1. Java Coding 3 – part2 David Davenport Computer Eng. Dept.,
Over & over again! for & do-while
David Davenport
Computer Eng. Dept.,
Bilkent University Ankara - Turkey.
Last revised: 20/10/2017 ~ part2 contains only for & do-while
Previously: 8/11/2016, 26/10/2016 ~ minor revisions
20/10/2016 ~ Added new animated slide for sentinel-controlled input, etc.
22/10/2012

2. IMPORTANT… Students… Instructors…
This presentation is designed to be used in class as part of a guided discovery sequence. It is not self-explanatory! Please use it only for revision purposes after having taken the class. Simply flicking through the slides will teach you nothing. You must be actively thinking, doing and questioning to learn!
Instructors…
You are free to use this presentation in your classes and to make any modifications to it that you wish. All I ask is an saying where and when it is/was used. I would also appreciate any suggestions you may have for improving it.
thank you, David.

3. Repetition Java repetition statements while (condition) statement; do
for ( init; condition; update)
statement;
Java has three forms of repetition statement: while, for & do-while
They can all essentially equivalent…
here we will concentrate on for & do-while (while is in first part!)
Statement to be repeated, input, output, assignment, decision, repetition!
Condition exactly as for if statements
where
statement is any Java statement
condition is a boolean expression

4. For & Do-while …other forms of repetition in Java
In Java the for loop can do anything the while loop can (this is not necessarily the case in other languages where
it is often only used with integer loop control variables).
We will use both for & do-while is special cases only:
For counting like loops
where the loop must be done at least once, eg. data validation & menus.
For & Do-while

5. Java for statements Same as while loop!
Use as short-hand counting style loop
statement
condition
true
false
update
init
for ( init; condition; update)
statement;
It has advantage of explicitly reminding you about init & update (which are easy to forget when writing a while loop).
In some languages, for can only use integer (enumerable) loop-control-variables
(suggest treating Java for statements same, i.e. only use for counting-style loops).
Notes:
variable i is often defined inside the for statement, e.g.
for ( int i = 0; i < 5; i++)
System;out.println( “*”);
System;out.println( “i = “ + i); // will not compile!
… if so, it will not be visible outside, i.e. after the loop.
Variables only visible inside block they are defined in.  important!
i++ is shorthand notation for i = i + 1;
Perhaps redo triangle drawing question from previous slide?
Example:
for ( i = 0; i < 5; i = i + 1) System.out.println( “*”);

6. Questions (1)
Write a program that asks the user to enter an integer number N, then prints a triangle with N lines of stars or numbers. For example, if N is 4 *
***
*****
*******
*******
*****
*** * 1
222
33333 1
212
32123
Try to apply top-down design to these problems
Then implement & test in Java.

7. Look at and learn to use Java API
Example
Ask the user for their name, print it out one character per line, then print “done”.
Enter your name: David D a v i d
done.
Look at and learn to use Java API
Note: This problem does not explicitly require for statements.
It is a good exercise though, and useful for looking at Java String class (and Math class too – precursor to static methods).
Possibly show the alternative for syntax too?
Solution:
Ask for and get name
for each character in name (from first to last)
print that character from name and move to next line.
print “done”
Alternatively:
while there are characters in name do
print the first character of name and move to next line
delete the first character from name
Note: another soln: simply insert “\n” btw each of the characters!
Extend, to print out in reverse order (i.e. “d, i, v, a, D”)
And then to building up as String which can be printed in one go!
(both as reverseNameString & nameString)
Important abilities:
should be able to divide and reconstruct String in either direction
should be able to divide & reconstruct an integer number in either direction.
print 1st char in name
“ 2nd “ “ “
“ 3rd “ “ “
“ : “ “ “
“ last “ “ “
Need to be able to:
“extract a character” from a String
find “number of characters” in String

8. Using Java methods… To use, need to know Method name
Inputs & their types
Output type
output
result type
answer = f( x, y, z );
Method name
input
parameters
// in Math class
public static double sin( double d)
double d = Math.sin( 0.5);
Reading … and applying … Java API documentation!
for Math & String only (static + Strings which are weird objects!)
- though lessons are generally valid.
Use ClassName.method(…) for “static” methods
// in String class…
public String substring( int beginIndex, int endIndex)
// usage…
shortString = longString.substring( 1, 3);
varName.method(…) for non-static methods

