UCLA CS 31 Lecture5 Post On Functions

Wednesday, October 20th

• Review Challenge• A new data type: bool• “Short Circuiting”• Introduction to Function

Calls

Review ChallengeWrite a program that reverses a string…

void main(void){ string s = “a man a plan a canal panama”;

// write your code here to reverse the string

cout << s; // prints “amanap lanac a nalp a nam a”;}

Boolean Variables: boolThe bool data type is another C++ type just

like int, float, or short.

bool variables can hold only two values: true or false

int main(void){ bool oldPerson; int age;

cin >> age; if (age > 33) oldPerson = true; else oldPerson = false; …}

int main(void){ bool bigger;

bigger = 10 > 3;

if (bigger == true) cout << “10 is > than 3\n”;}

oldPerson

?? age

-44

17

17false bigger ??true

More bool Examplesint main(void){ bool bigEars; ...

if (bigEars == true) cout << “U got big Ears!”;

if (bigEars) cout << “U got big Ears!”;}

There are two different ways to check if a boolean variable is true.

There are two different ways to check if a boolean variable is false.

int main(void){ bool bigEars; ...

if (bigEars == false) cout << “Small ears!”;

if ( ! bigEars ) cout << “Small ears!”;}

More bool Examples

#include <iostream>#include <cstdlib>

int main(void){ bool done = false; int myNum = rand( ); // 0 to 65535

while ( done == false) { int guess; cout << “Guess my #: “; cin >> guess; if (guess == myNum) done = true; } cout << “You got it!\n”;}

Boolean variables are often used to decide when to end a loop.

done

false

myNum

721

The rand function returns a random integer between

0 and 65535. rand is a function just like main is.

guess -10

Guess my #:721

721721 == 721??

true

true == false??

You got it!

Short Circuiting

if (cute == true || smart == true || rich == true)

cout << “Hey, want to go on a date?”; Consider the above expression…

If you’re either cute, or smart, or rich, then I’d like to go out with you.

Let’s say that I happen to know you’re cute.

bool cute = true;

Once I know you’re cute, I no longer need to waste time asking the other two questions…

Short Circuiting

if (cute == true || smart == true || rich == true)

cout << “Hey, want to go on a date?”; This is called short-circuiting.

In such an if statement, C++ will evaluate the expressions from left-to-right…

The moment C++ finds a condition that satisfies the if statement, it will skip the rest

of the cases and continue with the next statement.

bool cute = true;

This speeds

your program up!

Short Circuiting With OR ||

int eyes; cin >> eyes;

if (eyes < 2 || eyes > 8 || eyes == 5) cout << “FREAK!\n”;…

16

Case 1: Your “if statement” uses all || as above.

As soon as an expression (e.g. eyes > 8) is found to be true, C++ knows that it doesn’t

need to evaluate anything else.

16 < 2 16 > 8

Short Circuitingint age; cin >> age;

if (age > 18 && smart == true && cute == true) cout << “<witty pickup line here>.\n”;else cout << “Uh… See you later!\n”;

17

Case 2: Your “if statement” uses all && as above

As soon as an expression (e.g. age > 17) is found to be false, C++ knows that it doesn’t

need to evaluate anything else.

17 > 18??

Short Circuiting

What does this program print if the user types in

11 and 10?

int main(void){ int hw, sleep;

cout << "Enter hours of HW: "; cin >> hw; cout << "Enter hours of sleep: "; cin >> sleep;

if ( hw++ > 10 || ++sleep < 8 ) cout << “You nerd!\n";

cout <<"hw is " << hw << "\n"; cout <<"sleep is " << sleep << "\n";}

hw 50 sleep 69

Enter hours of HW:11Enter hours of sleep:10

1110

11 > 10???

12

hw is 12sleep is 10

You nerd!

Next Topic: Functionsint main(void){ int i, n, f;

n = 5; for (i=1,f=1;i<=n;i++) f *= i; cout << n << “factorial:” << f; ...

n = 15; for (i=1,f=1;i<=n;i++) f *= i; cout << n << “factorial:” << f;

...

n = 6; for (i=1,f=1;i<=n;i++) f *= i; cout << n << “factorial:” << f;}

What’s wrong with this program?

