1. M150: Data, Computing and Information
Unit Seven: An introduction to programming
using JavaScript – Part 2

2. 4- Programming for repetition: the while statement
The while statement (while loop), which has the effect of repeating execution of one or more program statements while a certain condition remains true
while (Boolean condition/expression) {
one or more statements }
The body of the while statement 2

3. 4- Programming for repetition: the while statement
Only years of birth between 1900 and 2008 are accepted as valid, and the user is repeatedly prompted until he or she enters a valid year.
(1) var currentYear, yearOfBirth;
(2) var age; // age this year
(3) currentYear = 2008;
(4) yearOfBirth = window.prompt('Please enter your year of birth','')
(5) yearOfBirth = parseFloat(yearOfBirth);
(6) while (yearOfBirth < 1900 || yearOfBirth > 2008)
(7) {
(8) document.write('You must enter a year of birth between 1900 and 2008' + '<BR>');
(9) yearOfBirth = window.prompt('Please enter your year of birth', '');
(10) yearOfBirth = parseFloat(yearOfBirth)
(11) };
(12) age = currentYear -yearOfBirth;
(13) if (age >= 60)
(14) {
(15) document.write(‘Free Tax')
(16) }
(17) else
(18) {
(19) document.write(‘ Tax is 15%')
(20) }
The initial statements requesting input and converting it into a number (lines (4) and (5)) are repeated inside the loop (as lines (9) and (10)). 3

4. 4- Programming for repetition: the while statement
You need to test the previous program with some valid years – between 1900 and 2008 – and some invalid years (at least one before 1900 and at least one after 2008). The boundary values are 1899, 1900, 1901, 2007, 2008 and 2009.
If the Boolean expression controlling a while loop evaluates to true, the whole of the body is executed. Then execution returns to the top of the while statement and the condition is tested again to see if it is still true. If it is, the body is executed again, and so on until the condition evaluates to false. The body of the while statement is then skipped over and execution of the rest of the program continues. 4

5. 4- Programming for repetition: the while statement
You are repeatedly prompted to enter a new password, and then retype it, until the two inputs match.
Example
var password, repeatPassword;
password = window.prompt('Please enter your password','');
repeatPassword = window.prompt('Please re-enter your password','');
while (password != repeatPassword) {
document.write('Your passwords don\'t match' + '<BR>');
repeatPassword = window.prompt('Please re-enter your password', ‘‘); }; 5

6. 4- Programming for repetition: the while statement
Example (Find the output of the following code)
myVar = window.prompt('Enter a number smaller than 30','');
myVar = parseFloat(myVar);
while (myVar > 30) {
document.write('Please enter a number smaller than 30!') }
The program would have displayed the line:
Please enter a number smaller than 30!
over and over again (will be stuck in a never ending (infinite) loop)
To correct the code you would need to include instructions in the body of the loop to read in the new input, assign it to myVar and convert it into a number. 6

7. 4- Programming for repetition: the while statement
Sentinel value (stopping value): needs to be terminating value, which indicates to the program that there are no more items (you don’t generally know in advance how many data items there will be)
Example
var word;
word = window.prompt('Please enter your first word','');
while (word != 'halt') {
document.write(word + ' ');
word = window.prompt('Please enter next word or halt to end',''); };
document.write('<BR>' + 'Thank you')
The program reads in each word, until it comes to the sentinel value, which is the string 'halt'.
Display each input string followed by a space
Thank you
halt is the sentinel value
If you input: halt;
the output will be:
Hello! nice to meet you
Thank you 7
If you input: Hello!, nice, to, meet, you, halt;
the output will be:

8. 4- Programming for repetition: the while statement
The program adds up a succession of positive numbers entered by the user. It displays the total when a negative number is entered.
(1) var total, nextNumber;
(2) total = 0;
(3) document.write('The current value of the total is ' + total + '<BR>');
(4) nextNumber = window.prompt('Please enter a positive number, any negative number to stop','');
(5) nextNumber = parseFloat(nextNumber);
(6) while (nextNumber >= 0)
(7) {
(8) document.write('Adding the new value of ' + nextNumber + '<BR>');
(9) total = total + nextNumber;
(10) document.write('The current value of the total is ' + total + '<BR>');
(11) nextNumber = window.prompt('Please enter a positive number, any negative number to stop','');
(12) nextNumber = parseFloat(nextNumber)
(13) };
(14) document.write('The total sum of your numbers is ' + total) 8

9. 4- Programming for repetition: the while statement
The current value of the total is 0
Adding the new value of 3
The current value of the total is 3
Adding the new value of 6
The current value of the total is 9
Adding the new value of 4
The current value of the total is 13
The total sum of your numbers is 13
If the inputs are: 3, 6, 4, -5;
the output will be:
The current value of the total is 0
The total sum of your numbers is 0
If you insert // at lines (4) and (5);
the output will be:
no value is assigned to nextNumber and so, when the condition is tested at line 6, JavaScript cannot compare it with 0, as required. In this situation JavaScript simply returns false. Hence the body of the while loop is skipped 9