9. Example
Ask the user for their name, print it out one character per line, then print “done”.
Enter your name: David D a v i d
done.
for (index = 0; index < name.length(); index++)
System.out.println( name.charAt( index) );
Note: This problem does not explicitly require for statements.
It is a good exercise though, and useful for looking at Java String class (and Math class too – precursor to static methods).
Possibly show the alternative for syntax too?
Solution:
Ask for and get name
for each character in name (from first to last)
print that character from name and move to next line.
print “done”
Alternatively:
while there are characters in name do
print the first character of name and move to next line
delete the first character from name
Note: another soln: simply insert “\n” btw each of the characters!
Extend, to print out in reverse order (i.e. “d, i, v, a, D”)
And then to building up as String which can be printed in one go!
(both as reverseNameString & nameString)
Important abilities:
should be able to divide and reconstruct String in either direction
should be able to divide & reconstruct an integer number in either direction.
for index = 0 to ___________
print charAt index in name
print 1st char in name
“ 2nd “ “ “
“ 3rd “ “ “
“ : “ “ “
“ last “ “ “
print charAt 0 in name
“ “ “ “
“ “ “ “
“ “ : “ “
“ “ last “ “

10. Example
Ask the user for their name, print it out one character per line, then print “done”.
Enter your name: David D a v i d
done.
now do it in reverse!
Enter your name: David d i v a D
done.
Note: This problem does not explicitly require for statements.
It is a good exercise though, and useful for looking at Java String class (and Math class too – precursor to static methods).
Possibly show the alternative for syntax too?
Solution:
Ask for and get name
for each character in name (from first to last)
print that character from name and move to next line.
Alternatively:
while there are characters in name do
print the first character of name and move to next line
delete the first character from name
Extend, to print out in reverse order (i.e. “d, i, v, a, D”)
And then to building up as String which can be printed in one go!
(both as reverseNameString & nameString)
Important abilities:
should be able to divide and reconstruct String in either direction
should be able to divide & reconstruct an integer number in either direction.
Finally…
From either sequence of characters, construct a String, reverseName

11. Question
Given an integer, print its digits in reverse sequence, one per line.
Enter integer: 7 3
done.
Enter an integer: 372
Reversed is: 273
Now reconstruct the reverse integer
Note: This problem does not explicitly require for statements.
It is a good exercise though, and useful for looking at Java String class (and Math class too – precursor to static methods).
Possibly show the alternative for syntax too?
Solution:
Ask for and get name
for each character in name (from first to last)
print that character from name and move to next line.
Alternatively:
while there are characters in name do
print the first character of name and move to next line
delete the first character from name
Extend, to print out in reverse order (i.e. “d, i, v, a, D”)
And then to building up as String which can be printed in one go!
(both as reverseNameString & nameString)
Important abilities:
should be able to divide and reconstruct String in either direction
should be able to divide & reconstruct an integer number in either direction.
Note: try doing it without using the power function!

12. Java do-while statements
Repeat 1 or more times do
statement;
while (condition);
statement
condition
true
false
loop
Note layout…
“{“ on same line as “while ();” can help avoid confusion!
Also, “;” is required here (in do-while statement), but not in “while” statement
Danger of this if used in “while” statement and in “if” statement.
Example:
i = 0; do {
System.out.println( “*”); i++;
} while ( i < 5);
Note: “}” on same line & “;” at end!

13. Examples (do-while) Data validation e.g. Read positive value from user
do ask for “a positive value” and get value while value is not positive
Read value btw 5 & 10 inclusive.
do { System.out.print( “Enter positive value: ”); value = scan.nextInt(); } while ( value <= 0); // assert: value > 0

14. Examples (do-while) Menus - set of options for user to select from
do display menu get selection from user
perform selection
while selection is not exit
print “goodbye”
ABC Trading Co
1 – sales 2 – stock
3 – admin
Select (0 to exit): _
Write algorithm for display menu & perform selection tasks
Translate into Java.
Remember to use named constants for selections!
if selection is SALES then // do sales things
else if selection is STOCK then // do stock things
else if selection is ADMIN then // do admin things
else if selection is not EXIT then print “invalid selection” msg