FunctionsIn the previous example, we repeated the

same logic over and over and over…

Not only does this make our program hard to read, but it also may introduce bugs!

for (i=1,f=1; i<=n ;i++) f *= i; // calculate n factorial cout << n << “ factorial:” << f;

...

for (i=1,f=1; i<n ;i++) f *= i; // calculate n factorial cout << n << “ factorial:” << f;}

We can solve these problems with C++ functions!

FunctionsA function is a special component of a program that performs a certain task (like computing a

factorial).

Each function is self-contained and independent of the other logic in your program.

Each function should perform a single, well-

defined task.

Using functions, we can simplify a program and split it up into smaller, more manageable

components.

Simplifying With Functionsint main(void){ int i, n, f;

n = 5; for (i=1,f=1;i<=n;i++) f *= i; cout << n << “factorial is ” << f; ... n = 15; for (i=1,f=1;i<=n;i++) f *= i; cout << n << “factorial is ” << f; ... n = 6; for (i=1,f=1;i<=n;i++) f *= i; cout << n << “factorial is ” << f;}

for (i=1,f=1;i<=n;i++) f *= i; cout << n << “factorial is ” << f;

void fact(int n){

}

int i,f;

int main(void){ fact(5); ... fact(15); ... fact(6);}

FunctionsIn a well-written program, the main function calls

other functions to do the work of the program.

main should not do all of the work itself:

int main(void){

GetTestScores(); // call to a functionComputeAverage();PrintScores();

}

A Simple Program With Functions

void praise(){ cout << “totally cool\n”;}

void tease(){ cout << “would like to be ”; praise();}

int main(void){ cout << “Carey is “; praise(); cout << “David S.“; tease(); cout << “The end.\n”;}

This program has three functions: main, praise, teaseAs always, our program starts in main!

Carey is

totally cool

David S.

would like to be totally cool

The end.

When we reach a function, we run the logic in the function and then continue with the next line of the program.

Passing Data To Functions

void cube( int n ){ int c = n*n*n;

cout << n << “ cubed is “ << c << “\n”;}

int main(void){ int value;

cout >> “Enter a value: ”; cin >> value;

cube(value);

cube(value+1);}

Sometimes you need to pass values to a function for

it to work on.

To do so, when we call the function, we place the

value(s) we want to send in between the parentheses.

When we define the function, we specify the type and name of each

parameter in between the parentheses.

value

93Enter a value: 2

2

2

2 n

2 c

2*2*2 8

2 cubed is 8

3

3 n

3 c

3*3*3 27

3 cubed is 27

Returning Data From Functions

float ComputeArea( float rad ){ float a; a = 3.14 * rad * rad;

return( a );}

int main(void){ float r, area;

cout >> “Enter radius: ”; cin >> r;

area = ComputeArea( r );

cout << “Area: “ << area;}

Sometimes you need to send a value back from a

function to the caller of the function.

To do so, when we use the return statement in our

function.

r 913

area71

Enter radius: 10

10

10

10 rad 10 a -1

3.14 * 10 * 10

314

314

314

314

Area: 314.00

The Function Definitionfloat ComputeArea( float rad ){ float a; a = 3.14 * rad * rad;

return( a );}

int main(void){ float r, area;

cout >> “Enter radius: ”; cin >> r;

area = ComputeArea( r );

cout << “Area: “ << area;}

The function definition is where you put the function’s actual logic.

1. A function header line that specifies:

a. The func’s name

Each func. def. has:

b. The func’s parametersb. The type of value the function returns.

I return floating-point numbers.

float ComputeArea( float rad )

2. The function body.

{ float a; a = 3.14 * rad * rad;

return( a );}

3. A return statement that sends a value back to the caller.

return( a );


return( a );}

int main(void){ float area;

area = ComputeArea( 4.0 ); cout << “Area is: “ << area;}

Function ParametersA function may have one

or more formal parameters.

They are treated just like any other local variable.

A value/variable passed to the function is called an actual parameter.

The actual parameter value is copied into the

formal parameter variable when you call the function.

4.0

You must make sure that your actual parameter and

formal parameter types match.

char x = ‘A’;x

// ERROR!

