mwag - 6 months ago 38

JSON Question

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]

Answer

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:

```
* using the --slurp option
* using "--arg a v" (or "--argjson a v" if available in your jq)
```

Here's an an even shorter def that's slower but often quite fast in practice:
```
def i(x;y):
(x|unique) as $x | (y|unique) as $y
| (($x + $y) | unique) - (($x - $y) + ($y - $x));
```

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 https://rosettacode.org/wiki/Set#Finite_Sets_of_JSON_Entities