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Tuesday, September 13, 2016

Home Home on the Elixir Range

Elixir Ranges

Last week we were digging around the Elixir docs and had some fun with strings. As a reminder, we tried to implement a function that chops off a prefix from a string. Something like this:


take_prefix.("Mr. John", "Mr. ")
# returns 'John'

We saw our initial implementation:


take_prefix = fn full, prefix ->
  base = String.length(prefix)
  String.slice(full, base, String.length(full) - base)
end

IO.puts take_prefix.("Mr. John", "Mr. ")

But one big problem were the multiple calls to


String.length

which as we saw last week, does a full traversal of the string. As per the Elixir docs, the first improvement they perform is replacing


String.slice(full, base, String.length(full) - base)


with


String.slice(full, base..-1)

Let's look at this a bit. What are those dots in the second argument? This is something called a range. From the docs, a range is:


A range represents a discrete number of values 
where the first and last values are integers.

Ranges can be either increasing (first <= last)
or decreasing (first > last). Ranges are also always inclusive.

Let's take an example to help illustrate this:


range = 1..5

The `range` variable holds 5 numbers (1, 2, 3, 4, 5). That's it! It's just a collection of sequential numbers. Now let's check the docs on the new slice method:


slice(string, range)

#Returns a substring from the offset given by the
#start of the range to the offset given by the end
#of the range

So our call to slice


# As we saw last week, base is 4
# base = String.length(prefix)
String.slice(full, base..-1)

really is:


String.slice("Mr. John", 4..-1)

This function takes the string starting at the 4th letter and goes to the first letter counting backwards.

To say this another way, it's saying to take the string starting from the 4th letter up to and including the last letter.

Again, the advantage of using the range is that we save having to do the `String.length` call in the Slice method that we did last week.

Ranges are fun and come in quite handy. One more quick example with Ranges


println = fn x ->
  IO.puts x
end

Enum.each 1..5, println
Enum.each [1, 2, 3, 4, 5], println

Both of the last two lines will print out the numbers 1 through 5. The first one does so using a range and the second one uses a list of numbers.

That's it this week on the second improvement to our `take_prefix` method. Next week we will look at the next improvement that we can make to this method. As a teaser, it will involve us knowing how strings are represented. Until then!

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