p12 - iterations

Computer Studies 2013 Syllabus

Mr. A. Gatt of 10

Iterations (12)

while, do-while, for

During programming, certain code (or a block of statements) might need to be repeated for several times,

for instance inputting the marks of a whole class. However, rather than typing the same statement over and

over again, loops are used.

There are three different types of loops (or iterations) which are for, while and do-while. First while and do-

while are compared with each other.

while do-while

Repeats statements while its condition is true. Repeats statements while its condition is true.

The condition is tested at the start. The condition is tested at the end.

So, there is a chance that the statements are not

executed at all if the condition starts as false.

So, the statements have to be carried out at least

once since the condition is checked at the end.

Is Condition

True?

Statements to

Execute

True

False

Is Condition

True?

Statements to

Execute

True

False


Mr. A. Gatt of 10

Syntax

while (condition) {

//do this

//do this

}

Syntax

do {

//do this

//do this

} while (condition);

Example

class While {

public static void main (String args[]){

int n=10;

while (n>0) {

System.out.println(“n=”+n);

n--;

}

}

}

Example

class DoWhile {


int n=10;

do {

System.out.println(“n=”+n);

n--;

} while (n>0);

}

}

The while loop

This loop keeps iterating while the condition is true. If the condition is false, the loop will stop and move on

to the next line after the loop (after the closing curly bracket) of the loop. Consider the following program.

class GuessNum {


final int NUM=7;

int guess=0;

while (guess!=NUM) {

System.out.println(“Guess number (1-10)”);

guess=Keyboard.readInt();

}

System.out.printl(“Well done! You guessed”);

}

}


Mr. A. Gatt of 10

This program will keep asking the user to input a number from 1 to 10 and keep on asking for a number

until 7 is inputted.

The do-while loop

Sometimes it may be necessary to execute the body of the loop at least once, even if the condition is false

at the beginning; this can be useful in a menu situation. Consider the following program.

class DoWhileGuess {


final int NUM=7;

int guess=0;

do {

System.out.println(“Guess number (1-10)…”);

guess=Keyboard.readInt();

} while (guess!=NUM);

System.out.println(“Well done! You Guessed”);

}

}

The do-while is very suitable for handling menu selection. The following flowchart depicts a program that

shows a menu and performs an arithmetic function depending on the user’s choice. The menu keeps on

showing until the user decides to quit.


Mr. A. Gatt of 10

Start

Input num1,

num2

Display

Menu

Input

Choice

Choice=1

?

Choice=2

?

Choice=3

?

Choice=4

?

num1+

num2

num1-

num2

num1*

num2

num1/

num2

End

Y Y Y Y

N N N N


Mr. A. Gatt of 10

class Menu {


char choice;

boolean quit = false;

System.out.println(“Enter first number “);

int num1 = Keyboard.readInt();

System.out.println(“Enter second number “);

int num2 = Keyboard.readInt();

do {

System.out.println(“Menu”);

System.out.println(“1. Addition”);

System.out.println(“2. Subtraction”);

System.out.println(“3. Multiplication”);

System.out.println(“4. Division”);

System.out.println(“5. Quit”);

System.out.println(“Enter your choice 1-5:”);

choice = Keyboard.readChar();

switch (choice) {

case ‘1’ : System.out.println(num1+num2); break;

case ‘2’ : System.out.println(num1-num2); break;

case ‘3’ : System.out.println(num1*num2); break;

case ‘4’ : System.out.println(num1/num2); break;

case ‘5’ : quit = true;

default : System.out.println(“Invalid Choice”);

}

System.out.println();

} while (!quit);

}

}


Mr. A. Gatt of 10

The for loop

This loop is used when you know exactly the number of times you want to repeat a set of statements. The

for loop is structured as

for (initialization; condition; iteration)

statement;

During the iteration of the for loop, if the result of the test in the condition is true, then the for loop

continues to iterate but if the test is false, it stops. The iteration expression determines how the loop

control variable is changed each time the loop iterates. Consider the statement

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

System.out.println(i);

The first part x=0 is used to initialize the start of the loop, in this case x=0; so it will start from 0.

