looping_and_control_statements.md

Looping and control statements

Conditional assignment

In JS, we sometimes want to only set the value of a variable if it's not already defined. To do that in JS, we would do something like this:

some_var = some_var || {}

In Ruby, there is an even cooler and more terse way (Yey for terseness!). Here is how to write the aforementioned statement in Ruby:

some_var ||= {} #This is similar to a += or /= operator

if keyword

This works just like the JavaScript version but with one small enhancement: it can be used inline with other statements (in a very cool readable way). See below:

puts "Hello Bruce" if name == 'Bruce'

unless keyword

This is similar to the if condition except it will execute the statement if the result is false

i = 0
unless i == 3
	print "Hooray it's not 3!" #will execute if the i == 3 is false which it is.
end
# => "Hooray it's not 3!"

You could also use the unless statement inline with other statements (in an almost backwards if statement). See below:

#Doing a type check to see if it's an integer. The `?` is just a naming convention to indicate it returns a boolean
puts "That's not an integer bro!" unless some_int.is_a? Integer 

switch/case statements

What is called Switch statements in JS and Java and other languages is called simply case statements in Ruby.

This in Ruby:

case language
when "JS"
	puts "Dat frontend scripting language!"
when "Java"
	puts "Aw man. The kingdom of nouns!"
else
	puts "Some cool language I don't know!"
end

or a more terse way to write it using when...then:

case language
	when "JS" then puts "Dat frontend scripting language!"
	when "Java" then puts "Aw man. The kingdom of nouns!"
	else puts "Some cool language I don't know!" 
end

is equivalent to in JS:

switch (language) {
	case "JS":
		console.log("Dat frotnend scripting language");
		break;
	case "Java":
		console.log("Aw man. The kingdom of nouns!");
		break;
	default:
		console.log("Some cool language I don't know!");
}

So switch keyword in JS translates to case in Ruby case keyword in JS translates to when in Ruby default keyword in JS translates to else in Ruby

for loops

unlike JS, Ruby has a different for syntax where it defines the range:

There are two forms for this:

• with two dots used in between the range (to be max bound inclusive)
• with three dots used in between the range (to be max bound exclusive)

See example below:

for i in 0..10 # same as "for(var i = 0; i <= 10; i++)" in JS. NOTE THE TWO DOTS FOR 10 inclusive
	#some code
end
for i in 0...10 # same as "for(var i = 0; i < 10; i++)" in JS. NOTE THE THREE DOTS FOR 10 exclusive
	#some code
end

CAVEAT: You cannot increment the variable in the for loop delcaration manually.

In other words,

// javascript
for (let i = 0; i < 100; i++) {
  console.log(i)
  i += 3
}
// => prints 0, 4, 8, 12...

# ruby
for i in (0..100)
  puts i
  i += 3
end
# => prints 0, 1, 2, 3, ... to 100

Using the loop keyword (i.e do..while statement in JS)

In Ruby, there is another way to loop through code called the loop statement. It takes in a code block and asks for a condition to exit out of the loop if the condition is met with the break keyword.

Like the do..while loop in other languages, the loop keyword always executes at least once

For example:

#Ruby way to do it
i = 0
loop do
	i += 1
	puts 'Hello Person #{i}'
	break if i > 9
end

This is equivalent to this do..while in JS:

var i = 0;
do {
	i += 1;
	console.log('Hello Person ' + i);
} while (i > 9);

Using until keyword in a while loop

Just like how the unless keyword is the opposite of the if statement, the while also has its opposite: until. It is used in a similar way to unless.

#The following code loops until j = 0 and then breaks out of the loop.
j = 3
until j == 0 do
	puts j
	j -= 1
end

Using the next keyword in loop

You can use the 'next' keyword to skip the loop if a condition is met. It is equivalent to the continue keyword used in combination with an if.

For example:

for i in 1..5
	next if i % 2 == 0 #skip if its an even number
	puts i
end

Equivalent to JS's continue used with an if statement:

for (var i = 0; i <= 5; i++) {
	if (i % 2 == 0) {
		continue;
	}
	console.log(i);
}

Using the each iterator for looping

The each iterator is similar to the Array's .forEach() or Underscore/Lodash's _.each function.

You specify an object to loop through and then specify a code block to run for each item in the object.

For example, the following in Ruby:

object = ['hello', 'goodbye']
object.each { |str| puts str }

#or written with do..end instead of {}
object.each do |str| 
	puts str
end

is equivalent to in JS:

var object = ['hello', 'goodbye'];
object.forEach(function(str) {
	console.log(str);
});

To iterate over hashes (i.e JS objects), you can specify 1 parameter (where the parameter can be both the key and value) or 2 parameters (one for key and one for value):

person = {
	'name' => 'Goku',
	'power_level' => 9000000000
}
#
#Using 2 parameter prints:
# name => Goku
# power_level => 9000000000
person.each { |key,val| puts "#{key} => #{val}" } 
#
# Using 1 parameter prints:
# name
# Goku
# power_level
# 9000000000
person.each { |key_or_val| puts |key_or_val| }

The reason for this is due to the way the iterator is setup for the each.

Using times iterator for simple looping

If you want to make a block of code execute a fixed number of times, the best way to do it would be through a "times" loop

3.times { puts "Hello!" } #Prints "Hello!" 3 times.