Exercise 32: Loops and Arrays

You should now be able to do some programs that are much more interesting. If you have been keeping up, you should realize that now you can combine all the other things you have learned with if-statements and boolean expressions to make your programs do smart things.

However, programs also need to do repetitive things very quickly. We are going to use a for-loop in this exercise to build and print various arrays. When you do the exercise, you will start to figure out what they are. I won't tell you right now. You have to figure it out.

Before you can use a for-loop, you need a way to store the results of loops somewhere. The best way to do this is with arrays. Arrays are exactly what their name says: a container of things that are organized in order from first to last. It's not complicated; you just have to learn a new syntax. First, there's how you make arrays:

hairs = ['brown', 'blond', 'red']
eyes = ['brown', 'blue', 'green']
weights = [1, 2, 3, 4]

You start the array with the [ (left bracket) which "opens" the array. Then you put each item you want in the array separated by commas, similar to function arguments. Lastly, end the array with a ] (right bracket) to indicate that it's over. Ruby then takes this array and all its contents and assigns them to the variable.

Warning

This is where things get tricky for people who can't code. Your brain has been taught that the world is flat. Remember in the last exercise where you put if-statements inside if-statements? That probably made your brain hurt because most people do not ponder how to "nest" things inside things. In programming nested structures are all over the place. You will find functions that call other functions that have if-statements that have arrays with arrays inside arrays. If you see a structure like this that you can't figure out, take out a pencil and paper and break it down manually bit by bit until you understand it.

We now will build some arrays using some for-loops and print them out:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
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18
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the_count = [1, 2, 3, 4, 5]
fruits = ['apples', 'oranges', 'pears', 'apricots']
change = [1, 'pennies', 2, 'dimes', 3, 'quarters']

# this first kind of for-loop goes through a list
# in a more traditional style found in other languages
for number in the_count
  puts "This is count #{number}"
end

# same as above, but in a more Ruby style
# this and the next one are the preferred 
# way Ruby for-loops are written
fruits.each do |fruit|
  puts "A fruit of type: #{fruit}"
end

# also we can go through mixed lists too
# note this is yet another style, exactly like above
# but a different syntax (way to write it).
change.each {|i| puts "I got #{i}" }

# we can also build lists, first start with an empty one
elements = []

# then use the range operator to do 0 to 5 counts
(0..5).each do |i|
  puts "adding #{i} to the list."
  # pushes the i variable on the *end* of the list
  elements.push(i)
end

# now we can print them out too
elements.each {|i| puts "Element was: #{i}" }

You should immediately see that Ruby has two kinds of loops that I am calling a for-loop. In programming the term for-loop just means "a loop that goes through each thing in an array of things." In Ruby this is both for number in the_count style, and the more common fruits.each style. You should use the .each version as it is more reliable and what other Ruby programmers expect you to write.

Warning

Ruby programmers are very particular about how their for-loops are written and will declare you a bad programmer for simply using this one construct wrong. They went so far as to break the for-each version of looping so that there are problems with using it, forcing you to conform to their culture. Heed my warning that you should always use .each and never for-each for fear of being forever branded bad and shunned. Yes, it is as ridiculous as it sounds.

What You Should See

$ ruby ex32.rb
This is count 1
This is count 2
This is count 3
This is count 4
This is count 5
A fruit of type: apples
A fruit of type: oranges
A fruit of type: pears
A fruit of type: apricots
I got 1
I got pennies
I got 2
I got dimes
I got 3
I got quarters
adding 0 to the list.
adding 1 to the list.
adding 2 to the list.
adding 3 to the list.
adding 4 to the list.
adding 5 to the list.
Element was: 0
Element was: 1
Element was: 2
Element was: 3
Element was: 4
Element was: 5

Study Drills

  1. Take a look at how you used (0..5) in the last for-loop. Look up Ruby's "range operator" (.. and ...) online to see what it does.
  2. Change the first for number in the_count to be a more typical .each style loop like the others.
  3. Find the Ruby documentation on arrays and read about them. What other operations can you do besides the push function? Try <<, which is the same as push but is an operator. fruits << x is the same as fruits.push(x).

Common Student Questions

How do you make a 2-dimensional (2D) array?
That's an array in an array like this: [[1,2,3],[4,5,6]]
Aren't lists and arrays the same thing?
Depends on the language and the implementation. In classic terms a list is very different from an array because of how they're implemented. In Ruby though they call these arrays. In Python they call them listss. Just call these arrays for now since that's what Ruby calls them.
Why is a for-loop able to use a variable that isn't defined yet?
The variable is defined by the for-loop when it starts, initializing it to the current element of the loop iteration, each time through.

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