How to filter array of objects by element property values using jq?

Question

I like to filter json files using jq:

jq . some.json

Given the json containing an array of objects:

{
  \"theList\": [
             


        

Answer 1

Using select(.id == (2, 4)) here is generally inefficient (see below).

If your jq has IN/1, then it can be used to achieve a more efficient solution:

.theList[] | select( .id | IN(2,3))

If your jq does not have IN/1, then you can define it as follows:

def IN(s): first(select(s == .)) // false;

Efficiency

One way to see the inefficiency is to use debug. The following expression, for example, results in 10 calls to debug, whereas only 9 checks for equality are actually needed:

.theList[] | select( (.id == (2,3)) | debug )

["DEBUG:",false]
["DEBUG:",false]
["DEBUG:",true]
{
  "id": 2,
  "name": "Fritz"
}
["DEBUG:",false]
["DEBUG:",false]
["DEBUG:",true]
{
  "id": 3,
  "name": "Walter"
}
["DEBUG:",false]
["DEBUG:",false]
["DEBUG:",false]
["DEBUG:",false]

index/1

In principle, using index/1 should be efficient, but as of this writing (October 2017), its implementation, though fast (it is written in C), is inefficient.