Overview/Questions Whats the point? Writing functions Calling - - PDF document

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Aaron Stevens

23 March 2009

CS101 Lecture 23: The Function of Functions

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Overview/Questions

– What’s the point? – Writing functions – Calling functions – Formal parameters – Actual parameters – Scope rules and name spaces – Return values

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Functions: what’s the point?

We’ve used a lot of functions in our programs, in several forms:

– Built-ins functions, like input()and float() – Library functions, like math.tangent()

Why did we use those? What was in it for us?

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Function Example

Example: happy birthday program

– Start with simple program with some repeated statements, group into function. – Add: formal parameter to receive name.

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Functions: Simple Example

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Functions: what’s the point?

Think of a a function as a subprogram – a program within a program. Function A sequence of statements which are given a name. This sequence can then be invoked by calling the name.

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Function Declaration Syntax

def function(<formal-parameter-list>): <statement> <statement> – The keyword def indicates the identifier that follows it is a new function name. – The <formal-parameter-list> is a list of 0 or more parameters required to invoke the function. – The parentheses are a required part of the definition, as is the colon after the parentheses.

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Formal Parameters

Functions take parameters for 2 reasons: – As changeable inputs to the function

• Example: name in happy birthday program

– Variables in a function are local to that function (scope)

• Example: name in birthday program

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Variable Scope

Scope refers to the places in a program where a variable name is visible (allowed to be read). – The variables in a function are local to that function. – To use a variable from outside the function it must be passed/received in as a parameter.

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Why not use global variables?

Global variables are declared at the top of a file, and are visible in the entire program. Why not use global variables all the time?

– Namespace gets cluttered – Naming conflicts, hard to remember which is which

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Actual Parameters

A function definition has the form:

def function(<formal-parameter-list>): <statement> <statement>

A function call has the form:

function(<actual-parameter-list>)

At run time, the actual parameters (passed by the caller) are (shallow) copied into the formal parameter list. Thus, the function can use the variables declared in its formal parameter list.

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Return Values

We think of parameters as inputs to a function. Sometimes we want to call a function and get data back. This data is called a return value. A function returns values by using the return statement:

return <expression>

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Return Values

Example: calculating area (single return value)

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Take-Home Points

– Why use functions – Variable Scope – Formal Parameters – Actual Parameters – Return Values

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Student To Dos

– HW09 due Tuesday 3/24 – QUIZ 4 is on FRIDAY 3/28

• Covers material in lectures (16-22)
• Flash concepts, motion tweens, images, audio
• Programming languages, python introduction
• Elements of python programs, computing with numbers

– Reading: How to Think Like A Computer Scientist: Learning with Python

Available online at: http://openbookproject.net//thinkCSpy/

• Ch03 (today)
• Ch04 (Wednesday/Friday)