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)‏ // foreach in other languages sum += nValue; 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 do- while? 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 listingStudentInterface.java

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 to go over all the main blocks of Java code you’ve seen so far.