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Loops and Iteration  ›  Lesson

while, until and loop

while 12 minwhile repeats as long as its condition is true and until as long as it is false, both checking before each pass, while loop with break is the clear way to run at least once.

while, until and loop

Bean Cart's espresso machine needs to heat up before the first order: keep checking the temperature until it reaches 93 degrees. You do not know in advance how many checks that takes. When a loop depends on a condition rather than a count, Ruby reaches for while, until and loop.

The rule

while runs as long as true; until runs as long as false.

A while loop checks its condition before each pass and stops the first time it is falsy:

temp = 20
while temp < 93
  temp += 25
  puts "Heating... #{temp}"
end
# Heating... 45, Heating... 70, Heating... 95

until is the mirror image, exactly as unless mirrors if. It keeps going while the condition is falsy and stops once it becomes true:

temp = 20
until temp >= 93
  temp += 25
end

Both have a one-line modifier form: temp += 25 while temp < 93.

Because the condition is checked first, the body may run zero times: with temp = 95, neither loop above runs at all.

Sometimes you need the body to run at least once, the classic "do/while". Ruby can write it by putting while after a begin ... end block:

begin
  reading = sensor_temperature
end while reading < 93

This form checks the condition after each pass. It works, but Ruby's creator has advised against it because it behaves differently from the modifier form, and style guides agree. The clearer Ruby idiom is loop with break:

loop do
  reading = sensor_temperature
  break if reading >= 93
end

loop do ... end repeats forever until something inside stops it, which makes the exit condition sit exactly where it is checked.

Whatever you choose, make sure the body changes something the condition depends on. If temp never changes, while temp < 93 never ends. Press Ctrl+C to stop a runaway loop in the terminal.

The shortcut

Say the stopping rule out loud. If it sounds like "keep going while ...", use while. If it sounds like "keep going until ...", use until. If the body must run before you can even test, use loop with break if.

Heating the Bean Cart machineStep 1 of 4

  1. Say the rule

    "Keep heating until it reaches 93." That is an until sentence.

Drills

Answer fast — trust the rule.

quick drill

When does until done stop looping?

quick drill

With n = 10, how many times does the body of while n < 5 run?

quick drill

Which loop checks its condition after the body, so the body always runs at least once?

quick drill

What does the recommended Ruby idiom for "run at least once" look like?

quick drill

With cups = 0, what is cups after cups += 4 until cups > 10?

Remember

  • while runs as long as true; until runs as long as false. Both test before each pass.
  • Say the stopping rule out loud to choose; use loop with break if for "at least once".
  • Trap: a condition the body never changes, which loops forever.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainHeating the machine

Write a loop that heats Bean Cart's machine by 25 degrees at a time from 20 until it reaches at least 93, explain how many passes it makes and why, and describe how you would make a loop that always runs at least once.