Correct way to test for the type of a JSON value

[tischwa]

Hi,

what is the correct way to test for the type of a value?

Example: I only want to get arrays or objects and unwrap the contained values. If I don't filter out scalars jq gives an error:

% echo '[1, 2] 1' | jq '.[]'
1
2
jq: error: Cannot iterate over number

Now I test for arrays and objects "hijacking" the >= operator:

% echo '[1, 2] 1' | jq 'select(. >= []) | .[]'
1
2

Is there a better way of doing this?

And the general question is: How to test for

• number
• string
• array
• ...

Regards,
Tilo

[ghost]

Thanks, i was looking for that! One hack to test for strings is:

echo '[1111] [] {"x":"y"} {} "z" 2222 true false null' | jq 'select(.==tostring)'
"z"

Based on your test for "object or array", just discovered a test for objects only:

echo '[1111] [] {"x":"y"} {} "z" 2222 true false null' | jq 'select(.>={})'
{
  "x": "y"
}
{}

Can combine to test for arrays:

echo '[1111] [] {"x":"y"} {} "z" 2222 true false null' | jq 'select(.>=[] and .<{})'
[
  1111
]
[]

Finally, can test for number by combining "not an object or array", string test and exhaustive explicit tests for not true/false/null:

echo '[1111] [] {"x":"y"} {} "z" 2222 true false null' |
jq 'select(.<[] and .!=tostring and .!=true and .!=false and .!=null)'
2222

Is there a shorter way?

[stedolan]

While not as ingenious as @13ren's solution, you can also use the type function :)

$ echo '[true, null, 42, "hello", []]' | ./jq 'map(type)'
["boolean","null","number","string","array"]

[tischwa]

On Wed, 23 Jan 2013 12:29:04 +0100, Stephen Dolan
notifications@github.com wrote:

While not as ingenious as @13ren's solution, you can also use the type
function :)

$ echo '[true, null, 42, "hello", []]' | ./jq 'map(type)'
["boolean","null","number","string","array"]

Great! Exactly what I was looking for.

I did search http://stedolan.github.com/jq/manual for such a function but
didn't find anything.

This function is probably useful for others too.

Regards,

Tilo

[ghost]

This is great! It enables recurse(filter) to traverse arbitrary json (i.e. without an explicit field holding child values, unlike the reddit json in #37 which inspired it).

echo '[true, null, 42, "hello", []]' | jq 'recurse(.[])'
[
  true,
  null,
  42,
  "hello",
  []
]
true
jq: error: Cannot iterate over boolean
42
jq: error: Cannot iterate over number
"hello"
jq: error: Cannot iterate over string
[]

echo '[true, null, 42, "hello", []]' | jq 'recurse(select(type|.=="array"or.=="object") | .[])'
[
  true,
  null,
  42,
  "hello",
  []
]
true
42
"hello"
[]

NB: recurse omits nulls (due to its definition), and Tilo's sweet hack recurse( select(.>=[]) | .[] ) is shorter.

Would it make sense to include something like the above, as a builtin traverse(filter)? It seems an important special-case of recursion for JSON.

Aside: recurse can (of course) do general recursion e.g.

jq -n '0 | recurse( .+1|select(.<3) )'
0
1
2

Sorry, getting carried away here, but we can define a for-loop function, with optional step:

def for(args): args as $a| ($a[2] // 1) as $step|   $a[0] | recurse( .+$step| select(.<$a[1]) );
for([0,4,2])
0
2
for([0,4])
0
1
2
3