Iteration and Simple Menus
Deterministic / Non-deterministic
loops and simple menus
Where are we?
The 6 Coding Concepts






Input – getting things into the program
Output – passing things to the outside world
Assignment – pass a value from one part of the
program to another
Sequence - do each thing in the order specified
Decision-making -check something then do the
appropriate action depending on that condition
Repetition - do the set of commands several
times
Deterministic loops (for loop)

Use if you know how many times the loop should
execute (deterministic loop)
int num = Console.readInt("How many lines?");
for (int i = 0; i < num; i++)
{ System.out.println("I must stay awake in class");
}
Breakdown of the for Loop
for (int i = 0; i < num; i++)


What does this mean?
The for loop consists of 3 items:





The initialiser (int i = 0)
The terminating condition (i < num)
The incrementor (i++)
These items are separated by a semicolon ;
Like the if statement, you can omit the {} that
follow it if there is only one statement to be
repeated
Nested for loops

You have seen how if-else statements can be nested
inside each other:
if(condition )
{ if(condition )
{ // do something
}
else
{ // do something different
}
}
else
{ // other stuff
}
Nested for loops

Well you can do the same with for loops:
for( int i = 0; i < 10; i++ )
{ System.out.println(“Outer loop”);
for( int j = 0; j < 5; j++ )
{ System.out.println(“Inner loop”);
}
}

Why use i and j as loop counters?
Loop examples
for( int i = 0; i < 10; i++ ) – will repeat 10 times by incrementing i
for( int i = 0; i <= 10; i++ ) – will repeat 11 times
for( int i = 0; i < 10; i+=2 ) – will repeat 5 times
for( int i = 10; i > 0; i-- ) – will repeat 10 times by decrementing i
for(;;) – will repeat infinitely! Hint: there may be a way out…
Alternative syntax: For-Each

There is another way of specifying the Java for loop
when using arrays and collections.
int [] myArray;
// or you could write int myArray[];
for(int nValue : myArray)
sum += nValue;

// foreach in other languages
instead of:
for(int i=0; i < myArray.length; i++)
sum += myArray[i];
A short digression – reading ints
If we need to read an integer value from the keyboard, we might do this:
import java.util.*;
private int getConsoleInt(String sPrompt)
{
System.out.print(sPrompt);
Scanner scan = new Scanner(System.in);
try{ return scan.nextInt(); }
catch (InputMismatchException ex) { return
Integer.MIN_VALUE; }
catch (NoSuchElementException ex){ return
Integer.MIN_VALUE; }
catch (IllegalStateException ex){ return Integer.MIN_VALUE;
}
} // note that a return of –2,147,483,648 means an error occurred!
Non-deterministic loops (while)


use if you want the loop to execute until a
condition is false (non-deterministic loop)
must allow tested variable to change within the
loop, otherwise you’ll have an infinite loop!
int num = getConsoleInt ("How many lines?");
while (num > 0)
{ System.out.println("I love Java!!!");
num--;
}
Non-deterministic loops (do-while)

There is a similar while loop called the do-while
loop:
int num = getConsoleInt ("How many lines?");
do
{ System.out.println("I love Java!!!");
num--;
}
while (num > 0);
Note the semi-colon!
Non-deterministic loops (do-while)




So what’s the difference between while and dowhile?
The contents of a while loop is not executed if
the test condition fails at the start.
The contents of a do-while loop is guaranteed to
execute at least once.
The test condition is only performed after the
first iteration through the loop.
Exiting a loop




The normal way to exit a loop is for the condition
that is tested to become false.
This is true of all three types of loops in Java:
for, while, and do-while.
However, there might be times when you want a
loop to end immediately, even if the condition
being tested is still true.
You can do this with a break statement, as
shown in the following code:
The break statement
int index = 0;
while (index <= 1000)
{ index = index + 5;
// or index+=5;
if (index == 400) break;
System.out.println("The index is " + index);
}

The break statement is especially handy if you
need to search a list and then exit the search
loop code if you found what you are looking for
Menus


Can use a combination of a while loop and a
switch statement to implement a menu
NB may be better to have “[0] Exit” - why?
What would you like to do?
[1]Register student
[2]Register student on program
[3]Exit
?
Simple menu display
public int showMenu()
{ int choice;
System.out.println("\nWhat would you like to do?");
System.out.println("\t[1]\tRegister student");
System.out.println("\t[2]\tChange student's program");
System.out.println("\t[3]\tExit");
choice = getConsoleInt ("? ");
return choice;
}
Simple menu control
int choice = showMenu();
while (choice != 3)
{ switch (choice)
{ case 1:
registerStudent();
break;
case 2:
changeProgram();
break;
default:
System.out.print(“Invalid, please try again");
}
choice = showMenu();
}
Implementing a
menu-based interface


Would have to write appropriate methods
 showMenu()
 registerStudent()
 changeProgram()
Look at StudentInterface.java listing
Summary


We have looked at:
 for loops
 Nested for loops
 while loops
 do-while loops
 break
We have put them together into a menu-driven
interface for the Student class (see tutorial).
Further work


Practical
Read http://www.faqs.org/docs/javap/c3/s1.html to go
over all the main blocks of Java code you’ve seen so far.