What is the difference between Cond and Case?

elixir, switch-statement

Solution

Let me put `if` to the club too. You use `if` with one condition and a possible `else`, that's it. You use the `cond` statement when you have more than one condition and an `if` statement isn't enough, by the end, the `case` statement is used when you want to pattern match some data.

Let's explain by examples: suppose you want to eat apple if today is raining or rice if not, then you could use:

if weather == :raining do
  IO.puts "I'm eating apple"
else
  IO.puts "I'm eating rice"
end

This is a limited world, so you want to expand your options and because of that you will eat different things on some conditions, so the `cond` statement is for that, like this:

cond do
  weather == :raining and not is_weekend ->
    IO.puts "I'm eating apple"
  weather == :raining and is_weekend ->
    IO.puts "I'm will eat 2 apples!"
  weather == :sunny ->
    IO.puts "I'm happy!"
  weather != :raining and is_sunday ->
    IO.puts "I'm eating rice"
  true ->
    IO.puts "I don't know what I'll eat"
end

The last `true` should be there otherwise it'll raise an exception.

Well so what about `case`? It is used to pattern match something. Let's suppose you receive the information about the weather and the day of week as a message in a tuple and you depend on that to take a decision, you could write your intentions as:

case { weather, weekday } do
  { :raining, :weekend } ->
    IO.puts "I'm will eat 2 apples!"

  { :raining, _ } ->
    IO.puts "I'm eating apple"

  { :sunny, _ } ->
    IO.puts "I'm happy!"

  { _, :sunday } ->
    IO.puts "I'm eating rice"

  { _, _ } ->
    IO.puts "I don't know what I'll eat"
end

So the `case` brings to you the pattern-matching approach to the data, that you don't have with `if` or `cond`.

Problem

Original source