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.

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

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34

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.

$ 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