ruby-104.md

More About Conditionals and Flow Control

In the calculator project, we talked about conditionals

if 1 > 0
  # do something
else
  # do something different
end

Comparisons

• <
• ==
• >=
• <=
• !=

Negating Comparisons

Ruby has an unless keyword that works just like if, but measured the other way.

# The following code executes identically
if !(number <= 10)
  puts "big number"
else
  puts "little number"
end

unless number <= 10
  puts "big number"
else
  puts "little number"
end

The if block executes if the conditional evaluates as true. The unless block executes if the conditional evaluates as false.

Therefore, unless x is the same as if !x.

Compound Conditions

Comparison expressions are often combined:

• &&
• ||

if command == "add"
  # add numbers
elsif command == "+"
  # also adds numbers
end

if command == "add" || command == "+"
  # adds numbers
elsif command == "subtract" || command == "-"
  # subtract the numbers
end

Wait a while

Execute the iterator while the condition is true.

i = 0

while i < 4
  puts i
  i += 1
end

The above code will output the values of i until i is no longer less than 4, resulting in the following output:

0
1
2
3

command = gets.chomp

while command != "add" && command != "+"
  puts "Please tell me to add (+)!"
  command = gets.chomp
end

puts "OMG It's about time!"

Make it clearer by using Until

Execute the iterator until the condition is true.

i = 0

until i == 4
   puts i
   i += 1
end

The above code will output the values of i until i is equal to 4, resulting in the following (equivalent to the above) output:

0
1
2
3

command = gets.chomp

until command == "add" || command == "+" || command == "subtract" || command == "-"
  puts "Please tell me to add (+) or subtract (-)!"
  command = gets.chomp
end

puts "OMG It's about time!"

Make it clearer by using include?

command = gets.chomp

until ["add", "+", "subtract", "-"].include? command
  puts "Please tell me to add (+) or subtract (-)!"
  command = gets.chomp
end

puts "OMG It's about time!"

Simplifying really complex conditionals

if command == "add" || command == "+"
  # adds numbers
elsif command == "subtract" || command == "-"
  # subtract the numbers
elsif command == "multiply" || command == "*"
  # multiply the numbers
elsif command == "divide" || command == "/"
  # divide the numbers
elsif command == "exponify" || command == "**"
  # exponify the number
elsif command == "sqrt"
  # find the square root of the number
end

The above code works, but it's kinda messy. We can trim it by using the case/when syntax:

case command
  when "add", "+"
    # adds numbers
  when "subtract", "-"
    # subtract the numbers
  when "multiply", "*"
    # multiply the numbers
  when "divide", "/"
    # divide the numbers
  when "exponify", "**"
    # exponify the number
  when "sqrt"
    # find the square root of the number
  else
    puts "What do you want from me?!"
end