How to "number" array items? [duplicate] - json

Question

This question already has answers here:
How do i add an index in jq
(3 answers)
Closed 2 years ago.
Considering input:
[
{
"a": 1
},
{
"a": 2
},
{
"a": 7
}
]
how do I add new field to each object, which value would be index in array? Producing:
[
{
"a": 1,
"index": 0
},
{
"a": 2,
"index": 1
},
{
"a": 7,
"index": 2
}
]

Answer 1

Using reduce, without disassembling/reassembling the input:
reduce range(length) as $index (.; .[$index] += {$index})
Online demo

Answer 2

Store the structure into a variable, then use keys to get the indices, retrieve the corresponding object from the variable using the index and add the index to it:
jq '[ . as $d | keys[] | $d[.] + {index:.} ]' file.json

Answer 3

to_entries takes an object and returns an array of key/value pairs.
It can be used effectively and intuitively.
jq/to_entries
input file
// file.json
[
{
"a": 1
},
{
"a": 2
},
{
"a": 7
}
]
commands
jq 'to_entries | map(.value+{index:.key})' file.json
results
[
{
"a": 1,
"index": 0
},
{
"a": 2,
"index": 1
},
{
"a": 7,
"index": 2
}
]

Answer 4

Here's also an alternative (non-jq) solution, using jtc:
bash $ <input.json jtc -w'[:]<I>k' -i'{"index":{I}}'
[
{
"a": 1,
"index": 0
},
{
"a": 2,
"index": 1
},
{
"a": 7,
"index": 2
}
]
bash $
PS. I'm a developer of jtc - unix JSON processor
PPS. The disclaimer is required by SO.

Answer 5

Try : Array[index_of_object].property = value
for example : array[0].a = 1