flatten ansible result using jinja2

I am getting a list of elements of the same structure using ansible, it looks a bit like the following:

results: [
  {
    "a": "foo",
    "b": "bar",
    "c": [
      {"c1": ...},
      {"c2": ...}
      {"c3": ...}]
  },
  {
    "a": "foo2",
    "b": "bar2",
    "c": [
      {"c4": ...},
      {"c5": ...}
      {"c6": ...}]
  }
]

I would need to iterate over all subelements of c - of all elements, so what I want is a list of the elements: c1, c2, c3, c4, c5, c6...

Normally I would use a nested loop, but since ansible is using jinja2s filters, I have no clue on how to accomplish that. I am totally new to this kind of data transformation.

To give some context, the actual code is:

- name: Find log files that are older than 1 day
  find:
    paths: "{{ item }}"
    age: 1d
    recurse: no
  register: oldLogs
  loop:
    - "/var/log"
    - "/home/user/log"

- name: print files
  debug:
    msg: "{{ oldLogs.results | <some filters here> }}\n"

Yes, I know I could pass a list of paths to find instead, but that's not the way I want to go, instead I would like to learn on how to use filters in such a situation.

Solution

For example,

results_c: "{{ (results|
                map(attribute='c')|
                flatten|
                combine).keys()|list }}"

gives the list

results_c: [c1, c2, c3, c4, c5, c6]

^{Example of a complete playbook for testing
- hosts: localhost
vars:
results:
- a: foo
b: bar
c:
- c1: val_1
- c2: val_2
- c3: val_3
- a: foo2
b: bar2
c:
- c4: val_4
- c5: val_5
- c6: val_6
results_c: "{{ (results|
map(attribute='c')|
flatten|
combine).keys()|list }}"
tasks:
- debug:
var: item
loop: "{{ results_c }}"

gives (abridged)
TASK [debug] *********************************************************************************
ok: [localhost] => (item=c1) =>
ansible_loop_var: item
item: c1
ok: [localhost] => (item=c2) =>
ansible_loop_var: item
item: c2
ok: [localhost] => (item=c3) =>
ansible_loop_var: item
item: c3
ok: [localhost] => (item=c4) =>
ansible_loop_var: item
item: c4
ok: [localhost] => (item=c5) =>
ansible_loop_var: item
item: c5
ok: [localhost] => (item=c6) =>
ansible_loop_var: item
item: c6}