1 9/18/06cs150 introduction to computer science 1 the if/else statement, boolean flags, and menus...

9/18/06 CS150 Introduction to Computer Science 1 1

The If/Else Statement, Boolean Flags, and Menus

Page 180

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Formally defined

What is an expression?

if( expression ){ statement 1; statement 2; . . . statement n;}

Just like a function, start at the top and execute in order tothe bottom

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More on Truth

Expressions that evaluate to non-zero are considered true

int x=5, y=0;// what will get executed?if ( x + y) {cout << “x+y is True” << endl;

}if ( y ) { cout << “y is True” << endl;}

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Floating Point and Relational Operators

Floating point math may not work out as you expect because of round off errors.

Math Class:o 6 * 2/3 = 4

C++:o 6.0 * 0.66666 =

o 6.0 * 0.66667 =

o 6.0 * 0.666666 =

o 6.0 * ( 2.0 / 3.0 ) =

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Example (page 180)

double result;

result = 6.0 * 0.666666;

if ( result == 4.0 ) { cout << “result == 4.0” << endl; }

cout << setprecision(6) << fixed; cout << result << endl; cout << setprecision(2) << result; cout << endl;

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Example

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Floating Points in Relational Operators

How can we get around this problem?

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Boolean Flags

We have seen how to store the value of a relational expression to a bool variable.

bool bIsSquare = ( length == width);

if ( bIsSquare ) {

}

Why would you want to do this?

Why not use the relational expression directly?

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Boolean Flags

This use of a bool variable is called a flag.

It is used to keep track of a condition so that the expression is evaluated only once

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If Statement

We may want to execute some code if an expression is true, and execute some other code when the expression is false.

This can be done with two if statements…

if( value >= LIMIT ) {// do something

}if( value < LIMIT ) {

// do something else}

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If/Else

C++ provides a shortcut to combine two if statements:

if( expression ){ // do stuff} else {

// do other stuff}

The statements in the else clause are executed only when the expression is false.

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Example

int number;cout << “Enter a number, I’ll tell you“;cout << “ if it is odd: “;cin >> number;

// use an if/else statement here

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If/Else: Syntax and Formatting

if( expression ){

// do stuff} else {

// do other stuff}

Note the braces with the else keyword and the alignment of the else under the if on its own line

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If/Else: Braces

if( expression ){

// do stuff} else

x += 9;

Always use braces with the else!

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If/Else: Commenting

// the expression I’m using here// checks for . . . if( expression ){

// if the expression is true// I need to ...

} else {

// if the expression is false// I need to ...

}

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Practice

Write a program that asks the user for two integers and output the floating point number produced by dividing the second number into the first. Be sure not to divide by zero!

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Practice

Turn this code into an if/else statement:int x, y;if ( x > y ){

x += y;} if ( y <= x){

y += x;}

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Practice

Are these two code snippets equivalent?

int x, y;if ( x > y ){

x += y;} if ( y < x){

y += x;}

int x, y; if ( x > y ) {

x += y; } else { y += x; }

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Nested Ifs

if ( x > y ) {}else{

if ( x == 9 ){}else {}

}

The second if is only executed if the first if conditional is false

Note the indentation of the inner if

There may be code between the { with the first else and the second if

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Using nested ifs…

Write a snippet of code that will:o add y to x if x > y

o add x to y if y > x

o add 1 to x if x == y

int x, y;if (

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C++ Shortcut

if ( x > y ) {

// do A}else{

if ( x == 9 ){

// do B}else {

// do C}

}

if ( x > y ) {

// do A}else if ( x == 9 ){

// do B}else {

// do C}

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Using nested ifs…

Write a snippet of code that will:o add y to x if x > y

o add x to y if y > x

o add 1 to x if x == y

int x, y;if (

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Chained If/Else statements

if ( x > y ) {

// do A}else if ( x == 9 ){

// do B}else if ( y > 1 ){

// do C}else{}

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Using nested ifs…

Write a snippet of code that will only do one of the following:o add y to x if x == y

o add x to y if y > x

o add 1 to x if (2 * x) == y

int x = 5, y = 10;if (

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Exercise

Write a snippet of code that will print the letter grade (A,B,C,D,F) of a student’s exam and set a bool flag to track if it is a passing grade.

int examScore; // from 0 to 100 bool bPassingGrade; if (