Add a single object to an array

Question

So I have this json structure:

{
  "dog": [
    {
      "name": "sam",
      "age": "2"
    },
    {
      "name": "billy",
      "age": "5"
    }
  ]
}

I've found that .dog[1] will return me the first object but not in the dog:[] array.

{
  "name": "billy",
  "age": "5"
}

and .[] |= .[$i] gives me an object:

{
  "dog": {
    "name": "billy",
    "age": "5"
  }
}

What I want is:

{
  "dog": [
    {
      "name": "sam",
      "age": "2"
    }
  ]
}

I plan to use this in a bash script, and write out to multiple files like:

jq -r --argjson i "$i" '.[] |= .[$i]' "$1"

Do you need the array slice? .[1:2]

Tirinoarim
– Tirinoarim

2021-08-25 10:06:58 +00:00
Commented Aug 25, 2021 at 10:06 — Tirinoarim
– Tirinoarim, Commented Aug 25, 2021 at 10:06

chepner · Accepted Answer · 2021-08-24 15:09:12Z

1

Instead of an integer index, use a slice. (Also, the array is 0-indexed, not 1-indexed.)

$ jq --argjson i 0 '{dog: .dog[$i:$i+1]}' < tmp.json
{
  "dog": [
    {
      "name": "sam",
      "age": "2"
    }
  ]
}

As you are asking for an object, not a string, the -r option doesn't do anything.

answered Aug 24, 2021 at 15:09

chepner

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Comments

Weeble · Accepted Answer · 2021-08-24 15:12:58Z

0

Try

jq --argjson i 2 '.dog|=[.[$i-1]]'

The .dog|=[.[$index]] part modifies just the array dog and replaces it with an array of just the item at position $index. This has the benefit of preserving anything else that might be in the top-level object. We use $i-1 since you indicate you want to provide 1-based indices as input.

answered Aug 24, 2021 at 15:12

Weeble

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1 Comment

dogman Over a year ago

I had wondered about that. As it was already in dog that seems more efficient, rather than re-creating dog. As it's being used in a bash script, the counter is already initialized to 0, so jq --argjson i 1 '.dog|=[.[$i]]' will work nicely too.

Daniel · Accepted Answer · 2021-08-24 15:04:37Z

0

JSON arrays starts from zero. you should use index 0 instead of 1: .dog[0]

edited Aug 24, 2021 at 15:04

answered Aug 24, 2021 at 15:02

Daniel

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4 Comments

chepner Over a year ago

I know JSON originated in Javascript, but there's no reason to mention it here. Arrays in JSON (like in many languages) are zero-indexed.

Daniel Over a year ago

@chepner I did adress js since it's a javascript object notation but you're correct, jq is for parsing JSON, and the question is not related to js. fixed :)

Weeble Over a year ago

Regardless, this doeesn't answer at least half of the question - how to keep the structure of the input.

Daniel Over a year ago

@Weeble He had the correct structure, his only issue was the indexing.

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