Exercise 33: While Loops

Now to totally blow your mind with a new loop, the while-loop. A while-loop will keep executing the code block under it as long as a boolean expression is true.

Wait, you have been keeping up with the terminology, right? You should know that Ruby has three kinds of code blocks you need to read. You have the kind that if-statements use, where code is started after the if, and the block of code is ended with end. You then have two kinds for .each style blocks. Either you use do ... end or { ... } when making a block of code. When I use the characters ... in that last description I do not mean type ... I mean them in the normal English way of "and then some stuff here". If you don't quite understand this go back and redo the last few exercises until you get it.

Later on we'll have some exercises that will train your brain to read these structures, similar to how we burned boolean expressions into your brain.

Back to while-loops. What they do is simply do a test like an if-statement, but instead of running the code block once, they jump back to the "top" where the while is, and repeat. A while-loop runs until the expression is false.

Here's the problem with while-loops: Sometimes they do not stop. This is great if your intention is to just keep looping until the end of the universe. Otherwise you almost always want your loops to end eventually.

To avoid these problems, there are some rules to follow:

1. Make sure that you use while-loops sparingly. Usually a for-loop is better.
2. Review your while statements and make sure that the boolean test will become false at some point.
3. When in doubt, print out your test variable at the top and bottom of the while-loop to see what it's doing.

In this exercise, you will learn the while-loop while doing these three checks:

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i = 0
numbers = []

while i < 6
  puts "At the top i is #{i}"
  numbers.push(i)

  i += 1
  puts "Numbers now: ", numbers
  puts "At the bottom i is #{i}"
end

puts "The numbers: "

# remember you can write this 2 other ways?
numbers.each {|num| puts num }

$ ruby ex33.rb
At the top i is 0
Numbers now:
0
At the bottom i is 1
At the top i is 1
Numbers now:
0
1
At the bottom i is 2
At the top i is 2
Numbers now:
0
1
2
At the bottom i is 3
At the top i is 3
Numbers now:
0
1
2
3
At the bottom i is 4
At the top i is 4
Numbers now:
0
1
2
3
4
At the bottom i is 5
At the top i is 5
Numbers now:
0
1
2
3
4
5
At the bottom i is 6
The numbers:
0
1
2
3
4
5