C++ Programming for Games Module I phần 4 ppt

To lustrate, suppose we have a program that must input the radius of a circle from the user and compute evaluates returns 0.707, sin 45° =0.707; that is to say sin x the sine of the give

Chapter 3

Functions

Introduction

A function is a unit of code designed to perform a certain task In order to perform its task, a function

me information and/or returns some information The concept is somewhat similar to

e trigonometric function

typically inputs so

mathematical functions Consider th sin( )x , which takes a parameter x and

) some value, namely the sine of x For example, the sine of 45° is approximately

, given 45° as a parameter, the function works to compute

hen returns or evaluates to the result 0.707

he utility of functions can be be easily demonstrated if we first consider a program without them To lustrate, suppose we have a program that must input the radius of a circle from the user and compute

evaluates (returns

0.707, sin( )45° =0.707; that is to say sin( )x

the sine of the given angle, and t

T

il

the area throughout the program (Recall the area of a circle is given by A=π ⋅r2, where r is the radius

of the given circle.) As a first attempt we might do the following:

Program 3.1: Program without Functions

float area = PI*radius*radius;

cout << "Area = " << area << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Enter a radius of a circle: ";

cin >> radius;

area = PI*radius*radius;

cout << "Area = " << area << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Enter a radius of a circle: ";

Press any key to continue

ate code which

to m to be made, it would be necessary to make the change

compute the area and return the result For the sake of discussion, let us assume such a unction exists and call it Area; moreover, assume the task of Area—that is inputting the radius from the user, computing the area and returning the result—is executed by simply writing “Area()” in a C++

program (When we execute a function we say that we call it or invoke it.) Program 3.1 can now be

rewritten like so:

Program 3.2: Revision of Program 3.1 using an Area function Note that this program will not compile yet because the function Area is not actually defined Note that we have bolded the calls to the area function

The main problem which Program 3.1 suffers is code duplication—there is duplic

nti lly performs the same task Besides bloating the code, prog

aintain because if changes or corrections need

orr ction in every duplicated instance In a large real world pr

e fi e and making changes is not only a waste of time, but it is prone to e

The problems of Program 3.1 could be resolved using a function whose sole task is to input the radius from the user,

// Input a radius and output the circle area

cout << "Area = " << Area() << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Area = " << Area() << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Area = " << Area() << endl;

// and so on

}

Program 3.2 is much cleaner and more compact There is no longer duplicate code for the input code

pute a and calculation code—we simply write “Area()” wherever necessary to input a radius and com

circle area Moreover, if a change or correction needs to be made to the code Area executes, it would

modification of the Area function With that, let us see how Area works

ons, let us examine the actual syntactic details of making

unction (i.e., create) the following needs to be specified:

pe of value the function evaluates to)

ƒ name (i.e., what you want to refer to it as)

eter list (i.e., what values does it take as input, if any)

ƒ

plem

only necessitate

3.1 User Defined Functions

Now that we understand the benefits of functi

and using them To define a f

ƒ return type (i.e., the ty

ƒ param

body (i.e., the code to be executed when the function is invoked)

3.1 shows the syntax of how the Area fun

Fi

im ented

Figure 3.1: Function definition

l compile and run

on of Program 3.2, this time with the Area function defined

lProgram 3.3 rewrites Program 3.2, this time defining Area so that the program wi

Program 3.3: Revisi

#include <iostream>

// Input a radius and output the circle area

cout << "Area = " << Area() << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Area = " << Area() << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Area = " << Area() << endl;

// and so on

}

Observ

functio lared or defined before it is called, as the compiler must recognize the

nction before you call it A function declaration (also called a function prototype) consists of the

return type, function name, and parameter list followed by a semicolon—there is no body in a function

