EGR 2261 Unit 6 - PowerPoint Presentation

Slide1

EGR 2261 Unit 6User-Defined Functions

Read Malik,

p

ages

345-382 in Chapter 6

.

Homework #6 and Lab #6 due next week.

Quiz next week (covers Units 5 and 6).Slide2

Introduction

Functions

are like miniature programs that can be combined to form larger programs.They let you divide complicated programs into manageable pieces. Advantages of this approach include:It’s easier to test small pieces.You may be able to reuse pieces in later projects.Functions are sometimes called subroutines, subprograms, or modules.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

2Slide3

Predefined Functions

The textbook distinguishes

predefined functions

(which someone else has written for us) from user-defined functions (which we write ourselves).As you know, predefined functions are organized into library files that you can include in your programs. For example:I/O functions are in <iostream>.Math functions are in <

cmath>.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

3Slide4

Predefined Functions (cont'd.)

Many predefined functions require one or more

parameters

(input values that you must provide when you call the function). Another name for parameters is arguments.Examples of some predefined functions: rand() takes no parameters.

sqrt(2.5) takes one parameter. pow(x, y) takes two parameters.See Table 6-1 in the text (next two slides) for some more predefined functions.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

4Slide5

Some Predefined Functions

Continued on next slide….Slide6

Some Predefined FunctionsSlide7

Using a Predefined Function

To use a predefined function, you must:

Include the appropriate header file in your program using the

#include directive.Know the following items:Name of the function.Number of parameters, if any.Data type of each parameter.Data type of the value returned (if any): we call this the function’s type or return type.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

7Slide8

Here is the information you need to know to use the power and square root functions.

Examples

Power

Square Root

Function namepowsqrtHeader file <

cmath

>

<

cmath

>

Number of parameters

2

1

Data types of parameters

double, double

double

Return type

double

doubleSlide9

Here are several attempts to call the

sqrt

function. Most of these fail for the reasons noted in the comments.

Examples of Errors When Calling a FunctionSlide10

Value-Returning Functions versus Void Functions

Every function is either value-returning or void.

Value-returning functions

have a return type.When the execute, they return a value of a specific data type.Void functions do not have a return type.When they execute, they do not return a value.All of the functions listed in Table 6-1 are value-returning functions.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

10Slide11

Value-Returning Functions

Here are some

ways

you can use the value returned by a value-returning function:You can display it: cout << pow(4, 3);You can assign it to a variable for later use.

myResult = pow(4, 3);You can use it as part of a larger expression. if (pow(4, 3) + x > 200)Common mistake: “Drop the ball” by not doing anything with the return value: pow(4, 3);

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

11Slide12

User-Defined (Programmer-Defined) Functions

Our textbook uses the term “user-defined function” to refer to functions that we create when we write our programs.

This is in contrast to predefined functions.

A better term might be “programmer-defined function,” since the user is the person who will be using our programs after we write them. That person doesn’t define functions—rather, we as programmers do.

12Slide13

How Many User-Defined Functions in a Program?

Every C++ program must contain at least one user-defined function, which must be named

main

, whose return type must be int.That’s why all of our programs contain a line like this: int main() {A program can also contain as many other user-defined functions as you wish.

13Slide14

Flow of Execution

When a program runs, execution always begins at the first statement in the function named

main.

Other functions are executed only when called.A function call transfers control to the first statement in the body of the called function.When the called function ends, control is passed back to the point immediately following the function call.The function’s returned value (if any) replaces the function call.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

14Slide15

Example of a Program with Two User-Defined Functions

Function call

Function definition

Execution starts here, even though the other function is listed first.Slide16

Single-Stepping: F10 versus F11

You know that F10 (or

Debug > Step Over

) lets you single-step a program. When you’re single-stepping and you reach a statement that calls a function, you have a choice:

Use F10 if you want to execute the entire function in one step. Use F11 (or Debug > Step Into) to step into the function and execute each of its statements one at a time.If you’ve stepped into a function and want to execute the rest of it as one step, use Shift+F11 (or Debug > Step Out).Slide17

Syntax: Value-Returning Function

Syntax to

define

a value-returning function:The formal parameter list can be empty:Or it can consist of a list of comma-separated dataTypes and identifiers:

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

17

functionType

functionName

(

dataType

identifier,

dataType

identifier, …)Slide18

Some Terminology

A function’s

heading

(or function header) is the first line of the function.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

18Slide19

Function Definition versus Function Calls

For a user-defined function other than main(), you

define

the function once

, and you call the function as many times as you wish.Function callsFunction definitionSlide20

