2.1 Program Construction In Java

1. 2.1 Program Construction In Java Joey Militti Brendan Tackney Period 1

2. Mistakes • The IB examination is not to picky on small code errors such as a missing semi-colon here and there. • However a mistake that leads to incorrect logic will result in a penalty • An example of how a missing SemiColon wont hurt is : If(x==3) { Y = y+4 //there is an error here Z=y-x; } X=x +1

3. Comments • If you want to add information that describes your code you should use comments // • Some comments may be graded in the IB exam. • Your program does not execute comments.

4. Basic Variables • Byte – used for small integers or whole numbers in the range -128 to +127 • Int is used for whole numbers negative or positive in the range -2146483648 to +2147483647 • Long used for whole numbers negative or positive that are much bigger then numbers used for ints • Double used for numbers with fractional parts in the approximate range ex – 56.55 • Char represents a signal symbol a letter other symbols or a number. Can also represent integer values in the range 0-65535. • Boolean – represents one of two states only.

5. Assignment • Assignment always takes the expression on the right side of the “equals” sign and places the results in the variable on the left side of the “equals” sign. number = number + 23; //now number is greater than it was • When it is executed, the right side is evaluated first. So 23 is added to the value in memory (4) and then 27 is placed in the memory. • The 4 that was there is now overwritten and can not be retrieved at any time.

6. Beyond Primitives • Classes, like String, begin with a capital letter. • Its important to remember that where objects are concerned the identifier is a reference as to where the object lies in the memory. String name = “Binh Nguyen Le”; int len = name.length(); • Primitives have no such extra information therefore “int len = name.length();” is not possible. • Primitives really consist just of a value whereas objects have a whole lot of associated methods.

7. Combining String • Combining Strings is a simple easily done task in most cases. • To combine a string we use a + sign. • We do not include the + sign in the “” instead we put it as follows “Brendan” + “in a non gay way Joey” • Backslashes also have a big use in strings. • \n – begins a new line in the string • \t – does a tab in the string • \” – puts a quote in a string.

8. Arithmetic Operators • In the IB test the main arithmetic operators we will use are + for addition, - for subtraction, * for multiplication, / for division, % for modulus • The order in which you place arithmetic operators can effect the final value. For Example: int x = 23 % 5 + 2 Int x = (23 % 5) +2 Parenthesis change the result because the first one equals 5 while the second question equals 2.

9. Mathematical Functions • double, int abs(x) – Returns the absolute value of its argument (x may be double or int). • double pow(x, y) – Returns x raised to the power y. • double sin(x) – Returns the sine of x, where x is in radians. • double cos(x) – Returns the cosine of x, where x is in radians. • Long round (x) – Rounds x to the nearest integer. • double floor(x) – Rounds x to the nearest integer not greater than x.

10. Sequence • Sequence of instructions are statements or actions in an algorithm simply flow from one to the next. public Add(){ double number1 = inputDouble(“Input the first number: “); double number2 = inputDouble(“Input the next number: “); double total = 0; output(“The total is: “ + total); double total = number1 + number2; }

11. Constructing A Modular Program • IB Computer Science program requires students to analyze and solve problems rather than to write code solutions to simple programming exercises. • As the programming exercises increase in complexity, other stages can also be brought in.

12. Parameters • One way of passing a variable to another method is to use a parameter. • The identifier or expression where the function is “called” is often referred to as the “argument.” • The parameter acts like a local identifier whose scope is the method body.

13. Method Signatures • The line where the function parameters and return type are defined is known as the method signature.

14. Selection • This is the use of different sets of instructions depending on a given condition. • Selection is also often referred to as “branching.”

15. Nesting • Nesting occurs when one control structure is placed inside another. • if and if else statements are control structures. int x = 9; int y = 0; int z = 3; char c = ‘x’; if (x ==9) if (y < 3) if (c != ‘x’) output (“flip”); else if (z >= 3) output (“flop”); else output (“fly”);

16. Multiple Selections • When making multiple selections, if statements can become quite complicated. • Instead of: if if else • You can use: if else if else if • This is very helpful when multiple choices need to be made.

17. Repetition • Repeating things again and again until some condition is met or while the condition is true or false is the basic structure of a loop. • A While loop keeps executing while the condition is true • EX: While(brendanownz= true) { Own(); } • A different while loop can check the condition at the end so the loop executes once before checking the condition • EX: Int x = 1; { Output(X); X *= 2; } While(x < 10);

18. Tracing Loops • In order to sum values a list of numbers must be input ending with a 0 or -1 • Tracing an algorithm means to work out what the values of variables will be in each step as a means of understanding how it works.

19. Classes, User-Defined methods, and Objects • Java has built-in functions like the one that returns the length of a string. • Sometimes the term “function” is used to distinguish a method that returns a single value and thus has a return type.

20. Exercise • Devise an algorithm which finds the largest value in a given array of numbers. To help you here is an algorithm which finds the smallest in a given array of numbers listed called listA: • // initialise an int array with values int[] listA = new int[] {3, 67, -9, 304, -56, 2}; Int pos = 1; Int smallest = listA[ 0]; Whiile( pos < 6) { if(listA[ pos] < smallest) { smallest = listA[pos]; } pos = pos +1; } Output(“the smallest number in the list is: “ + smallest);