How to iterate every two elements of a list in an Ansible loop?

I have list variable with disks names which сan be off variable amount - 4, 8, 16 etc. elements, but even and I want to print every two disks in loop at all iteration. How I can do this?

---
- hosts: all

  vars:

    disks:
      - sdc
      - sdd
      - sde
      - sdf
  
  tasks:
   - name: Iterate every two elements (disks) in loop
     shell: echo '/dev/sdc + /dev/sdd' (and in second iteration of loop must be echo '/dev/sde + /dev/sdf')
     loop: "{{ disks }}"

Solution

I have a list with a variable amount of elements (4, 8 or 16, but even) and I want to print chunks of two in a loop at every iteration.

Although Ansible isn't a programming language such construct is possible in example in the following way

---
- hosts: localhost
  become: false
  gather_facts: false

  vars:

    disks:
      - sdc
      - sdd
      - sde
      - sdf
      - sdg
      - sdh
      - sdi
      - sdj

  tasks:

  - name: Input validation
    assert:
      that:
        - disks | length != 0
        - disks | length >= 4
        - disks | length <= 16
        - disks | length is even
      success_msg: "PASSED: List length is OK"
      fail_msg: "FAILED: List length doesn't fulfill requirements!"

  - name: Iterate every two elements (disks) in loop
    debug:
      msg: "/dev/{{ disks[item] }} and /dev/{{ disks[item + 1] }}"
    loop: "{{ range(0, disks | length, 2) | list }}"

will result into an output of

TASK [Input validation] ********************************
ok: [localhost] => changed=false
  msg: 'PASSED: List length is OK'

TASK [Iterate every two elements (disks) in loop] ******
ok: [localhost] => (item=0) =>
  msg: /dev/sdc and /dev/sdd
ok: [localhost] => (item=2) =>
  msg: /dev/sde and /dev/sdf
ok: [localhost] => (item=4) =>
  msg: /dev/sdg and /dev/sdh
ok: [localhost] => (item=6) =>
  msg: /dev/sdi and /dev/sdj

It seemed easier for me to loop over a range of integers in steps and use them to catch the list elements directly, instead of iterating over the list elements itself and mess around with ansibe_loop.index.

Further Documentation

loop and the range function

To take into account the comment about How do I split a list into equally-sized chunks? and other examples, another approach is simply

  - name: Iterate every two elements (disks) in loop
    debug:
      msg: "/dev/{{ item | first  }} and /dev/{{ item | last }}"
    loop: "{{ disks | batch(2) }}"

resulting into an output of

TASK [Iterate every two elements (disks) in loop] ******
ok: [localhost] => (item=[u'sdc', u'sdd']) =>
  msg: /dev/sdc and /dev/sdd
ok: [localhost] => (item=[u'sde', u'sdf']) =>
  msg: /dev/sde and /dev/sdf
ok: [localhost] => (item=[u'sdg', u'sdh']) =>
  msg: /dev/sdg and /dev/sdh
ok: [localhost] => (item=[u'sdi', u'sdj']) =>
  msg: /dev/sdi and /dev/sdj

Further Documentation