looping through json using sh+ jq [duplicate] - shell

Question

I like to filter json files using jq:
jq . some.json
Given the json containing an array of objects:
{
"theList": [
{
"id": 1,
"name": "Horst"
},
{
"id": 2,
"name": "Fritz"
},
{
"id": 3,
"name": "Walter"
},
{
"id": 4,
"name": "Gerhart"
},
{
"id": 5,
"name": "Harmut"
}
]
}
I want to filter that list to only show the elements with id having the value 2 and 4, so the expected output is:
{
"id": 2,
"name": "Fritz"
},
{
"id": 4,
"name": "Gerhart"
}
How do I filter the json using jq? I have played around with select and map, yet didn't got any of those to work, e.g.:
$ jq '.theList[] | select(.id == 2) or select(.id == 4)' array.json
true

Answer 1

From the docs:
jq '.[] | select(.id == "second")'
Input [{"id": "first", "val": 1}, {"id": "second", "val": 2}]
Output {"id": "second", "val": 2}
I think you can do something like this:
jq '.theList[] | select(.id == 2 or .id == 4)' array.json

Answer 2

You could use select within map.
.theList | map(select(.id == (2, 4)))
Or more compact:
[ .theList[] | select(.id == (2, 4)) ]
Though written that way is a little inefficient since the expression is duplicated for every value being compared. It'll be more efficient and possibly more readable written this way:
[ .theList[] | select(any(2, 4; . == .id)) ]

Answer 3

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.

Answer 4

Here is a solution using indices:
.theList | [ .[map(.id)|indices(2,4)[]] ]