► |  | The Racket Guide

1 | Welcome to Racket
2 | Racket Essentials
3 | Built-In Datatypes
4 | Expressions and Definitions
5 | Programmer-Defined Datatypes
6 | Modules
7 | Contracts
8 | Input and Output
9 | Regular Expressions
10 | Exceptions and Control
11 | Iterations and Comprehensions
12 | Pattern Matching
13 | Classes and Objects
14 | Units (Components)
15 | Reflection and Dynamic Evaluation
16 | Macros
17 | Creating Languages
18 | Performance
19 | Running and Creating Executables
20 | More Libraries
21 | Dialects of Racket and Scheme
22 | Command-Line Tools and Your Editor of Choice
| Bibliography
| Index

▼ | 4 | Expressions and Definitions

4.1 | Notation
4.2 | Identifiers and Binding
4.3 | Function Calls (Procedure Applications)
4.4 | Functions (Procedures): lambda
4.5 | Definitions: define
4.6 | Local Binding
4.7 | Conditionals
4.8 | Sequencing
4.9 | Assignment: set!
4.10 | Quoting: quote and '
4.11 | Quasiquoting: quasiquote and `
4.12 | Simple Dispatch: case
4.13 | Dynamic Binding: parameterize

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#### 4.12`` Simple Dispatch: case

The case form dispatches to a clause by matching the result of an expression to the values for the clause:

> > (case expr
> > [(datum ...+) expr ...+]
> > ...)

Each datum will be compared to the result of the first expr using eqv?. Since eqv? doesn’t work on many kinds of values, notably strings and lists, each datum is typically a number, symbol, or boolean.

Multiple datums can be supplied for each clause, and the corresponding expr is evaluated if any of the datums match.

Example:

> > (let ([v (random 6)])
> (printf "~a\n" v)
> (case v
> [(0) 'zero]
> [(1) 'one]
> [(2) 'two]
> [(3 4 5) 'many]))

> 1
> 'one

The last clause of a case form can use else, just like cond:

Example:

> > (case (random 6)
> [(0) 'zero]
> [(1) 'one]
> [(2) 'two]
> [else 'many])

> 'many

For more general pattern matching, use match, which is introduced in Pattern Matching.

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