Control Structures Week 4: Control Structures Week 4: Monchai - - PowerPoint PPT Presentation

204111: Computer and 204111: Computer and Programming Programming Week 4: Week 4: Control Structures

Control Structures

Monchai Monchai Sopitkamon Sopitkamon, Ph.D. , Ph.D.

Overview Overview

• Types of control structures

Types of control structures

• Using selection structure

Using selection structure

• Using repetition structure

Using repetition structure

Types of control structures Types of control structures

• C# provides 8 types of control structures

C# provides 8 types of control structures

• ne sequence structure
• ne sequence structure –

– statements are executed statements are executed

• ne after the other in the order they appear in the
• ne after the other in the order they appear in the

program. program.

• three selection structure

three selection structure – – statement(s statement(s) is/are ) is/are executed if a condition is true or skipped if the executed if a condition is true or skipped if the condition is false. E.g., condition is false. E.g., if if, , if/else if/else, , switch switch

• four repetition structure

four repetition structure – –statement(s statement(s) is/are ) is/are executed over and over again (loops) until or while a executed over and over again (loops) until or while a specific condition is satisfied. E.g., specific condition is satisfied. E.g., while while, , do/while do/while, , for for, , foreach foreach. .

Selection Structures Selection Structures

• if

if (single

(single-

• selection structure)

selection structure)

• if/else

if/else (double

(double-

• selection structure)

selection structure)

• switch

switch (multiple

(multiple-

• selection structure)

selection structure)

Selection Structure Selection Structure

condition False True

statement

if (condition)

statement; condition False True statement2 statement1

if (condition)

statement1;

else

statement2;

IF IF/ELSE

Selection Structure Selection Structure

• switch

switch (multiple

(multiple-

• selection structure)

selection structure)

• syntax:

syntax:

< constant-expression> must be integral or

string type.

switch (< expression> ) { case < constant-expression> :

< statements> ;

break; : default:

< statements> ;

break; }

• ptional

required

Switch statement Switch statement

• <

< expression expression> > is evaluated. is evaluated.

• Result is compared to each case, sequentially.

Result is compared to each case, sequentially.

• If a match is found, the statements after the

If a match is found, the statements after the case keyword are executed. case keyword are executed.

• If no match is found, the (optional) default case

If no match is found, the (optional) default case takes effect. takes effect.

• “

“break; break;” ” is required after each block, since C#

is required after each block, since C# does not support an explicit does not support an explicit “ “fall fall-

• through

through” ” from from

• ne case label to another. However, it is
• ne case label to another. However, it is

allowed to stack case labels to specify that allowed to stack case labels to specify that certain statements are to be executed for certain statements are to be executed for several cases. several cases.

switch (< expression> ) { case < constant-expression> :

< statements> ;

break; : default:

< statements> ;

break; }

Switch example Switch example -

• Simple Calculator

Simple Calculator

double a, b; char op; double sol=0; a = double.Parse(Console.ReadLine());

• p = char.Parse(Console.ReadLine());

b = double.Parse(Console.ReadLine()); switch(op) { case "+": sol = a+b; break; case "-": sol = a-b; break; case "*": sol = a*b; break; case "/": sol = a/b; break; } Console.WriteLine("{0} {1} {2} = {3}", a, op, b, sol);

If we want 'x' to be If we want 'x' to be another multiplicative another multiplicative

• perator as well, we can
• perator as well, we can

insert a line as follows: insert a line as follows:

case "* ": case "x": sol = a* b; break;

A “fall- through” case example

do/while Statement : Syntax do/while Statement : Syntax

do { do {

Statement; Statement; Statement; Statement; Statement; Statement;

} while } while ( ( condition

condition)

) ;

;

condition true false

Statement Statement Statement

could be boolean constant, boolean variable, or boolean expression

do/while Statement : Example do/while Statement : Example

Output: Hello World Hello World Output: 3

Do/ while statement is a conditional iteration

statements

The loop body of a do/ while statement will be

executed first before the exit condition is checked

Do/while loop is also called post-conditional because the condition is checked after the loop body A do/while statement always gets executed at least once

do/while Statement : Operation

do/while statement: do/while statement: Example1

Example1

int int n = 0; n = 0;

do { do {

Console.Write( Console.Write(“ “{ 0} { 0} “ “, n); , n); n+ + ; n+ + ;

} while } while ( ( n < 5)

n < 5); ;

(n < 5)? False True

n+ + write n

(n = 0)

