Learning Exercise
A vector in Clojure is a sequential, indexed, immutable collection of zero or more values. This means that once a vector has been created, it cannot be modified. Functions for operating on vectors will return a new vector, while the original vector remains unchanged. The values in a vector may be of heterogenous types. Vectors can be defined as follows:
(def empty [])
(def single-value [5])
(def single-value-alternative (vector 5))
(def three-values [a b c])
Elements can be retrieved from a vector using an index. Clojure vectors are zero-based, meaning that the first element's index is always zero:
(def numbers [2 3 5])
;; Read value from vector
(get numbers 2)
;;=> 5
;; Update value in vector
(assoc numbers 2 9)
;;=> [2 3 9]
The original vector is unchanged:
numbers
;;=> [2 3 5]
To remember the updated value, we need to pass it along or capture it in a var:
(def updated-numbers
(assoc numbers 2 9))
(get updated-numbers 2)
;;=> 9
You're an avid bird watcher that keeps track of how many birds have visited your garden in the last seven days.
You have six tasks, all dealing with the numbers of birds that visited your garden.
For comparison purposes, you always keep a copy of last week's counts nearby, which were: 0, 2, 5, 3, 7, 8 and 4. Create a vector containing last week's counts:
last-week
;;=> [0 2 5 3 7 8 4]
Implement the today function to return how many birds visited your garden today. The bird counts are ordered by day, with the first element being the count of the oldest day, and the last element being today's count.
(def birds-per-day [2 5 0 7 4 1])
(today birds-per-day)
;;=> 1
Implement the inc-bird function to increment today's count:
(inc-bird birds-per-day)
;;=> [2 5 0 7 4 2]
Implement the day-without-birds? predicate function that returns true if there was a day at which zero birds visited the garden; otherwise, return false:
(day-without-birds? birds-per-day)
;;=> true
Implement the n-days-count function that returns the number of birds that have visited your garden from the start of the week, but limit the count to the specified number of days from the start of the week.
(n-days-count birds-per-day 4)
;;=> 14
Some days are busier than others. A busy day is one where five or more birds have visited your garden.
Implement the busy-days function to return the number of busy days:
(busy-days birds-per-day)
;;=> 2
Over the last year, you've found that some weeks for the same, odd pattern, where the counts alternate between one and zero birds visiting. Implement the odd-week? function that returns true if the bird count pattern of this week matches the odd pattern:
(odd-week? [1 0 1 0 1 0 1])
;;=> true
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