# Class 48: Variable-Arity Procedures

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This outline is also available in PDF.

Held: Friday, April 27, 2007

Summary: Today we consider how and why to write procedures that can take different numbers of arguments in different situations.

Related Pages:

Due

Notes:

• Exam 3 distributed.
• EC for Monday's Alumna Scholar talk by Dr. Nesbitt.

Overview:

• Definition of arity.
• Why have variable-arity procedures.
• How to write variable-arity procedures.
• Lab.

## Variable-Arity Procedures

• Def. Arity: The number of parameters a procedure expects.
• Some procedures naturally take varying numbers of parameters:
• `+` takes 0 or more parameters.
• `display` takes 1 or 2 parameters.
• `map` takes 2 or more parameters.
• It would be nice to be able to write our own procedures of variable arity (that is, varying numbers of parameters).
• Zero or more parameters:
```(lambda params
body)
```
• `params` is not parenthesized.
• In your body, treat `params` as a list.
• One or more parameters:
```(lambda (first . remaining)
body)
+ In your body, treating `remaining` as a list.
```
• Two or more parameters:
```(lambda (first second . remaining)
body)
+ In your body, treating `remaining` as a list.
```

## Lab

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Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

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Samuel A. Rebelsky, rebelsky@grinnell.edu

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