Q&A

How to represent arrays in YAML

By AlgoInsight Team2026-06-07arraysyaml

Overview

YAML ("YAML Ain't Markup Language") is a human-friendly data-serialization format widely used for configuration (Kubernetes, Docker Compose, CI pipelines, Ansible). An array in YAML terminology is a sequence: an ordered list of items. YAML offers two syntaxes that produce identical data.

graph TD
    A[YAML Sequence / Array] --> B[Block style]
    A --> C[Flow style]
    B --> B1["- item one<br/>- item two<br/>- item three"]
    C --> C1["[item one, item two, item three]"]
    B & C --> D[Same parsed result:<br/>an ordered list]
    style A fill:#1e1b4b,color:#fff

Block Style (most common)

Each element starts with a hyphen and a space, all at the same indentation level:

fruits:
  - apple
  - banana
  - cherry

Parsed, fruits is a list of three strings. Indentation matters in YAML - items must align. Block style is preferred for readability in config files.

A top-level array (no key) is also valid:

- apple
- banana
- cherry

Flow Style (inline, JSON-like)

Compact lists fit on one line with brackets:

fruits: [apple, banana, cherry]

This is handy for short lists and is exactly how JSON would express it (YAML is a superset of JSON).

Arrays of Different Types

Array of numbers / mixed scalars

ports: [80, 443, 8080]
mixed:
  - 42
  - "forty-two"
  - true
  - null

Array of mappings (objects)

A very common pattern - a list where each item is a key/value object:

servers:
  - name: web-1
    ip: 10.0.0.1
    role: frontend
  - name: db-1
    ip: 10.0.0.2
    role: database

Here servers is a list of two maps. Note the indentation: the - introduces each item, and the keys of that item align under it.

Nested arrays (array of arrays)

matrix:
  - [1, 2, 3]
  - [4, 5, 6]
# or block style:
grid:
  -
    - a
    - b
  -
    - c
    - d

Empty array

tags: []

Real-World Example (Docker Compose style)

services:
  web:
    image: nginx:latest
    ports:
      - "80:80"
      - "443:443"
    environment:
      - NODE_ENV=production
      - DEBUG=false
    depends_on: [db, cache]

This combines block-style sequences (ports, environment) and a flow-style sequence (depends_on) in one file - both are valid.

Common Gotchas

• Indentation must use spaces, never tabs. Tabs are forbidden in YAML and cause parse errors.
• Consistent indentation - all items in a sequence must align at the same column.
• Quoting - quote strings that contain special characters (:, #, leading -) or that look like other types ("yes", "3.14", "true") when you want them treated as strings.
• The hyphen needs a trailing space - -apple is a scalar string, - apple is a list item.
• Don't mix styles confusingly within one list (it's legal but harms readability).

Why This Matters in Interviews

DevOps and backend interviews often involve reading or writing YAML for Kubernetes, CI/CD, or Ansible. Know both array styles, how to express a list of objects (the most common real-world shape), and the tab/indentation rules that cause the majority of YAML errors.

Common Pitfalls

• Using tabs → instant parse failure.
• Forgetting the space after the hyphen.
• Misaligned indentation merging or breaking nesting.
• Unquoted values that YAML coerces to the wrong type (e.g. no becoming boolean false).

References

• YAML specification: https://yaml.org/spec/
• YAML on Wikipedia for a quick overview: https://en.wikipedia.org/wiki/YAML

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