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Learning Exercise
Recursive functions are functions that call themselves.
A recursive function needs to have at least one base case and at least one recursive case.
A base case returns a value without calling the function again. A recursive case calls the function again, modifying the input so that it will at some point match the base case.
In Elm, recursive functions are typically defined using pattern matching with case expressions or multiple function clauses.
count : List a -> Int
count list =
case list of
[] ->
0 -- base case
_ :: tail ->
1 + count tail -- recursive case
Due to immutability, loops in Elm are written differently from imperative languages. For example, loops commonly look like:
for(i = 0; i < array.length; i++) {
// do something with array[i]
}
In a functional language like Elm, mutating variables is not possible. Thus, loops have to be implemented with recursion.
The equivalent of a for loop in Elm would look like this:
loop : List a -> (a -> b) -> List b
loop list f =
case list of
[] ->
[]
head :: tail ->
f head :: loop tail f
In practice, iterating over lists and other data structures in Elm is most often done using functions from the List module, such as List.map, List.filter, and List.foldl.
Under the hood, these functions are implemented using recursion.
You're an avid bird watcher that keeps track of how many birds have visited your garden on any given day.
You decided to bring your bird watching to a new level and implement a few tools that will help you track and process the data.
You have chosen to store the data as a list of integers. The first number in the list is the number of birds that visited your garden today, the second yesterday, and so on.
In real-world applications, you should always use core modules whenever applicable, however for the purpose of learning about recursion, do not use any of the List functions in this exercise.
Implement the today function.
It should take a list of daily bird counts and return today's count.
If the list is empty, it should return Nothing, otherwise the count will be wrapped in a Just.
BirdCount.today [2, 5, 1]
-- Just 2
Implement the incrementDayCount function.
It should take a list of daily bird counts and increment today's count by 1.
If the list is empty, return [1].
BirdCount.incrementDayCount [4, 0, 2]
-- [5, 0, 2]
Implement the hasDayWithoutBirds function.
It should take a list of daily bird counts.
It should return True if there was at least one day when no birds visited the garden, and False otherwise.
BirdCount.hasDayWithoutBirds [2, 0, 4]
-- True
BirdCount.hasDayWithoutBirds [3, 8, 1, 5]
-- False
Implement the total function.
It should take a list of daily bird counts and return the total number that visited your garden since you started collecting the data.
BirdCount.total [4, 0, 9, 0, 5]
-- 18
Some days are busier than others. A busy day is one where five or more birds have visited your garden.
Implement the busyDays function.
It should take a list of daily bird counts and return the number of busy days.
BirdCount.busyDays [4, 5, 0, 0, 6]
-- 2
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