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Tutorial Reference

YAML Mappings

Learn how YAML mappings work, including block and flow styles, simple mappings, sequence mappings, nested mappings, and mixed mappings with key-value pairs.

A YAML file is rarely used to describe a simple scalar. Most of the time, it describes a collection.

YAML collections can be a sequence or a mapping of elements.

In YAML, the collection is represented with proper sequence styles.

YAML Mappings

A mapping is a container consisting of key-value pairs where the keys must be unique.

YAML mappings are also known as associative arrays, hash tables, key: value pairs.

Mapping Styles

Like sequences, there are two ways to write a mapping.

Block Style version

In the block style, each key is written on a new line, followed by a colon (:). The value can be placed either on the same line, or on the following line, indented one level:

person: 
    name: "Tom" 
    surname:
        "Nolan" 
    age:  23

Flow Style version

In the flow style, the mapping is enclosed in curly brackets ({...}), colons (followed by spaces) (: ) are used to separate keys and values, and key-value pairs are separated by commas (,):

person: {name: "Tom", surname: "Nolan", age: 23}

Type of Mappings

We can classify mappings in four different types:

  • Simple mapping
  • Sequence mapping
  • Nested mapping
  • Mixed mapping

Simple Mapping

In simple mapping, we will provide a key in the first part, and then separate with a colon and space (,):

---
host: tutorialreference-host
student: Tom

Sequence Mapping

We can map the value of the key in sequence.

Block Style Exampleyaml
---  
student: Tom 
hobbies:  
  - Maths 
  - ICT
Flow Style Exampleyaml
---  
student: Tom 
hobbies:  [Maths, ICT]

Nested Mapping

The mappings can be nested within each other.

Block Style Exampleyaml
---  
host: tutorialreference-host  
datacenter:  
  location: italy    
  cab: 10  
  cab_unit: 2
Flow Style Exampleyaml
---  
host: tutorialreference-host  
datacenter: {location: italy, cab: 10, cab_unit: 2}

Mixed Mapping

In mixed mapping, we have both mappings and sequences as values.

Block Style Exampleyaml
---  
student: Tom 
details:  
  fatherName: Ryan  
  motherName: Lucy  
more:   
    - Maths  
    - ICT
Flow Style Exampleyaml
---  
student: Tom 
details: {fatherName: Ryan, motherName: Lucy}
more: [Maths, ICT]
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