10. 4- Programming for repetition: the while statement
Continue:
The current value of the total is undefined
Adding the new value of 3
The current value of the total is NaN
Adding the new value of 6
The total sum of your numbers is NaN
If you insert a comment (//)
at line (2) with inputs 3, 6 and -5;
the output will be:
If line (2), which initialises the variable total, is deleted, the value of total is still undefined when line (3) is execute. When, in line (9), JavaScript tries to add a new value to this undefined variable, the current value of total is displayed as above 10

11. 4- Programming for repetition: the while statement
The program multiplies together all the positive numbers you input, stopping when the sentinel 0 is input
Example
var product, nextNumber;
product = 1;
document.write('The current value of the product is '+ product + '<BR>');
nextNumber = window.prompt('Please enter a positive number, 0 to stop','');
nextNumber = parseFloat(nextNumber);
while (nextNumber != 0) {
document.write('Multiplying by ' + nextNumber + '<BR>');
product = product * nextNumber;
document.write('The current value of the product is ' + product + '<BR>');
nextNumber = parseFloat(nextNumber) };
document.write('The product of your numbers is ' + product)
Initializing product to 0 wouldn’t work as 0 multiplied by any other number will always give 0. It must be initialized by 1 11

12. 4- Programming for repetition: the while statement
String
In JavaScript, strings are object types
They have a number of methods associated with them.
charAt() method: (stringName.charAt(n) )
will return the character in the position after n in the string stringName. The characters in a JavaScript string are numbered (indexed) from 0 to one less than the number of characters. So stringName.charAt(0) returns the character that is the first in the string (starting from the left); stringName.charAt(4) returns the character in the fifth position from the left. 12

13. 4- Programming for repetition: the while statement
Example
var myString;
myString = window.prompt('Please enter a string with exactly 5 characters','');
document.write('You have chosen the following word ‘+myString + '<BR>');
document.write('The character in the first position is '
+ myString.charAt(0) + '<BR>');
document.write('The character in the fifth position is '
+ myString.charAt(4) + '<BR>')
The output of the following statement if you input ‘hello’ is:
document.write('The character in the last position is ' +
myString.charAt(5)+ '<BR>')
You have chosen the following word hello
The character in the first position is h
The character in the fifth position is o
If you input ‘hello’;
the output will be:
The character in the first position is
No character in the sixth position 13

14. 4- Programming for repetition: the while statement
a program which searches a word for a particular letter, in this case e, and writes out its position in the string.
Example
var myWord, index;
myWord = window.prompt('Please enter a word which has an e init','');
index = 0;
while (myWord.charAt(index) != 'e') {
index=index+1 };
document.write('The first occurrence of letter e is in position '
+ (index + 1) + ' in ' + myWord)
The first occurrence of letter e is in position 2 in tea
If the input is tea, the output will be 14

15. 5- Programming for repetition: the for statement
Syntax of a for loop
for (declare and initialize counter;
test value of counter;
change value of counter) {
statement (s) } 15

16. 5- Programming for repetition: the for statement
for statement (for loop)
Repeat the execution of a block of statements a predetermined number of times
The for statement is controlled by a counter, and can be used only when the number of repetitions is known before execution of the statement
To use a JavaScript for loop, you need to know:
the starting value of the counter;
the final value of the counter for which the loop body is executed;
the fixed number by which the counter is increased or decreased after each repetition;
the statement (s) which make up the body of the loop (as for a while loop). 16

17. 5- Programming for repetition: the for statement
The code writes out the value of
count, and then increases it by 1. It does this 10 times.
Count is 1
Count is 2
Count is 3
Count is 4
Count is 5
Count is 6
Count is 7
Count is 8
Count is 9
Count is 10
Thank you
The final value of count is 11
The output will be: 17

18. 5- Programming for repetition: the for statement
The program writes out the value of count in increasing order.
The initial value, the end value and the step will be entered by the user
Example:
initialValue = window.prompt('Please enter an initial value for count','');
initialValue = parseFloat(initialValue);
endValue = window.prompt('Please enter a stopping value for count','');
endValue = parseFloat(endValue);
stepValue = window.prompt('Please enter a step value','');
stepValue = parseFloat(stepValue);
for (var count = initialValue; count <= endValue; count = count +stepValue) {
document.write('Count is ' + count + '<BR>') };
document.write('Thank you'+ '<BR>')
Count is 3
Count is 5
Count is 7
Count is 9
Thank you
If the inputs are: 3, 9, 2; the output will be: 18

