Ruby Intro

SENG2021

Loops

Like the if statements you saw 2 posts prior, there are many ways to construct loops. Not that there is no increment/decrement, ++/--, operation in Ruby.

There is the while loop you’re used to

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i = 0
while i < 3 do
  puts i
  i += 1
end
# 0
# 1
# 2

While loops with the test at the end

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i = 0
begin
  puts i
  i += 1
end while i < 3
# 0
# 1
# 2

i = 0
begin
  puts i
  i += 1
end while i < 0
# 0

There’s the until loops which run while the condition is false

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i = 0
until i > 2
  puts i
  i += 1
end
# 0
# 1
# 2

i = 0
begin
  puts i
  i += 1
end until i >= 0
# 0

And finally, my favourite – the for loops. If you already have a list to iterate through, great.

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a = [0, 1, 2]
for i in a
  puts i
end
# 0
# 1
# 2

If not, it’s great too

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for i in 0..2
  puts i
end
# 0
# 1
# 2

(0..2).each do |i|
  puts i
end
# 0
# 1
# 2

There’s also 4 keywords to help you manage the control of flow break, next, redo and finally retry which I’ll bring up in the Error Handling section. break exits the inner most loop

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for i in 0..2
  puts i
  if i == 1
    break
  end
end
# 0
# 1

While next jumps to the next iteration of the loop

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for i in 0..2
  if i == 1
    next
  end
  puts i
end
# 0
# 2

for i in 0..2
  if i == 2
    next
  end
  puts i
end
# 0
# 1

And redo jumps back to the beginning of the current iteration in this loop

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j = 0
for i in 0..2
  puts "i is #{i}"
  puts "j is #{j}"
  j += 1
  if j > 2
    break
  end
  if i == 1
    redo
  end
end
# i is 0
# j is 0
# i is 1
# j is 1
# i is 1
# j is 2