loops george mason university. loop structure loop- a structure that allows repeated execution of a...

Loops

George Mason University

Loop Structure

Loop- A structure that allows repeated execution of a block of statements

Loop body- A block of statements; as long as the expression is true, the loop body executes

Iteration- One execution of any loop

Loop Choices

Definite Loop• #iterations known• for loop

Indefinite Loop• #iterations unknown (conditional)

– while loop – condition checked at top

– do/while loop – condition checked at bottom will always execute at least once

Repetitions

for Loops

while Loops

do-while Loops

break and continue

Using a for Loop

for loop- A special loop that is used when a definite number of loop iterations is required

Keyword for

Set of parentheses

Three sections within parentheses• Initializing the loop control variable

• Testing the loop control variable

• Updating the loop control variable

for Loopsfor (initial-action; loop-

continuation-condition; action-after-each-iteration) {

// loop body; statement(s);}

for (i = 0; i < 100; i++) { document.write( "Welcome to JavaScript!

 "); }

initial-action

loop-continuation-condition

true

false

action-after-each-iteration

statement(s);

i=0

i<100true

false

i++

print(“welcome”)

Trace for Loop

for (i = 0; i < 2; i++) { document.write( "Welcome to JavaScript! "); }

i=0

i<2true

false

i++


????????

i

MEMORY

WEB PAGE

Trace for Loop, cont.

for (i = 0; i < 2; i++) { document.write("Welcome to JavaScript! "); }

Execute initializeri is now 0

i=0

i<2true

false

i++


????????

i

MEMORY

WEB PAGE

00

for (i = 0; i < 2; i++) { document.write( "Welcome to JavaScript! "); }

(i < 2) is true since i is 0

i=0

i<2true

false

i++


00

i

MEMORY

WEB PAGE

int i;for (i = 0; i < 2; i++) { document.write("Welcome to JavaScript ! "); }

Print Welcome to JavaScript

i=0

i<2true

false

i++


00

int i

MEMORY

WEB PAGE

Welcome to JavaScript!Welcome to JavaScript!

Execute adjustment statement i now is 1

i=0

i<2true

false

i++


00

i

MEMORY

CONSOLE


11

(i < 2) is still true since i is 1

i=0

i<2true

false

i++


11

i

MEMORY

WEB PAGE

i=0

i<2true

false

i++


11

i

MEMORY

CONSOLE

Welcome to Java!Welcome to Java!Welcome to JavaScript!Welcome to JavaScript!Welcome to JavaScript!Welcome to JavaScript!

Execute adjustment statement i now is 2

i=0

i<2true

false

i++


11

i

MEMORY

WEB PAGE

Welcome to JavaScript!Welcome to JavaScript!Welcome to JavaScript!Welcome to JavaScript!

22

(i < 2) is false since i is 2

i=0

i<2true

false

i++


22

i

MEMORY

WEB PAGE

for (i = 0; i < 2; i++) { dcoument.write("Welcome to JavaScript! "); }

Exit the loop. Execute the next statement after the loop

i=0

i<2true

false

i++


22

i

MEMORY

CONSOLE

Nested For Loops

• for loops can be nested, e.g.

for (row=1; row<=9; row++)

for (column=1; column<=9; column++)

document.write(row*column + " ");

The above example would write out 81 numbers, going row by row and then by column within a row

Indefinite Loop

Don’t always have access to the number of iterations ahead of time

If a condition (user-response, limit reached, …) controls the number of iterations – indefinite loop is appropriate

While Loop

Using the while Loop

while Loop- To execute a body of statements continually as long as the Boolean expression continues to be true• Consists of the keyword while followed by a

Boolean expression within parentheses followed by the body of the loop

• Use when you need to perform a task a undetermined number of times

while Loop

while (loop-continuation-condition) {

// loop-body;

Statement(s);

}

count = 0;

while (count < 100) {

document.write("Welcome to JavaScript! ");

count++;

}

Loop Continuation Condition?

true

Statement(s) (loop body)

false (count < 100)?

true

document.write("Welcome to JavaScript! "); count++;

false

(A) (B)

count = 0;

Trace while Loop

count = 0;

while (count < 2) {


count++;

}

Initialize count

00

count

MEMORY

WEB PAGE

Trace while Loop, cont.

count = 0;

while (count < 2) {


count++;

}

(count < 2) is true

00

count

MEMORY

WEB PAGE

count = 0;

while (count < 2) {


count++;

}


00

count

MEMORY

WEB PAGE

count = 0;

while (count < 2) {


count++;

}

Increase count by 1count is 1 now

00

count

MEMORY

WEB PAGE


11

count = 0;

while (count < 2) {


count++;

}

(count < 2) is still true since count is 1

count

MEMORY

WEB PAGE


11

count = 0;

while (count < 2) {


count++;

}


count

MEMORY

CONSOLE

Welcome to Java!Welcome to Java!

11

count = 0;

while (count < 2) {

document.write("Welcome to JavaScript !");

count++;

}

Increase count by 1count is 2 now

int count

MEMORY

CONSOLE

11


22

count = 0;

while (count < 2) {


count++;

}

(count < 2) is false since count is 2 now

count

MEMORY

CONSOLE


22

Trace while Loop

count = 0;

while (count < 2) {


count++;

}

The loop exits. Execute the next statement after the loop.

count

MEMORY

WEB PAGE


22

Ending a Loop with a Sentinel Value Often the number of times a loop is executed is not predetermined. You may use an input value to signify the end of the loop. Such a value is known as a sentinel value.

do…while loop

Checks at the bottom of the loop after one repetition has occurred

Loop body executes at least one time

The loop starts with the keyword do

The body of the loop is contained within curly braces

do-while Loop

do {

// Loop body;

Statement(s);

} while (loop-continuation-condition);

Loop Continuation Condition?

true

Statement(s) (loop body)

false

Which Loop to Use?The three forms of loop statements, while, do-while, and for, are expressively equivalent; that is, you can write a loop in any of these three forms. For example, a while loop in (a) in the following figure can always be converted into the following for loop in (b):

A for loop in (a) in the following figure can generally be converted into the following while loop in (b) except in certain special cases

for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; }

(a)

Equivalent

(b)

initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; }

while (loop-continuation-condition) { // Loop body }

(a)

Equivalent

(b)

for ( ; loop-continuation-condition; ) { // Loop body }

Which Loop to Use?

Interchangeable – BUT, only one is the right choice!• Definite: for loop• Indefinite and at least one iteration: do/while• Indefinite and not necessarily at least one

iteration: while

Common Logic Error

Semicolon to terminate loops (for/while) prior to the body!

for (i=0; i<10; i++);

document.write(" i is " + i);

---------------------------------------------------

play = prompt(“Play Game? Enter y for yes, n for no");

while (play == "y"); // play Scrabble

Caution, cont.

Similarly, the following loop is also wrong: i=0; while (i < 10);{ document.write(" i is " + i); i++;}

In the case of the do loop, the following semicolon is needed to end the loop.

i=0; do { document.write(" i is " + i); i++;} while (i<10);

Logic Error

Correct

Use of break and continue

Used to alter the intended flow of execution

• break; //end the control structure

• continue; //end current iteration

loops george mason university. loop structure loop- a structure that allows repeated execution of a...

Documents