Lesson 15 of 59

5 mins read

Optional Positional Parameters & Default Values

In Plain English

While named parameters use curly braces `{ }`, Optional Positional parameters are wrapped in square brackets `[ ]`. Callers do not label them by name; instead, they are supplied in strict positional order. Optional positional parameters can have default values or be nullable.

The Mental Model

"Optional positional parameters are like ordering a combo meal: you must order the burger first, then you can optionally choose fries, and optionally a drink in that exact order."

Deep Dive: How It Works

Positional Sequence: Callers cannot skip the first optional positional argument to supply the second one.

Mutual Exclusivity: A function signature can have named parameters OR optional positional parameters, but NEVER both.

Defaults: `[String prefix = "INFO", int indent = 0]` assigns fallback constants when omitted.

Syntax Blueprint

String log(String message, [String level = "INFO", int code = 0]) {
  return "[$level:$code] $message";
}

Positional arguments followed by bracketed optional positional parameters.

Core Rules to Remember

Bracket Notation [ ]: Square brackets define optional positional arguments.

Ordered Omission: Arguments are omitted from right to left.

No Mixing with Named: Choose either `{ }` named parameters or `[ ]` optional positional parameters per function.

Common Beginner Traps & How to Fix Them

Mistake:Trying to mix `{ }` named parameters and `[ ]` optional positional parameters in the same function signature.

Why it happens: Dart syntax disallows combining both styles in a single parameter list.

How to fix: Standardize on named parameters `{ }` for functions with more than 2 optional arguments.

Pro Tip: Use optional positional parameters for simple 1-argument extensions like `String repeat(String str, [int times = 1])`.

Live Interactive Example

Hit Run Code to see it live

Optional Positional Parameters in Action

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: The formatCurrency function supplied defaults for currency and decimals when omitted.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Create Function with Optional Positional Arguments

Define `String wrap(String text, [String tag = "div"]) => "<$tag>$text</$tag>";`.
Inside `main()`, print `wrap("Header", "h1")` and `wrap("Content")`.

Sandbox Output

Click "Run & Check" to test your solution.

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