Foundations

Variables

Ruby variables can hold numbers and strings, then feed later expressions.

variable assignment A variable name stores the result of the expression on the right side of `=`.

Variables

unit_price
variables.rb
Replay: real traced execution (multi-file project)
unit_price = 12
quantity = 3
total = unit_price * quantity

puts "unit=#{unit_price}"
puts "total=#{total}"

unit_price = 8
quantity = 3
total = unit_price * quantity

puts "unit=#{unit_price}"
puts "total=#{total}"

unit_price = 20
quantity = 3
total = unit_price * quantity

puts "unit=#{unit_price}"
puts "total=#{total}"

  1. unit_price ← 12, quantity ← 3, total ← 36

    1unit_price→ 12 = 12  #@unit_price=8, 202quantity→ 3 = 33total→ 36 = unit_price12 * quantity345puts "unit=#{unit_price12}"6puts "total=#{total36}"
    outputunit=12
total=36

  1. unit_price ← 8, quantity ← 3, total ← 24

    1unit_price→ 8 = 82quantity→ 3 = 33total→ 24 = unit_price8 * quantity345puts "unit=#{unit_price8}"6puts "total=#{total24}"
    outputunit=8
total=24

  1. unit_price ← 20, quantity ← 3, total ← 60

    1unit_price→ 20 = 202quantity→ 3 = 33total→ 60 = unit_price20 * quantity345puts "unit=#{unit_price20}"6puts "total=#{total60}"
    outputunit=20
total=60

Follow the Total

  1. unit_price starts at 12.
  2. quantity starts at 3.
  3. total = unit_price * quantity multiplies 12 * 3.
  4. total becomes 36.
  5. The program prints unit=12 and total=36. | unit_price | quantity | total | | --- | --- | --- | | 8 | 3 | 24 | | 12 | 3 | 36 | | 20 | 3 | 60 |

Exercise: variables.rb

Reproduce unit=12 and total=36, then try unit_price 8 and 20 and predict each total before running it.