Given arrays X and Y (preferably both as inputs, but otherwise, with one as input and the other hardcoded), how can I use jq to output the array containing all elements common to both? e.g. what is a value of f such that
echo '[1,2,3,4]' | jq 'f([2,4,6,8,10])'
would output
[2,4]
?
I've tried the following:
map(select(in([2,4,6,8,10]))) --> outputs [1,2,3,4]
select(map(in([2,4,6,8,10]))) --> outputs [1,2,3,4,5]
A simple and quite fast (but somewhat naive) filter that probably does essentially what you want can be defined as follows:
# x and y are arrays
def intersection(x;y):
( (x|unique) + (y|unique) | sort) as $sorted
| reduce range(1; $sorted|length) as $i
([]; if $sorted[$i] == $sorted[$i-1] then . + [$sorted[$i]] else . end) ;
If x is provided as input on STDIN, and y is provided in some other way (e.g. def y: ...), then you could use this as: intersection(.;y)
Other ways to provide two distinct arrays as input include:
--slurp option--arg a v (or --argjson a v if available in your jq)Here's a simpler but slower def that's nevertheless quite fast in practice:
def i(x;y):
if (y|length) == 0 then []
else (x|unique) as $x
| $x - ($x - y)
end ;
Here's a standalone filter for finding the intersection of arbitrarily many arrays:
# Input: an array of arrays
def intersection:
def i(y): ((unique + (y|unique)) | sort) as $sorted
| reduce range(1; $sorted|length) as $i
([]; if $sorted[$i] == $sorted[$i-1] then . + [$sorted[$i]] else . end) ;
reduce .[1:][] as $a (.[0]; i($a)) ;
Examples:
[ [1,2,4], [2,4,5], [4,5,6]] #=> [4]
[[]] #=> []
[] #=> null
Of course if x and y are already known to be sorted and/or unique, more efficient solutions are possible. See in particular Finite Sets of JSON Entities