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# Data Structures — GDScript


**Skill codes on this page**

| Code | Level | Skill |
|---|---|---|
| C635 | 5–6 | arrays |
| C636 | 5–6 | records |
| C638 | 5–6 | identifies, in the internal documentation, why the selected data types and data structures were used |

**Definition.** A *data structure* is a method of organising data to allow particular operations to be performed on it efficiently.

## C635 — Arrays

**Definition.** A list of elements indexed by position. In most programming languages, the first element has index zero.

```gdscript
# C635 — Array[float]: fares need decimal places
var fares: Array[float] = [5.30, 8.00, 11.20]

func show_fare() -> void:
    $ResultLabel.text = "Zone 1: $" + str(fares[0])
```

Typed (`Array[float]`) earns it. Bare `var fares = []` does not — no type for the marker to see.

**In tile-matching-game** (`board.gd`) — a typed array of your own class:

```gdscript
# tile-matching-game — board.gd
var found: Array[Card] = []   # C635 — Array[Card]: only cards can get in
for child in get_children():
    if child is Card:
        found.append(child)
```

**Two-dimensional array** — a grid organised into rows and columns, accessed with two indices:

```gdscript
# C635 — 2D array: fare by [zone][day type], 0 = weekday, 1 = weekend
var fare_grid: Array = [
    [5.30, 3.60],
    [8.00, 5.40],
]

var weekend_zone2: float = fare_grid[1][1]
```

## C636 — Records

**Definitions.**

- *Record:* a complete set of fields relating to an entity, such as a person.

- *Field:* a single data item in a record (e.g. `FamilyName`).

GDScript has no `record` keyword — a **Dictionary** is the everyday record, one key per field:

```gdscript
# C636 — record: one journey, fields of different types
var journey: Dictionary = {
    "date": "2026-08-11",
    "zone": 2,
    "fare": 8.00,
    "concession": false,
}

print(journey["fare"])   # access one field
```

**In tile-matching-game** (`deck.gd`) — each CSV row becomes a record, and the deck is an array of them:

```gdscript
# tile-matching-game — deck.gd
cards.append({
    "pair": pair,   # C636 — fields: which card matches which
    "text": row[1].strip_edges(),
    "image": row[2].strip_edges() if row.size() > 2 else "",
})
```

An **array of records** is a table — the shape most SAT data layers need:

```gdscript
# C635 + C636 — a table: array of journey records
var journeys: Array[Dictionary] = []

func add_journey(zone: int, fare: float) -> void:
    journeys.append({"zone": zone, "fare": fare})
```

A **custom class** is the stronger record — typed fields the editor can check — and doubles as level 9–10 evidence (see [OOP Concepts](/sd/C06/OOP%20Concepts)).

**In club-ladder** (`team.gd`) — the class-as-record, real: its own doc comment says "A Team is plain data."

```gdscript
# club-ladder — team.gd
class_name Team
extends RefCounted

var team_name := ""   # C636 — the fields of the record
var wins := 0
var draws := 0
var losses := 0
```

An `Array[Team]` (`season.gd`) is then the table — the class version of the array-of-dictionaries above.

## C638 — Why these types and structures

Level 5–6 asks you to **identify** the reason in internal documentation — one line on each structure:

```gdscript
# C638 — Array[Dictionary]: journeys arrive one at a time in order (array),
# and each journey has mixed-type fields (record)
var journeys: Array[Dictionary] = []
```

The "why" comes from how the structure behaves — see the input/storage/output table in [Data Type Characteristics](/sd/C06/Data%20Type%20Characteristics).

**Model documentation in the wild:** the header comment of `tile-matching-game/deck.gd` identifies the whole design — one CSV row per card face, rows sharing a `pair` value match — before any code runs. That is C638 done as a file-level comment.

## Check Your Understanding

1. Why does `var scores = []` fail C635?

>| ### Answer
>| It is untyped — `Array[int]` (or a comment naming the type) is required, so the marker can assess the choice.

2. Name two ways to build a record in GDScript.

>| ### Answer
>| A Dictionary with one key per field, or a custom class with typed fields (the stronger version).

3. What everyday shape is an "array of records"?

>| ### Answer
>| A table — one record per row, like journeys or teams.