The second part contains the condition that needs to be tested, in this case it is checking if it is less

than 10 (if it is the loop will continue)

The third part is to increment (add a one) the variable used in the loop

So, if this statement in incorporated in a small program:

class ForLoop {

public static void main(String args[]){

int x;

for (x=0; x<10; x++) {

System.out.println(“x =”+x);

}

}

}

The following program shows another example of a loop which prints letters from a to z.

class ForLoop {

public static void main(String args[]){

for (char low=’a’; low<=’z’; low++) {

System.out.println(low);

}

}

}


Mr. A. Gatt of 10

Activities

1. Draw a flowchart and then write an application using the while loop (condition at the start) to ask

the user to input a list of integer marks until -1 is inputted. Note that -1 is a rogue (sometimes called

dummy or terminator value) and is not to be considered as a mark.

2. Now change the application (1) to input also a list of integer marks and use a do-while loop to stop

entry of data when -1 is inputted.

3. Use application (2) above to find the sum of the marks entered.

4. Create a password program and include a while loop which allows the user three attempts to enter

the password. The program stops either if the password is correct (“Access Granted”) or if the user

enters the wrong password (“Access Denied”) after three consecutive tries.

5. Develop a program using a for loop to generate 5 random numbers between 1 and 43 for the Super

Five lottery.

6. Develop an application using a for loop to simulate the tossing of a coin 100 times and count how

many times you get Heads and how many times you get Tails.

7. Consider the program

1 class UseFor {

2 public static void main (String args[]){

3 int choice = 1;

4 System.out.println(“Enter a list of numbers, type 0 to stop”);

5 for (int i=1; (i<=10) && (choice!=0); i++) {

6 System.out.print(“ i =”+i+” : “);

7 choice = Keyboard.readInt();

8 }

9 }

10 }

a) In which lines are variables directly initialized?

b) Which type of loop is being used in this program?

c) Write down the line numbers that represent a loop.

d) Which line is outputting a prompt?

e) What happens if the semicolon at the end of line 3 is left out?

f) To what value is the control variable of the loop initialized in line 5?

g) Write down the condition part of the loop in line 5.

h) What is the i++ doing in line 5?


Mr. A. Gatt of 10

i) What do the && mean in the condition and what is it called?

j) Explain exactly when the double condition will result in false.

k) What does the user have to type to stop data entry?

l) What test data would you use to check that the program is working properly?

m) What happens if the user keeps on entering the data without typing a zero?

8. Draw a flowchart to ask the user to input 10 marks, to find out how many marks are equal to or

above 50 and how many marks are below 50. Hence, output the number of failures and the number

of passes. Code the program, try it and test it.

9. Ask the user to input two integer numbers, num1 and num2, and output all the multiples of four

between those two numbers. You need to use a for loop from num1 to num2.


Mr. A. Gatt of 10

10. Write a program to input 10 integer marks in the range of 0 to 100 and output the highest based on

the following flowchart.

11. Change the flowchart for question 10 above to output the lowest mark. Then change the program

as well.

12. Now combine the flowcharts in question 10 and 11 above to input 10 integer marks in the range of

0 to 100 and the output the highest and the lowest, then write the program to do it.

13. Develop the program in question 12 to input 10 marks, find and output the sum, the average, the

highest and the lowest mark.

Start

N=0 HI =0

Input Next Num

Num>HI

?

HI=NUM

N=N+1

N<10?

Output HI

Start

N

Y

Y

N


Mr. A. Gatt of 10

14. Draw a flowchart then develop a program to create the following application to display the

following menu:

a. Enter Marks

b. Display Statistics

c. Display Grades

d. Quit

Then using a do-while, ask the user to enter a choice and through the use of a switch, depending on

the choice, output a single line as follows:

If 1 is chosen, output “You chose to enter marks”

If 2 is chosen, output “You chose to display sum, highest, lowest, average”

If 3 is chosen, output “You chose to display the grades”

If 4 is chosen output “End of Program”

15. Study carefully this pseudo-code:

a. Generate a number between 1 and 100

b. Ask the user to enter a guess

c. If number is guessed, then output “Number guessed” and go to step f

d. If the number is not guessed, then output “Number is higher” if the guess is lower or

output “Number is lower” if the guess is higher

e. Go to step (b)

f. Stop

Draw a flowchart to do the same task as the pseudo-code above.

Develop an application to implement this algorithm.

The number of tries can be reduced if the person guessing the number uses the techniques of a

‘binary search’. Find out what this is and try it out when running the program to try and guess

the number in as few tries as possible. If properly applied then you can guess the number in 7

or at most 8 tries!

***

p12 - iterations

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