Output: 0 1 2 3 4

do/while statement: do/while statement: Example2

Example2

bool bool flag = flag = true true; ; string s; string s; int int n; n;

do { do {

s= s= Console.ReadLine Console.ReadLine(); (); n= n= int.Parse(s int.Parse(s); ); if (n > 0) if (n > 0) Console.Write( Console.Write(“ “{ 0} { 0} “ “ , n); , n); else else flag = flag = false false; ;

} while } while (flag = = true);

(flag = = true);

(flag= = false)? false true read n (n > 0)? true write n false set flag = false

(flag = true)

While Statement : Syntax While Statement : Syntax

while while (condition ) (condition ) {

{

Statement; Statement; Statement; Statement; Statement; Statement;

} }

could be boolean constant, boolean variable, or boolean expression

condition

false true

Statement Statement Statement

While Statement : Example While Statement : Example

Output: Hello World Hello World Output: 3

Since the condition is checked before the execution of the loop body, a while loop is also called pre-conditional. As long as the entry condition remains true, the while loop will keep repeating. No loop will be executed if the entry condition is false from the beginning.

While Statement : Operation

While Statement : Example1 While Statement : Example1

int int n = 0; n = 0;

while while (n < = 5)

(n < = 5) {

{

Console.Write( Console.Write(“ “{ 0} { 0} “ “, n); , n); n+ + ; n+ + ;

} }

(n < = 5) ?

False True

write n n+ +

(n=0)

Output: 0 1 2 3 4 5

While Statement : Example2 While Statement : Example2

bool bool flag = true; flag = true; string s; string s; int int n; n;

while while (flag= = true)

(flag= = true) {

{

s= s= Console.ReadLine Console.ReadLine(); (); n= n= int.Parse(s int.Parse(s); ); if (n > 0) if (n > 0) Console.Write(n Console.Write(n) ) else else flag = flag = false false; ;

} }

flag = = true

False

true read n (n > 0) ?

True

Write n

False

flag = false

Sentinel Sentinel-

• Controlled Loops

Controlled Loops

• Template:

Template:

Read the first value of input variable Read the first value of input variable

while while (input variable is not equal to sentinel)

(input variable is not equal to sentinel) {

{

... ... Read next value of input variable Read next value of input variable

} }

Sentinel Sentinel-

• Controlled Loops

Controlled Loops

• Example: Accumulating exam scores

Example: Accumulating exam scores

• Steps

Steps

1.Initialize 1.Initialize total to 0 to 0 2.Read the first value and assign it to 2.Read the first value and assign it to score score 3.While score is not the sentinel (e.g. 3.While score is not the sentinel (e.g. -

• 1) do

1) do 3.1 Add 3.1 Add score score to to total 3.2 Read the next value and assign it to 3.2 Read the next value and assign it to score score

Use of Sentinel

• A sentinel is a value

used to control the termination of a loop.

• E.g. User can

choose to use “–1” as the sentinel value to terminate the iteration.

int total =0, score; Convert.Write("Enter score (-1 to end): "); score=int.Parse((Console.ReadLine()); while (score != -1) { total := total + score; Convert.Write("Enter score (-1 to end): "); score=int.Parse((Console.ReadLine()); }

Sentinel Sentinel-

• Controlled Loops

Controlled Loops

Sentinel Sentinel-

• Controlled Loop: Example1

Controlled Loop: Example1

Loops Controlled by Boolean Flags Loops Controlled by Boolean Flags

• Flag:

Flag:

• A Boolean variable whose value is changed from False

A Boolean variable whose value is changed from False to True when a particular event occurs to True when a particular event occurs

• Boolean Flags Template:

Boolean Flags Template:

Initialize flag to false Initialize flag to false

while while (flag is still False)

(flag is still False) {

{

... ... Reset flag to True if event being monitored Reset flag to True if event being monitored

• ccurs
• ccurs

} }

Loops Controlled by Flags : Example Loops Controlled by Flags : Example

• A program that checks whether the input

A program that checks whether the input charater charater is a digit between 0 is a digit between 0-

• 9.

9.

DigitRead DigitRead = false; = false;

while while (!

(!DigitRead DigitRead) ) {

{

Console.Write Console.Write ( (“ “Please enter a numeric : "); Please enter a numeric : "); string string st st = = Console.ReadLine Console.ReadLine(); (); char char NextChar NextChar = = Char.Parse(st Char.Parse(st); ); DigitRead DigitRead= (('0'< = = (('0'< = NextChar NextChar)&& ( )&& (NextChar NextChar< = '9')) < = '9'))

} }

For Statement : Syntax For Statement : Syntax

for( for( [

[ initializers initializers] ] ; ; [condition] [condition] ; ; [ [ iterators iterators] ] )

) { {

statement; statement; statement; statement; statement; statement;

} }

condition false true

Statement Statement Statement

iterators

Intializers

For Statement : Syntax For Statement : Syntax

• initializers

initializers

• A comma separated list of expressions or assignment

A comma separated list of expressions or assignment statements to initialize the loop counters statements to initialize the loop counters.