19. 5- Programming for repetition: the for statement
About the previous example:
If you enter an initial value which is greater than the final value, then the condition will evaluate to false the first time it is tested, so the body of the loop will never be evaluated and the value of count will never be displayed.
You can change the previous program to writes out the value of count in descending order by replace the for statement as follows:
for(var count = initialValue; count >= endValue; count = count - stepValue)
(the comparison operator has been changed from <= to >= and the counter is decremented rather than incremented).
Count is 9
Count is 7
Count is 5
Count is 3
Thank you
After replace the previous statement;
If the inputs are: 9, 3, 2; the output will be: 19

20. 5- Programming for repetition: the for statement
The counter is only initialized once regardless of the number of iteration
Example:
for (var count = 1; count <= 6; count = count + 1) {
document.write('*') };
document.write('I\'ve finished!')
The condition will need to evaluate to true six times; on the seventh attempt it will evaluate to false
******
I’ve finished
The output is: 20

21. 5- Programming for repetition: the for statement
In this program, the user will enter the length of the character (line), and the character to be output
Example
var length;
var character;
length = window.prompt('Please enter the length of your line','');
length = parseFloat(length);
character = window.prompt('Please enter your chosen character','');
for (var count = 1; count <= length; count = count + 1) {
document.write(character + ’<BR>’) } +
If you input 3, + ; the output will be: 21

22. 5- Programming for repetition: the for statement
In this program, the user is prompted to enter a score and this score is added to the total (average exam score)
Example:
var total; // total of scores so far
var score; // score for a single student
var average; // average score
total = 0;
/* insert code for a for loop to read in and add the 5 scores to total */
for (var count = 1; count <= 5; count = count + 1) {
score = window.prompt('please enter next score','');
score = parseFloat(score);
total = total + score };
average = total / 5;
document.write('average score is ' + average + '<BR>') 22

23. 5- Programming for repetition: the for statement
Convert the for statement to while statement
It is always possible to write a while loop that is equivalent to for loop, but we recommend that when you know how many iterations are required you use a for loop.
It is not usually possible to write a for loop that is equivalent to a while loop, because while loops are normally used where you don’t know in advance how many iterations will be required
for(var count =1;count<=10; count =count+1) {
document.write('Count is ' + count + '<BR>') };
document.write('Thank you' + '<BR>')
var count;
count = 1
while (count <= 10)
document.write('Count is ' + count +'<BR>');
count = count + 1 23

24. 5- Programming for repetition: the for statement
More about String
String objects are associated with:
methods (behaviors) as charAt()
properties, which hold information related to their structure and state (value).
String objects have one such property called length which, holds the number of characters in the string
StringName.length: ‘dot’ notation is used to indicate a property associated with an object
Note:
The index of the last character in the string is the length of the string minus 1.
lastIndex = StringName.length – 1 24

25. 5- Programming for repetition: the for statement
The program repeatedly prompts the user to enter a string, and displays its length until the user enters a terminating string, end.
Example
var myString; // string entered by user
// initialise myString to an empty string
myString = '';
while (myString != 'end') {
myString = window.prompt('Please input a string; enter end to finish','');
document.write('The length of ' + myString + ' is ' + myString.length + '<BR>') };
If the input is the output will be
hello The length of hello is 5
(empty string) The length of is 0
****** The length of ****** is 6
hello world The length of hello world is 11
The length of is 9
end The length of end is 3 25

26. 5- Programming for repetition: the for statement
The program reads a string from the user and outputs the characters, each followed by an asterisk. For example, if the user inputs the string hello, the output will be: h*e*l*l*o*
Example:
var myString;
myString = window.prompt('Please enter a string','');
//Remember -the arguments to charAt() start at 0, not 1
for (var count = 0; count < myString.length; count = count + 1) {
document.write(myString.charAt(count) + '*') }
Control structure: is control the flow of execution of the program (the order in which statements are executed). Such as if statement, while statements and for statements
Flow of control: is the flow of execution, which is dependent on the evaluation of Boolean conditions 26

27. 5- Programming for repetition: the for statement
The program reads in a word from the user, and displays it with any occurrences of the letter e replaced by asterisks
Example:
var myString;
var word, letter;
word = window.prompt('Please enter a word','');
for (var count = 0; count < word.length; count = count + 1) {
letter = word.charAt(count);
if (letter == 'e')
document.write('*') }
else
document.write(letter) }; 27

28. 5- Programming for repetition: the for statement
The program displays the number of occurrences of the letter e in the input word
Example:
var word, total;
word = window.prompt('please enter a word','');
total = 0;
for (var count = 0; count < word.length; count = count + 1) {
if (word.charAt(count) == 'e')
total = total + 1 } };
document.write('The number of occurrences of e in the word '
+word + 'is'+ total) 28