Replacing select condition with jq's `IN`

December 6, 2024

Given the simplified input:

[
  [ "gpu" ],
  [ "disk" ]
]

I would like to select the arrays for which the first element is "gpu".

I can do it with a simple condition:

map(select(.[0] == "gpu"))

[
  [ "gpu" ]
]

But I don’t understand what’s wrong when I try to use the IN builtin instead:

map(select(.[0] | IN(["gpu"])))

[]

I’m using jq 1.6

>Solution :

IN/1 is defined as def IN(s): any(s == .; .);, so map(select(.[0] | IN(["gpu"]))) translates to map(select(.[0] | any(["gpu"] == .; .))) but . inside the map is [ "gpu" ], thus .[0] is "gpu", which is not equal to [ "gpu" ], and the filter consequently fails.

Instead, compare .[0] with "gpu":

map(select(.[0] | IN("gpu")))

[
  [
    "gpu"
  ]
]

Demo

Using IN, however, only makes sense for compacting multiple comparisons. For this single comparison, of course, a simple map(select(.[0] == "gpu")) would suffice.