Reverse flatten nested arrays with jq - json

Question

Suppose I have the following nested data structure
cat nested.json
[
{
"a": "a",
"b": [
{"c": "c"}
]
},
{
"a": "a",
"b": [
{"c": "c"}
]
}
]
I can flatten it like this
cat nested.json | jq '
[. as $in | reduce paths(scalars) as $path ({};
. + { ($path | map(tostring) | join(".")): $in | getpath($path) }
)]
' > flat.json
cat flat.json
[
{
"0.a": "a",
"0.b.0.c": "c",
"1.a": "a",
"1.b.0.c": "c"
}
]
To reverse the flatten operation with jq I tried this
cat flat.json | jq '
.[0] | reduce to_entries[] as $kv ({};
setpath($kv.key|split("."); $kv.value)
)
'
{
"0": {
"a": "a",
"b": {
"0": {
"c": "c"
}
}
},
"1": {
"a": "a",
"b": {
"0": {
"c": "c"
}
}
}
}
However, I want to convert numbers in the setpath param to create arrays. This doesn't quite work, but I think it's close?
cat flat.json | jq '
def makePath($s): [split(".")[] | if (test("\\d+")) then tonumber else . end];
.[0] | reduce to_entries[] as $kv ({}; setpath(makePath($kv.key); $kv.value))
'
jq: error (at <stdin>:8): split input and separator must be strings
The desired output is the same as the original data in nested.json

Answer 1

Wouldn't it be simpler to do it this way:
Encode your input with
jq '[path(.. | scalars) as $path | {($path | join(".")): getpath($path)}] | add' nested.json
{
"0.a": "a",
"0.b.0.c": "c",
"1.a": "a",
"1.b.0.c": "c"
}
And decode it with
jq 'reduce to_entries[] as $item (null; setpath($item.key / "." | map(tonumber? // .); $item.value))' flat.json
[
{
"a": "a",
"b": [
{
"c": "c"
}
]
},
{
"a": "a",
"b": [
{
"c": "c"
}
]
}
]
However, if you don't care about your special dot notation (e.g. "0.b.0.c") for the encoded keys, you can simply convert the path array into a JSON string instead, having albeit uglier virtually the same effect. Moreover, it would automatically enable the handling of input object field names that include dots (e.g. {"a.b":3}) or look like numbers (e.g. {"42":"Panic!"}).
Using JSON keys, encode your input with
jq '[path(.. | scalars) as $path | {($path | tojson): getpath($path)}] | add' nested.json
{
"[0,\"a\"]": "a",
"[0,\"b\",0,\"c\"]": "c",
"[1,\"a\"]": "a",
"[1,\"b\",0,\"c\"]": "c"
}
And decode it with
jq 'reduce to_entries[] as $item (null; setpath($item.key | fromjson; $item.value))' flat.json
[
{
"a": "a",
"b": [
{
"c": "c"
}
]
},
{
"a": "a",
"b": [
{
"c": "c"
}
]
}
]