Formal Parameters versus Actual Parameters

A

formal parameter

is a variable declared in a function heading. Example: double addOne(double numberIn)

An actual parameter is an expression or variable listed in a call to a function.Examples: modified = addOne(original); cout << addOne(156.7);

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

20Slide21

Function Call

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

Syntax to

call

a function:

Syntax of the actual parameter list

:

If the function’s formal parameter list is empty, a call to the function looks like:

21Slide22

Example of a Function with No Parameters

Call to a function with no parameters

Definition of a function with no parametersSlide23

return Statement

A value-returning function returns its value via the

return

statement.It passes this value outside the function to the statement that called it.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

23Slide24

Syntax: return Statement

Syntax:

When a

return statement executes:The function immediately terminates.Control goes back to the caller and the function call in the calling statement is replaced by the function’s return value.When a return statement executes in the function main, the program terminates.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

24Slide25

More Than One return

Statement?

A function can have more than one

return statement. On any particular function call, only one of these will be executed.Example:

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

25Slide26

Caution #1: Make Sure that Value-Returning Functions Always Execute a Return

If the

return

statements in a function are located inside if statements or other conditional code, your function may not execute any return statements when it runs.This is almost always a mistake that you want to avoid.See example on next slide.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

26Slide27

Caution #1: Make Sure that Value-Returning Functions Always Execute a Return

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

27

In this code we’re guaranteed to execute a

return

for every possible value of

x

.

In this code we won’t execute a

return

for some values of

x

.Slide28

Caution #2: A Value-Returning Function Can Only Return One Value

Although you can pass as many values as you want

into

a function (via the parameter list), you can only return one value from a function (via a return statement).See next slide for two examples of functions that try (but fail) to return more than one value.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

28Slide29

Caution #2: A Value-Returning Function Can Only Return One Value

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

29Slide30

Summary: Parts of a User-Defined Function

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

30Slide31

Nested Function Definitions Not Allowed

Recall that you can nest loops within other loops,

if

statements within other if statements, loops within if statements, etc.You can also call one function from within another function.But in C++ you cannot define one function within another function. See example on next slide…

31Slide32

Example: Nested Functions Not Allowed

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

32Slide33

Function Prototype

User-defined functions can be placed either before or after the function

main

.Although execution always starts with main, compilation proceeds from top to bottom.So if a user-defined function is placed after the function main, the function’s prototype must be placed before the function

main. The compiler can then correctly translate a function call.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

33Slide34

Function Prototype (cont’d.)

A

function prototype

is a copy of the function heading, ending in a semicolon.Syntax:In the parameter list you must at least specify each parameter’s data type. You don’t have to specify each parameter’s name, but you can.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

34Slide35

Function Prototype (cont’d.)

Example: Consider this function definition:

double

addOne(double numberIn)

{ double numberOut; numberOut = numberIn + 1; return numberOut;

}

Here are two acceptable function prototypes:

double

addOne

(double

numberIn

);

double

addOne

(double);

The following is not an acceptable prototype:

double

addOne

();

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

35Slide36

Review: Value-Returning Functions versus Void Functions

Recall that every function is either value-returning or void.

Value-returning functions

have a return type.They return a value of a specific data type using one or more return statements.Void functions do not have a return type.They may contain one or more return statements, but these return statements cannot return values.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

36Slide37

Void Functions (cont’d.)

Void function definition syntax:

As

with value-returning functions, a void function might not take any formal parameters, in which case the formal parameter list is empty.

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

37Slide38

Summary: Several Possibilities

A function’s heading tells you whether the function takes parameters and whether it returns a value. We have at least four possibilities:

Doesn’t take parameters, doesn’t return a value:

void myFunc1()Takes parameters, doesn’t return a value:

void myFunc2(int num1, double z)Doesn’t take parameters, returns a value: double myFunc3()Takes parameters, returns a value: int myFunc4(double sum, bool finished)

38Slide39

Void Functions (cont’d.)

Function call

syntax for a void function:

A call to a void function is a stand-alone statement. You can’t use it as part of an assignment statement or as part of a larger expression, as we did with value-returning functions in these earlier examples: cout << pow(4, 3);

myResult = pow(4, 3); if (pow(4, 3) + x > 200)

C++ Programming: From Problem Analysis to Program Design, Seventh Edition

39Slide40

Example of a Void Function

This

return

statement does not return a value.

This function call is a standalone statement, not part of a bigger statement.