Learning Exercise
JSON defines an array as:
An array is an ordered collection of values. An array begins with
[left bracket and ends with]right bracket. Values are separated by,comma.
Arrays can contain zero or more of any kind of JSON values. Array elements do not need to be all the same type.
Use brackets to collect elements into an array.
[1, 2, 3]
The elements of a stream can be captured into an array by enclosing it in brackets.
range(5) # => a stream of 5 numbers
[range(5)] # => the array [0, 1, 2, 3, 4]
The length function returns the number of elements in an array.
Array indexing is zero-based.
Retrieve an element from an array with a bracket expression:
.[2] gets the third element.
Negative indexes count backwards from the end of the array:
.[-1] gets the last element; .[-2] is the second last.
A slice is a sub-sequence of the array.
.[10:15] returns 5 elements starting from index 10; the end index is not included.
There are some convenience functions:
first gets the first element.last gets the last element.nth(n) gets the element at index n.jq provides many functions to cover common iteration functionality:
map(expr) returns a new array where the expr is applied to each element in turn.
[1, 2, 3] | map(. * 10) # => [10, 20, 30]
select(expr) is a function that applies the expr to a single value;
if the result of the expression is true, then the value is returned;
if the result is false, nothing is returned -- not null, actually nothing.
To apply that to an array, combine it with map.
[range(10)] | map(select(. % 2 == 0)) # => [0, 2, 4, 6, 8]
Alternately, apply select to a stream and collect the results.
[range(10) | select(. % 2 == 0)] # => [0, 2, 4, 6, 8]
any(condition) and all(condition) return a boolean value whether any/all of the elements in the array pass the condition.
[1, 2, 3, 4] | any(. > 4) # false
[1, 2, 3, 4] | any(. >= 4) # true
[1, 2, 3, 4] | all(. >= 4) # false
[1, 2, 3, 4] | all(. <= 4) # true
You are an avid bird watcher that keeps track of how many birds have visited your garden in the last seven days.
You have five tasks, all dealing with the numbers of birds that visited your garden.
You store your bird counts in JSON format. A "main" array stores arrays of weekly counts. Each weekly count is an array of seven integers, one number for each day of the week. You always append new weeks at the end of the main array. Implement the filter that returns last week's counts.
$ cat bird-counting-data.json
[
...,
[0, 2, 5, 3, 7, 8, 4],
[4, 5, 9, 10, 9, 4, 3]
]
$ jq -f bird-count.jq < bird-counting-data.json
{
"last_week": [0, 2, 5, 3, 7, 8, 4],
...
Implement the filter to return how many birds visited your garden yesterday. 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.
$ jq -f bird-count.jq < bird-counting-data.json
{
...
"yesterday": 4,
...
Implement the filter to return the total number of birds that have visited your garden this week.
$ jq -f bird-count.jq < bird-counting-data.json
{
...
"total": 44,
...
Some days are busier than others. A busy day is one where five or more birds have visited your garden. Implement the filter to return the number of busy days this week.
$ jq -f bird-count.jq < bird-counting-data.json
{
...
"busy_days": 4,
...
Implement the filter that returns true if there was any day this week when zero birds visited the garden; otherwise, return false.
$ jq -f bird-count.jq < bird-counting-data.json
{
...
"has_day_without_birds": false
}
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