However, once call the function Program 3.4 rewrites Program 3.3 using a function declaration

e from Program 3.3 that the function Area is defined before any calls to that function A

n must be either dec

fu

declaration Once a function is declared, it can be defined elsewhere in the program

declared, the function definition can come even after you

Program 3.4: Revision of Program 3.3, this time using a function declaration

#include <iostream>

using namespace std;

// Function declaration The function declaration just tells the

// compiler the function exists (it will be defined later), and

// its name, return type and parameters

float Area();

int main()

{

// Input a radius and output the circle area

cout << "Area = " << Area() << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Area = " << Area() << endl;

// Do some other work

cout << "Other work " << endl;

// Input another radius and output the circle area

cout << "Area = " << Area() << endl;

// and so on

}

// Function definition The function definition contains the

// fun ction body and consists of the code that specif ies what

// the function actually does

The Area function did not have a parameter But let us look at an example which does have a parameter

A useful function might be one that cubes (x ) the given input, as follows: 3

rogram 3.5: Function with a parameter We have bolded the function calls to

Paramete

Cube P

#include <iostream>

using namespace std;

// Declare a function called 'Cube' which has a parameter

// of type 'float' called 'x', and which returns a value

// of type 'float'

float Cube( float x);

// Provide the definition of Cube it computes x^3

float Cube( float x)

Press any key to continue

The Cube function is similar to the function except that it takes a parameter (i.e., its parameter list side the parentheses of the function declaration/definition is not empty) A parameter is not a value in Area

riable) That is, the function caller will “pass in” or

le for the function to use The actual value passed into a

alled an argument

0) Here input0 is the argument—it stores a

Cube hat is, the value stored in input0 is copied into

meter x The word “copied” is important, as input0 and x are not the same variables but

the function is called This copying of argument value to rame he argument is copied to the parameter, the code inside the

e value that was passed into it

gure code relative to the calling program code Think of a

eed data into it (copy arguments into the parameters), it

s so n body), and it outputs a result (returns something back to u) Again, functions are useful primarily because they prevent code duplication and provide a level of

in

and of itself, but rather a value placeholder (va

“input” a value into this placeholder variab

particular function call is c

For example, in program 3.5, we write Cube(input

ecific value which is input into the function; t

sp

the Cube para

will contain copies of the same value when

ter is called passing by value Once t

pa

body of Cube can execute, where x contains th

Fi 3.2 shows how you can think of function

function as a separate “machine” where you f

mething with those parameters (functio

doe

yo

code organization; that is, breaking up programs into more manageable parts, where each part does a specific task

Figure 3.2: Calling functions with parameters and returning results

3.1.3 Functions with Several Parameters

Functions are not limited to zero or one parameter, but can have several parameters The following program uses a function named PrintPoint, which takes three parameters: one each for the x-coordinate, y-coordinate and z-coordinate of a point The function then outputs the coordinate data in a convenient point format

Program 3.6: Functions with several parameters

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As Program 3.6 shows, additional parameters could be added and

is called, an argument for each parameter is provided separated with the comma operator

other thing to notice about the PrintPoint function is that because its sole task is to output a point the console window, it does not need to return a value It is said that the function returns void, and

specified for the return type Observe that you do not need to write a return

a function that returns void

nctions themselves can contain other code units such as if statements and loops Furthermore, these

s now to see how this vocabulary is used

code units can be nested This brings up the topic of variable scope Variable scope refers to what

variables a code unit can “see” or “know about” A variable defined inside a particular code unit is said

to be a local variable relative to that code unit Additionally, a variable defined outside a particular code unit is a global variable relative to that code unit (A subunit of code is not considered to be

“outside” the unit of code that contains the subunit.) A unit of code can “see” variables that are global and local, relative to it Let us look at a couple of example

Press any key to continue

The first variable defined is gPI Because gPI is outside the code units of main and SphereVolume,

it is global relative to both of them, and both functions can “see”, use, and modify gPI

