BC101 Lecture 4 - Program Structures

8/3/2019 BC101 Lecture 4 - Program Structures

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if Statements


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Slide 2

Introduction

Sequence

Statements in the given order

Condition statement (branching)

Chooses between two (or more) sequences depending on some conditionif {

}

else {

}

Iteration statement (looping)repetitively execute a given sequence

while {

}

Three program structures:


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Slide 3

Structured programming

Any program can be written as a sequence ofthree basic program structures!!!

1. sequences,

2. conditionals,3. and iterations


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Slide 4

The fundamental if-else Statemen

Syntax

if (Expression)

Action1else

Action2 If Expression is true then

execute Action1 otherwise

execute Action2

Exampleif(v == 0)

cout


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Slide 5

if


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Slide 6

Example: Finding the Big One

int main() {

int value1;

int value2;

int larger;

cout > value1 >> value2;

if(value1 > value2)

larger = value1;

else

larger = value2;

cout


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Slide 7

Example: Absolute Value (1st )// program to read number & print its absolute value

#include

int main(){

int value;

int absvalue;

cout > value;

if(value < 0)

absvalue = -value;

else absvalue = value;

cout


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Slide 8

When the action is more than

one statement

Put multiple action statements within braces

if


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Slide 9

Example: Absolute Value (2nd)// program to read number & print its absolute value

#include

int main(){

int value;

int absvalue; // absolute value

cout > value;

if (value < 0) {

absvalue = -value;

cout


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Slide 10

How to create an

expression for a test?

Using relational operators

Using boolean (logical) operators

An expression for a test (a boolean expression) has one of

the two values: true or false.


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Slide 11

Using Relational Operators

Relational operators are used to compare two values

Math C++ Plain English

= == equals [example: if(a==b) ]

[ (a=b) means put the value ofb into a ]< < less than

> greater than

>= greater than or equal to

!= not equal to


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Slide 12

Relational Expressions

Examples:

numberOfStudents < 200

10 > 20

20 * j == 10 + i


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Slide 13

Using Boolean (logical)

operators

Logical AND operator && Logical OR operator || Logical NOT operator !

Examples: (x>5) && (x10) || (x5)

Warning!

& and | are also operators

Boolean operators can be used to form more complexconditional expressions


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Slide 14

A Boolean Type

Example logical expressions

bool P = true;

bool Q = false;

bool R = true;

bool S = P && Q;

bool T = !Q || R;

bool U = !(R && !Q);

C++ contains a type (new!!!) named bool

which can have one of two values

true (corresponds to non-zero value)

false (corresponds to zero value)


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Slide 15

Operator Precedence

Which comes first?

( )

* / %

+ -

< = >

== !=

=

Answer:


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Slide 16

Summary of Operator

Precedence

Precedence of operators (from highest to lowest) Parentheses ( )

Unary operators !

Multiplicative operators * / %

Additive operators + -

Relational ordering < = >

Relational equality == !=

Logical and &&

Logical or ||

Assignment =

arithmetic

relational

logical


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Slide 17

5 != 6 || 7


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Slide 18

Boolean expressions

Arithmetic expression: use arithmetic operators +,-

,*,/, to produce a number as the final result

Boolean expression: use relational operators , ==,

and boolean operators AND (&&), OR (||), NOT (!)to produce one of the two values true (1) and false

(0) as the final result

New type: bool, true, false

Example: bool cond;

cond = true;cond = (x>y);

Old versions of C++, simulated boolean type by int with 0/1

If (BOOLEAN EXPRESSION)


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Slide 19

Alternative ways of

writing choice statements

If statement

Nested if

If-else-if statement

Switch statement


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Slide 20

The if Statement

Syntax

if(Expression)

Action

If the Expression is true

then execute Action Action is either a single

statement or a group ofstatements within braces

Example: absolute value

if(value < 0)

value = -value;

Expression

Action

true false


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Slide 21

Example: Absolute Value (3rd )

// program to read number & print its absolute value

#include

int main(){

int value;

cout > value;

if(value < 0)

value = -value;

cout


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Slide 22

Example: Sorting Two

Numbers

int value1;

int value2;

int temp;

cout > value1 >> value2;

if(value1 > value2){

temp = value1;

value1 = value2;

value2 = temp;

}cout


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Slide 23

Nested if Statements

Nested means that one complete statement is insideanother

if {

if {

if {

}

}

}


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Slide 24

Nested if Statements

Example:

if {

if


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Slide 25

if-else-if Statements

if {

}

else if {

}

else if {

}

else{

}

Q

R

TS


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Slide 26

if {

}

else if {

}

else if {

}

else{

}


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Slide 27

Selection

Often we want to perform a particular actiondepending on the value of an expression

Two ways to do this

if-else-if statementif-else statements glued together

switch statement

An advanced construct


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Slide 28

switch StatementAn alternative way of writing nested if-else-if statements for multiple choic

switch (Expression) {

case constant_1: action1; break;

case constant_2: action2; break;

case constant_n: actionN; break;

default: actionDefault

}

if Exp==constant_1 {

action1;

}

else if Exp==constant_2 {

action2;}

else if {

}

else{

actionDefault

}

=


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Slide 29

if(score >= 90)cout


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Slide 30

Example using switch:

switch(int(score)/10){

case 10:

case 9: cout


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Slide 32

Dangling Else Problem

Problem: Nested if statements can seemambiguous in their meaning.

What is the value of c after the following is

executed?int a=-1, b=1, c=1;

if(a>0)

if(b>0)

c = 2;

else

c = 3;


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Slide 33

C++ groups a dangling else with the most

recent if. The following indentation shows how C++

would group this example (answer: c=1).

int a=-1, b=1, c=1;

if(a>0)

if(b>0)

c = 2;

else // dangling else grouped to nearest if

c = 3;


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Slide 34

Use extra brackets { } to clarify the intended

meaning, even if not necessary.

int a=-1, b=1, c=1;

if(a>0){

if(b>0)

c = 2;

else // parenthesis avoid dangling else

c = 3;

}


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Slide 35

Summary on condition

statement The fundamental If-else statement

if, nested if, if-else-if, switch

Bool type and boolean expression for a test

true/false

relational operators (==,

Documents

BC101 Lecture 4 - Program Structures