).map { |interval| interval.reduce(:+) / n.to_f }

```



##### Reduce an array `a` of Boolean values



For *and*, *or*, and *xor*, we can use `Enumerable#reduce` paired with the appropriate logical operator, i.e., `a.reduce(:&)`, `a.reduce(:|)`, and `a.reduce(:^)`, respectively.



For reducing over *and* and *or*, I find `a.all?` and `a.any?` to be more idiomatic than explicit use of `reduce`.



##### "Map compact" or "select map"



This one is best illustrated by an example. Note, I don't think there is a particularly elegant way to do what I'm about to describe in Ruby. A common idiom is to map over a collection and select the resulting values for those elements which conform to some predicate.



For example, suppose we have an array `a = [1, 2, 3, 4, 5]` and want to get a resulting collection that adds 1 to each element only if that element is even. We can achieve this in several ways:



```ruby

a = [1, 2, 3, 4, 5]



a.map { |l| l + 1 if l.even? }.compact                      # => [3, 5]

a.select(&:even?).map { |l| l + 1 }                         # => [3, 5]

a.each_with_object([]) { |l, res| res << l + 1 if l.even? } # => [3, 5]

a.reduce([]) { |res, l| res.push(l + 1) if l.even?; res }   # => [3, 5]

```



All of these one-liners return the desired result, all of them feel clumsy.



### Hashes



##### Create a hash `h` from two arrays `k` and `v` of equal length that represent `h`'s keys and values, respectively:

```ruby

h = Hash[k.zip(v)]

```

##### Decompose a hash `h` into two arrays `k` and `v` that respresent `h`'s keys and values, respectively:



1) The straightforward way:

```ruby

k, v = h.keys, h.values

```

2) Or more cryptically for impressing your nerdy friends:

```ruby

k, v = *h.to_a.transpose

```



##### Remove key-value pairs in hash `h` given keys in array `a`:



ActiveSupport has a nifty little method for this called `Hash#except`. The best I could come up with for doing this in one line with Ruby non-destructively is

```ruby

hash.tap { |h| a.map { |k| h.delete(k) } }

```



##### Weighted random sampling without replacement of a hash `h`'s keys whose values are weighted probabilities that sum to 1:

```ruby

h.max_by { |_, weight| rand ** (1.0/weight) }.first

```



This is essentially the weighted analogue of `Array#sample`. With Ruby 2.2+, we also have the analogue of `Array#sample(n)`:

```ruby

h.max_by(n) { |_, weight| rand ** (1.0/weight) }.map(&:first)

```



### Regular Expressions



### Calculations



##### Compute `n!` for all nonnegative integers n (returns `nil` otherwise):



```ruby

(1..n).reduce(1, :*) if n.is_a?(Integer) && n > -1

```



### Miscellaneous



##### Given an `m`x`n` matrix, return an array that represents the clockwise spiral path from the top left to the center. For example, given



    matrix = [[1,2,3],

              [8,9,4],

              [7,6,5]]

              

`spiral(matrix)` should return `[1, 2, 3, 4, 5, 6, 7, 8, 9]`



```ruby

def spiral(matrix)

  matrix.empty? ? [] : matrix.shift + spiral(matrix.transpose.reverse)

end

```



Note that this implementation has one major drawback — it mutates its argument.



##### `Symbol#to_proc` with arguments



This isn't a one-liner nor is it something I recommend without extreme caution as it patches a core class, but it is one of the slickest — if not the slickest — Ruby hacks I've ever seen.



Open up the `Symbol` class and add a `call` method to it that contains the following lambda:



```ruby

class Symbol

  def call(*args, &block)

    ->(caller, *rest) { caller.send(self, *rest, *args, &block) }

  end

end

```



This gives `Symbol#to_proc` the superpower of accepting arguments. so, instead of doing, e.g.,



```ruby

nums = [1, 2, 3, 4]

text = %w(this is a test)



nums.map { |num| num ** 1i }

text.map { |word| word.gsub('s', '*') }

```



we can do



```ruby

nums.map(&:**.call(1i))

text.map(&:gsub.call('s', '*'))

```



But there's one more trick here. By naming our method `call`, we can use the shorter `.()` syntax:



```ruby

nums.map(&:**.(1i))

text.map(&:gsub.('s', '*'))

```