Function Parameters#include <iostream>#include <cmath>using namespace std;

float TotalFluid(int zits, float ozPerZit){ float totalOunces; totalOunces = zits * ozPerZit;

return(totalOunces);}

int main(void){ int num_zits = 4, total;

total = TotalFluid( num_zits , .52 ); cout << “Ounces of fluid: “ << total;}

Functions may have multiple parameters.

Make sure that the type of each actual

parameter matches the type

of each formal parameter!

Each formal parameter should be separated by a comma and must have its own type

and name specified .

Function Parameters

Functions may also have no parameters.

Then, make sure you don’t pass any parameters to the function when you

call it.

In this case, you should place the word “void” in between the

parentheses in the function definition.

void praise(void){ cout << “totally cool\n”;}

void tease() // ( ) is the same as void{ cout << “would like to be ”; praise();}

int main(void){ cout << “Carey is “; praise(); cout << “David S.“; tease(); cout << “The end.\n”;}


return( a );}

long ComputeBoogies(int age){ long boogers = 10 * age*age; return(boogers);}

int main(void){ float area; long snot; area = ComputeArea( 10 ); snot = ComputeBoogies(32); ...

In this case, our top function has a return type of “float” so it should return float-type values.

float

For instance, it shouldn’t try to return char values.

char your_grade; your_grade = ‘A’; return(your_grade);

char your_grade; your_grade = ‘A’; return(your_grade); // ERROR!

This may result in either a compiler error or a run-time error!

Returning from a FunctionA function can return any single type of value it likes: int, char, float, double, long, etc.

int ComputeNoseHairs(int age){ if (age < 33) return(age * 10);

}

int main(void){ int nh;

nh = ComputeNoseHairs(18); cout << “Will has “ << nh << “ nose hairs!\n”;}

When the program reaches a return

statement…

The current function ends immediately and sends its result value

back to the caller.

cout << “Old and hairy!\n”;return(age*100);

Returning from a Function

nh 9991

18age 18

18 < 33?

180

180

Will has 180 nose hairs!

18*10180

Returning from a Functionvoid PrintGrade(int grade){ if (grade >= 90) { cout << “A”; return; } if (grade >= 80) { cout << “B”; return; } cout << “Study harder!”;}

int main(void){ PrintGrade(85); cout << “Done!\n”;}

In this case, you may still use the return statement to immediately exit the function, but you can’t

specify a value.

If a function doesn’t need to return a value, it

should have a void return type.

This means: “I don’t return any

value.”

85 grade 85


return( a );}

int main(void){ float area;

area = ComputeArea( 4 );

cout << “Area is: “ << area;}

Variable Rules

A function can only access its own variables, but not other function’s

variables.

Why is this important?

So ComputeArea can access a and rad but can’t access the area variable.

cout << area; // ERROR!

cout << rad; // ERROR!Similarly, main can access area but cannot access the rad or a variables.

A function may have its own private “local”

variables.

Function Prototypes

void UCLARules(int x){ int j;

for (j=0;j<x;j++) cout << “UCLA Rules\n”;}

int main(){ int n; cin >> n; UCLARules(n);}




What’s the difference between these two programs?

Function Prototypes




In this example, a function (main) calls

another function (UCLARules) that is defined later in the

program.This is like a novel that refers to a character, but the character is introduced in a later

chapter.

It’s confusing!

Function Prototypes




This is also confusing for the C++ compiler.

When compiling the main function, it sees

the function call to UCLARules but it

doesn’t know about this function yet.

This results in an error.

Function Prototypes




If you would like to call a function defined later in the program, you must add a

special line called a prototype above your call to

the function.

void UCLARules(int x)Simply copy the function’s header line above the first reference to the function.

Then add a semicolon at the end.

;

Think of this as an introduction to your new function. It tells your other functions how to use it, even if its

defined later.

Functions: Case Sensitivity#include <iostream>#include <cmath>using namespace std;

float GetArea(float width, float height){ float ar; ar = width * height;

return(ar);}

int main(void){ float x = 3, y = 4, area;

area = GetArea(x,y); cout << “The square’s area is “ << area;}

Note: function names are

case sensitive in C++.

gETaREA // ERROR!

UCLA CS 31 Lecture5 Post On Functions

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