[Ruby] Behavior of evaluation of conditional expression in while

Introduction

I encountered the first behavior while using while, so make a note of it.

environment

ruby: 2.7.1

phenomenon

Phenomenon_A

a = 0

while
  if a == 5
    break
  end
  a += 1
end

puts a
#Execution result: 0

Phenomenon_B

b = 0

while
  b += 1
  if b == 5
    break
  end
end

puts b
#Execution result: 5

Phenomenon_C

c = 0

while c < 5 do
  c += 1
end

puts c
#Execution result: 5

Phenomenon_D

d = 0

loop do
  if d == 5
    break
  end
  d += 1
end

puts d
#Execution result: 5

Phenomenon_E

e = 0

loop do
  e += 1
  if e == 5
    break
  end
end

puts e
#Execution result: 5

As a result I expected, I thought that 5 would output even with phenomenon _A.

Why this happens

(It was solved by @ scivola's comment.)

Apparently, the expression immediately after while is evaluated as a conditional expression. In other words, in this phenomenon_A,

if a == 5
  break
end

This part is evaluated for the first time as a conditional expression of while, and of course ʻa == 5 becomes false and the value of ʻif expression becomes nil, so it seems that it has escaped from the loop.

By the way, in Ruby, all values except nil and false are true.

When I gave a conditional expression as follows, the processing in the loop was executed.

a = 0

while true do #do is optional
  if a == 5
    break
  end
  a += 1
end

puts a
#Execution result: 5

The pitfall of this time was that the expression inside the loop was actually a conditional expression.

Edit history

2020/07/04 17:11 In the comment, @scivola explained the cause of the problem, so I edited "Why this happens". 2020/07/04 17:31 I changed the title to match the content and proofread the text a little.

Remarks

I don't usually use while or loop, but use another iterative method.