.

• expression

expression

• An expression that can be implicitly converted to

An expression that can be implicitly converted to bool

bool

• r a type that contains overloading of the
• r a type that contains overloading of the true

true and and false false operators

• perators.

. The expression is used to test the

The expression is used to test the loop loop-

• termination criteria

termination criteria.

.

• iterators

iterators

• Expression statement

Expression statement(

(s

s)

) to increment or decrement

to increment or decrement the loop counters the loop counters.

.

For Statement : Operation For Statement : Operation

The The for

for statement executes the

statement executes the statement statement repeatedly as follows repeatedly as follows:

:

• First

First,

, the

the initializers initializers are evaluated are evaluated.

.

• Then

Then,

, while the

while the expression expression evaluates to evaluates to true

true, ,

the the statement statement(

(s

s)

) are executed and the

are executed and the iterators iterators are evaluated are evaluated.

.

• When the

When the expression expression becomes becomes false

false, , control is

control is transferred outside transferred outside of

• f the loop

the loop.

.

For Statement : Example1 For Statement : Example1

public class ForLoopTest public class ForLoopTest { { public static void Main public static void Main() () { { for for( (int int i i = = 1 1; ; i i <= <= 5 5; ; i i++ ++) ){ { Console Console. .WriteLine( WriteLine(“ “{0} {0}” ”, , i); i); } } } } } } Output

1 2 3 4 5

For Statement : Example2 For Statement : Example2

• Output A

Output A…

…Z alphabets on the screen using

Z alphabets on the screen using

• nly one
• nly one WriteLine

WriteLine() statement. () statement.

A A B B . . . . . . Z Z

For Statement : Example3 For Statement : Example3

• Write a program that finds a summation of a

Write a program that finds a summation of a sequence of numbers from 1 sequence of numbers from 1-

• 99 and show the

99 and show the result. result. 1+ 2+ 1+ 2+ …

…..+ 99 = 4950

..+ 99 = 4950

do/while vs. while do/while vs. while

condition

true false

Statement Statement Statement

do/while statement while statement

condition

false true

Statement Statement Statement

while vs. for while vs. for

while statement for statement

condition

false true

Statement Statement Statement

condition false true

Statement Statement Statement

iterators

Intializers

A nested loop is formed by placing an inner loop inside an outer loop. The outer loop is executed first. If the inner loop is reached, the outer loop is suspended and the inner loop is executed. After the inner loop is terminated, the program continues to execute the remaining statement(s) in the outer loop

Nested Loops

for (int outer = 1; outer <= 4; outer++) { for (int inner = 1; inner <=5; inner++) { Console.WriteLine("Outer = {0} & Inner = {1}", outer, inner); } }

Nested for loops

• Do not use the same control variable in the outer loop as in

the inner loop

Nested Loops : Example Nested Loops : Example

int outer = 1; while (outer <= 4) { int inner = 1 ; while (inner <= 5) { Console.WriteLine("Outer = {0} & Inner = {1}", outer, inner); inner = inner +1; }

• uter = outer +1;

}

Nested while loops

Nested Loops : Example Nested Loops : Example

int outer = 1; do { int inner = 1; do { Console.WriteLine("Outer = {0} & Inner = {1}", outer, inner); inner++; } while (inner < 6);

• uter++;

} while (outer < 5);

Nested Loops : Example Nested Loops : Example

Nested do/while loops

Infinite looping

No statements in the loop body can update the value of the entry condition. (E.g. omit counter = counter +1) The statement(s) in the loop body make(s) the variable become far away from the stopping condition. (E.g. counter = counter – 1)

Break Statement Break Statement

• The

The break

break statement

statement terminates terminates the the closest enclosing closest enclosing loop loop or

• r switch

switch

statement in which it appears statement in which it appears

• Control is passed to the statement that

Control is passed to the statement that follows the terminated statement follows the terminated statement

Break Statement : Example Break Statement : Example

Continue Statement Continue Statement

• The

The continue

continue statement passes control to the

statement passes control to the next iteration next iteration of the enclosing

• f the enclosing iteration

iteration statement in which it appears statement in which it appears.

.

Continue Statement : Example Continue Statement : Example