# Chapter 6
```elixir
Mix.install([
{:json, "~> 1.4"}
])
```
## Exercise: ModulesAndFunctions-1 & 3
```elixir
defmodule Times do
def double(n), do: n * 2
def triple(n), do: n * 3
def quadruple(n), do: n * 4
end
```
## Exercise: ModulesAndFunctions-4
```elixir
defmodule Sum do
def from_1_to(1), do: 1
def from_1_to(n) when is_integer(n) and n > 1, do: n + from_1_to(n - 1)
end
5050 = Sum.from_1_to(100)
```
## Exercise: ModulesAndFunctions-5
```elixir
defmodule Math do
def gcd(x, 0) when is_integer(x) and x >= 0, do: x
def gcd(x, y) when is_integer(x) and x >= 0 and is_integer(y) and y >= 0, do: gcd(y, rem(x, y))
end
6 = Math.gcd(54, 24)
```
## Exercise: ModulesAndFunctions-6
```elixir
defmodule Chop do
def guess(actual, range = low..high) do
guess = div(low + high, 2)
IO.puts("Is it #{guess}")
guess(actual, range, guess)
end
defp guess(actual, low.._, guess) when guess > actual, do: guess(actual, low..(guess - 1))
defp guess(actual, _..high, guess) when guess < actual, do: guess(actual, (guess + 1)..high)
defp guess(_, _, guess), do: guess
end
Chop.guess(273, 1..1000)
```
## Exercise: ModulesAndFunctions-7
```elixir
# Convert a float to a string with two decimal digits. (Erlang)
:io_lib.format("~.2f", [3.1415]) |> IO.puts()
# Get the value of an operating-system environment variable. (Elixir)
System.get_env("ELIXIR_EDITOR") |> IO.puts()
# Return the extension component of a file name (so return .exs if given "dave/test.exs"). (Elixir)
Path.extname("dave/test.exs") |> IO.puts()
# Return the process’s current working directory. (Elixir)
File.cwd!() |> IO.puts()
# Convert a string containing JSON into Elixir data structures.
JSON.decode!(~s({"key":"this will be a value"})) |> IO.inspect()
# Execute a command in your operating system’s shell.
System.cmd("say", ["Hello world"])
:os.cmd('say Hello world')
```
