How can I count the number of occurrences of a certain object in a hash that matches a condition in Ansible?

Given this list in an ansible variable:

---
hosts:
- address: host1.local
  cluster:
    name: RED
- address: host2.local
  cluster:
    name: RED
- address: host3.local
  cluster:
    name: GREEN
- address: host4.local
  cluster:
    name: BLUE

I now want to count somehow the number of hosts in each cluster. So I want to end up with:

hosts_per_cluster:
   RED: 2
   BLUE: 1
   GREEN: 1

How would one go about this?

My first attempt was something like this:

- name: Get number of hosts per cluster
      set_fact: 
        hosts_per_cluster[item.cluster.name]={{ hosts_per_cluster[item.cluster.name] | default(0) | int +1 }}
      loop: "{{ hosts }}"

That did however not work...

Solution

The task below creates the dictionary

    - set_fact:
        hpc: "{{ hpc | d({}) | combine({item.0: item.1 | length}) }}"
      loop: "{{ _hosts | groupby('cluster.name') }}"

gives

  hpc:
      BLUE: 1
      GREEN: 1
      RED: 2

You don't have to iterate the list. Group the cluster names

  ca: "{{ _hosts | groupby('cluster.name') }}"

gives

  ca:
    -   - BLUE
        -   -   address: host4.local
                cluster:
                    name: BLUE
    -   - GREEN
        -   -   address: host3.local
                cluster:
                    name: GREEN
    -   - RED
        -   -   address: host1.local
                cluster:
                    name: RED
            -   address: host2.local
                cluster:
                    name: RED

and create the lists

  cluster: "{{ ca | map('first') }}"
  address: "{{ ca | map('last') | map('map', attribute='address') }}"
  size: "{{ address | map('length') }}"

gives

  cluster: ['BLUE', 'GREEN', 'RED']
  address: [['host4.local'], ['host3.local'], ['host1.local', 'host2.local']]
  size: [1, 1, 2]

Then, create the dictionary

hpc: "{{ dict(cluster | zip(size)) }}"

gives the same result

  hpc:
      BLUE: 1
      GREEN: 1
      RED: 2

^{Example of a complete playbook for testing
- hosts: localhost

vars:

_hosts:
- address: host1.local
cluster: {name: RED}
- address: host2.local
cluster: {name: RED}
- address: host3.local
cluster: {name: GREEN}
- address: host4.local
cluster: {name: BLUE}

ca: "{{ _hosts | groupby('cluster.name') }}"
cluster: "{{ ca | map('first') }}"
address: "{{ ca | map('last') | map('map', attribute='address') }}"
size: "{{ address | map('length') }}"
hpc2: "{{ dict(cluster | zip(size)) }}"

tasks:

- debug:
var: _hosts | groupby('cluster.name')

- set_fact:
hpc: "{{ hpc | d({}) | combine({item.0: item.1|length}) }}"
loop: "{{ _hosts | groupby('cluster.name') }}"

- debug:
var: hpc

- debug:
msg: |
cluster: {{ cluster }}
address: {{ address }}
size: {{ size }}

- debug:
var: hpc2}