The next set of variables occurs inside the main function unit of code These variables, input0 and V,

re local to main and global to no code unit Therefore, main is the only code unit that can “see” them

ot “see” input0, and if you try to use input0 in SphereVolume,

error Note, however, that you can define a variable with the same variable

er code unit, as we do with V Because the variables V are defined in separate are completely independent of each other

ned, SphereVolume defines its own separate version of V This V is local to

lume is the only code unit that can “see” this V Additionally,

e When the argument input0 is passed into variable radius It is important to understand

t input0 and radius are separate variables in memory

t are destroyed when the program exits that code unit For example, when SphereVolume is invoked, the program will create memory for the variable V After

a

For example, SphereVolume cann

you will get a compiler

name as a variable in anoth

code units they

Finally, as already mentio

th m variable radius is local to SphereVolum

in input0 is copied to the

e para eter

the function, the value stored

es place and thathat this copy tak

Important: Variables declared in a code uni

the function ends (after V is returned) the memory for V is deleted

rogram 3.8 also has a logic error Namely, the counting variable cnt is

er of loop cycles Recall the “important” note from the Section 3.2.1

id

neither local nor glo

“undeclare

Besides the compilation error, P

ot keeping track of the numb

n

Variables declared in a code unit are destroyed when the program exits that code unit Every time the for-loop repeats, cnt is re-created and re-destroyed after that loop cycle, and therefore, the value does not persist The program needs to be rewritten as follows:

Program 3.9: Revision of Program 3.8

Note that

re

Press any key to continue

Recall that we can create varia

u

bles of t t in ifferent code units This is nctions b f op statements Program 3.10 eclares a variable called var local to main and assigns 5.0 to it The program then asks if var is greater

ut However, this presents a dilemma: Which

ar is used in the cout statement: the one local to the ‘if’ statement or the one local to main? As a

he same name if they exis dstraightforward with separate f ut can be tricky when using i /lo

3.3 Math Library Functions

s for many of the elementary math functions and operations,

oot and absolute value ust be included The

ble 3.

The C++ standard library provides function

such as trigonometric, logarithmic, and exponential functions, as well as square r

functions To use the standard math functions, the <cmath> header file m

following table summarizes some of the most commonly used math functions:

Ta 1: Some Standard Library Math Functions

( )x

ln

Remark 1: The trigonometric functions work in radians and not degrees A number x can be converted

from radians to degrees by multiplying it by 180° π For example:

x Likewise, a number x can be converted from degrees to radians

y multiplying it by π 180° For example: 360°=360°⋅π 180°=2π

b

Remark 2: The functions above work with floats, hence the ‘f’ suffixes The standard math library

t work with doubles The double versions are the same except that the ‘f’ example, the double version of the cosine function would be double x) In real-time 3D computer game graphics floats are typically used, which is the

e float versions were given in the above table

s how to call some of these “calculator” functions The results can be mputations on a calculator

also provides versions tha

ffix is omitted For

su

cos(double

son why th

rea

The following program show

verified by performing the co

P r 3.11: Examples of using the standard math

#include <iostream>

cout << "cosf(0.0f) = " << cosf(0.0f) << endl;

cout << "sinf(quarterPI) = " << sinf(quarterPI) << endl;

cout << "sqrtf(2.0f) = " << sqrtf(2.0f) << endl;

cout << "logf(expf(1.0f)) = " << logf(expf(1.0f)) << endl;

cout << "powf(2.0f, 3.0f) = " << powf(2.0f, 3.0f) << endl;

cout << "fabsf(-5.0f) = " << fabsf(-5.0f) << endl;

cout << "floorf(2.3f) = " << floorf(2.3f) << endl;

cout << "ceilf(2.3f) = " << ceilf(2.3f) << endl;

Press any key to continue

3.4 Random Number Library Functions

The C++ standard library provides a function called rand (include <cstdlib>), which can be used to generate a pseudorandom number This function returns a random integer in the range [0,

Program 3.12: Random numbers without seeding