code for chapter 4 in ruby

04_quicksort/ruby/01_loop_sum.rb

@@ -0,0 +1,10 @@
+def sum(arr)
+  total = 0
+  arr.each do |x|
+    total += x
+  end
+
+  total
+end
+
+puts sum([1, 2, 3, 4])

04_quicksort/ruby/02_recursive_sum.rb

@@ -0,0 +1,4 @@
+def sum(list)
+  return 0 if list.empty?
+  list[0] + sum(list[1..-1])
+end

04_quicksort/ruby/03_recursive_count.rb

@@ -0,0 +1,4 @@
+def count(list)
+  return 0 if list.empty?
+  1 + count(list[1..-1])
+end

04_quicksort/ruby/04_recursive_max.rb

@@ -0,0 +1,9 @@
+def max(list)
+  if list.length == 2
+    # condition ? then : else
+    list[0] > list[1] ? list[0] : list[1]
+  else
+    sub_max = max(list[1..-1])
+    list[0] > sub_max ? list[0] : sub_max
+  end
+end

04_quicksort/ruby/05_quicksort.rb

@@ -0,0 +1,18 @@
+def quicksort(array)
+  if array.length < 2
+    # base case, arrays with 0 or 1 element are already "sorted"
+    array
+  else
+    # recursive case
+    pivot = array[0]
+    # sub-array of all the elements less than the pivot
+    rest = array[1..-1]
+    less = rest.select { |i| i <= pivot }
+    # sub-array of all the elements greater than the pivot
+    greater = rest.select { |i| i > pivot }
+
+    quicksort(less) + [pivot] + quicksort(greater)
+  end
+end
+
+print quicksort([10, 5, 2, 3])