reduce & foreach

Document

{
  "team": "Platform",
  "office": { "city": "Berlin", "floor": 3 },
  "members": [
    { "name": "Ada", "age": 36, "role": "lead", "active": true,
      "email": "[email protected]", "skills": ["rust", "sql"] },
    { "name": "Linus", "age": 28, "role": "dev", "active": true,
      "email": null, "skills": ["c", "shell"] },
    { "name": "Grace", "age": 45, "role": "dev", "active": false,
      "skills": ["cobol"] },
    { "name": "Alan", "age": 41, "role": "qa", "active": true,
      "email": "[email protected]", "skills": [] }
  ]
}

Query

reduce .members[] as $m ({}; .[$m.role] += 1)

1. reduce .members[] as $m Loop over the stream, binding each element to $m.
2. {} The starting value: an empty object.
3. .[$m.role] += 1 Each round: bump the counter stored under this member's role (a missing key counts as 0).

Result

{"lead":1,"dev":2,"qa":1}

Document

{
  "team": "Platform",
  "office": { "city": "Berlin", "floor": 3 },
  "members": [
    { "name": "Ada", "age": 36, "role": "lead", "active": true,
      "email": "[email protected]", "skills": ["rust", "sql"] },
    { "name": "Linus", "age": 28, "role": "dev", "active": true,
      "email": null, "skills": ["c", "shell"] },
    { "name": "Grace", "age": 45, "role": "dev", "active": false,
      "skills": ["cobol"] },
    { "name": "Alan", "age": 41, "role": "qa", "active": true,
      "email": "[email protected]", "skills": [] }
  ]
}

Query

[foreach (.members[] | .age) as $a (0; . + $a)]

1. foreach Like reduce, but outputs the state after every step instead of only the last.
2. (.members[] | .age) as $a The stream to loop over, and the variable holding each item.
3. 0 The starting state.
4. ; . + $a The update: new state = old state (.) plus this age.

Result

[36,64,109,150]