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Exercise: ModulesAndFunctions-7

Find the library functions to do the following, and then use each in iex. (If the word Elixir or Erlang appears at the end of the challenge, then you'll find the answer in that set of libraries.)

  • Convert a float to a string with two decimal digits. (Erlang)
  • Get the value of an operating-system environment variable. (Elixir)
  • Return the extension component of a file name (so return .exs if given "dave/test.exs"). (Elixir)
  • Return the process's current working directory. (Elixir)
  • Convert a string containing JSON into Elixir's data structures. (Just find; don't install.)
  • Execute a command in your operating system's shell.

Solutions

Float Conversion

The problem ends with Erlang so let's look at the Erlang docs. In io there is a method called format, but this alone doesn't look like what we want as it's coupled closely with fwrite. However, in io_lib I also see a format method which looks more promising; this one just returns a character list. Let's try it:

iex> print_float = fn n -> :io_lib.format("~.2f", [n]) end  
#Function<6.50752066/1 in :erl_eval.expr/5>  
iex> print_float.(2.123)  
['2.12']

Almost got it, except it returned a list of strings instead of just the string. I believe this is because the data parameter we passed in was a list. Anyways, let's just take the head:

iex> print_float = fn n ->  
...>   [head | _] = :io_lib.format("~.2f", [n])  
...>   head  
...> end  
#Function<6.50752066/1 in :erl_eval.expr/5>  
iex> print_float.(2.123)  
'2.12'  
iex> print_float.(273.1834)  
'273.18'

Environment Variable

A quick Google search of "Elixir Environment Variable" brought me to the System module in the Elixir docs and get_env looks promising. Let's try it:

iex> System.get_env "SHELL"  
"/bin/bash"

File Extensions

So I immediately look in the File and File.Stat modules thinking those were likely places for this, but after no success, a search in the docs led me to Path, where an extname function resides:

$ touch ~/test.txt
$ iex
iex> Path.extname("~/test.txt")
".txt"

Process CWD

Again, I thought I new exactly where this one would be: Process. But, a quick search again led me to the right place: File. It has the function cwd:

iex> File.cwd
{:ok, "/home/paul/dev/programming-elixir-exercises"}

JSON Deserialization

Ironically, working on a side project I already did this using a library I found on Github called Poison. See it here: https://github.com/devinus/poison

OS Shell Execution

Looking in the Elixir docs again, System.cmd looks like it should work:

iex> System.cmd "echo", ["Hello world"]
{"Hello world\n", 0}

Righteous.