CODEWITHSUNDEEP
jq
phoniel
phoniel

Reputation: 1

jq: Why do two expressions which produce identical output produce different output when surrounded by an array operator?

I have been trying to understand jq, and the following piuzzle is giving me a headache: I can construct two expressions, A and B, which seem to produce the same output. And yet, when I surround them with [] array construction braces (as in [A] and [B]), they produce different output. In this case, the expressions are:

A := jq  '. | add'
B := jq -s `.[] | add`

Concretely:

$ echo '[1,2] [3,4]' | jq  '.'
[1,2]
[3,4]
$ echo '[1,2] [3,4]' | jq  '. | add'
3
7
# Now surround with array construction and we get two values:
$ echo '[1,2] [3,4]' | jq '[. | add]'
[3]
[7]

$ echo '[1,2] [3,4]' | jq -s '.[]'
[1,2]
[3,4]
$ echo '[1,2] [3,4]' | jq -s '.[] | add'
3
7
# Now surround with array construction and we get only one value:
$ echo '[1,2] [3,4]' | jq -s '[.[] | add]'
[3,7]

What is going on here? Why is it that the B expression, which applies the --slurp setting but appears to produce identical intermediate output to the A expression, produces different output when surrounded with [] array construction brackets?

Upvotes: 0

Views: 112

Answers (1)

pmf
pmf

Reputation: 36391

When jq is fed with a stream, just like [1,2] [3,4] with two inputs, it executes the filter independently for each. That's why jq '[. | add]' will produce two results; each addition will separately be wrapped into an array.

When jq is given the --slurp option, it combines the stream to an array, rendering it just one input. Therefore jq -s '[.[] | add]' will have one result only; the multiple additions will be caught by the array constructor, which is executed just once.

Upvotes: 1

Related Questions