1. CS 106 Introduction to Computer Science I 10 / 08 / 2007 Instructor: Michael Eckmann

2. Michael Eckmann - Skidmore College - CS 106 - Fall 2007 Today’s Topics Comments and/or Questions? multidimensional arrays useful Math class methods

3. Two dimensional arrays If instead of a list of items, you wanted to store a grid of items, you could use a 2d array. A grid has rows and columns. A two dimensional array is declared with two pairs of brackets. int array2d [] [] = new int[ 2 ][ 3 ]; // This line creates a 2-by-3 array (2 rows, 3 columns)

4. Two dimensional arrays int array2d [ ] [ ] = { { 5, 7, 11 }, { 62, 3, 48 } }; 48362 1175 to access the element at row 0, column 1, we use: array2d [ 0 ] [ 1 ] // this holds the value 7 in our example.

5. Two dimensional arrays A two dimensional array might be useful for something like keeping track of all the students in a class and their grades on each of the homeworks. So, each student could be represented by a number (0 to 15, for 16 students) and each homework represented by a number (0 to 9, for 10 hw's). Each student could be a row in the 2d array and each homework could be a column. The values stored in the array elements are the grade a particular student got on a particular homework.

6. Two dimensional arrays) int student_hw_grades [] [] = new int [ 16 ] [ 10 ]; //The outer loop will be for the 16 students and the inner loop will be for the 10 grades. for (int row = 0; row < array2d.length; row++) { for (int col = 0; col < array2d[row].length; col++) { user_str = JOptionPane.showInputDialog( "Enter student " + row + "'s homework #" + col + "grade"); array2d[ row ] [ col ] = Integer.parseInt(user_str); } // note the use of.length in the two loop conditions

7. Multi-dimensional arrays Not only can we have one dimensional, and two dimensional arrays but we can have n dimensional arrays, where n is any positive integer. Example when n=3: double temperatures[][][] = new double [12][31][24]; take a guess as to what might be stored in this array and what the indices mean

8. Multi-dimensional arrays Unfortunately the first index must go from 0 to 11, the second from 0 to 30 and the third from 0 to 23. What if we wanted the index to represent exactly the month (1 to 12), day (1 to 31) and hour (0 to 23)? Is there anything we could do to this line? double temperatures[][][] = new double [12][31][24];

9. Math class Let's look at the Java API for the Math class. Specifically these methods: abs – guess what this does. cos, sin, tan ceil – returns smallest whole number >= parameter. pow – takes two parameters – raises first to second and returns the result. random – returns random # in the range: [0.0, 1.0) sqrt Michael Eckmann - Skidmore College - CS 106 - Fall 2007

10. A few more methods in the Math class max ( x, y ) method that returns the larger of x and y min ( x, y ) method that returns the smaller of x and y There are versions of these methods that work for floats, doubles, ints and longs and return a result that is one of these types.

11. example calls to static methods in the Math class double w = 5.1, z = 10.56, a, b, c; a = Math.max ( w, z ); // what value would a have? a = Math. max ( z, w ); // what value would a have, now? b = Math. min ( z, w ); c = Math. sqrt ( z );

12. random( ) method in the Math class double rand_num; rand_num = Math.random ( ); // what value might rand_num have after this line of code? // is 0.34452 a possible value? // is 2 a possible value? // is -14.555423 a possible value?

13. random( ) method in the Math class random ( ) returns a double whose value is >= 0 and < 1, but sometimes we want a random integer How might we do that?

14. random( ) method in the Math class random ( ) returns a double whose value is >= 0 and < 1, but sometimes we want a random integer One way to do that is to first multiply the result by some integer to get a value that isn’t necessarily between 0 and 1. Then, cast this new value to an int by using the (int) cast operator.

15. random( ) method in the Math class // example: int some_random_int; double some_random_dbl; some_random_dbl = Math.random ( ) * 25; // this will result in a value >= 0 and < 25. some_random_int = (int) (Math.random ( ) * 25); // what is the range of values for some_random_int here?

16. random( ) method in the Math class int random_card_value; int random_card_suit; random_card_value = 1 + (int) (Math.random ( ) * 13); random_card_suit = (int) (Math.random ( ) * 4); Let’s put this code in a program and execute it.

17. random( ) method in the Math class What if I put the cast to int without using parentheses around the rest of the expression? e.g. random_card_suit = (int) Math.random ( ) * 4;

18. random( ) method in the Math class What if I put the cast to int without using parentheses around the rest of the expression? e.g. random_card_suit = (int) Math.random ( ) * 4; since the cast operator (int) has higher precedence than the multiplication operator *, it would be done first, which means what?

19. random( ) method in the Math class random_card_suit = (int) Math.random ( ) * 4; the Math.random() method call would return a double value and immediately this value would be cast to an int. Casting a double to an int causes the truncation of any decimal portion. Recall that the double that is returned by Math.random() is >= 0.0 and < 1.0 So, what's the possible values of (int) Math.random() ?

20. random( ) method in the Math class (int) Math.random( ) would always be zero.

21. Math class Let's write some code using these methods. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

22. what is a method A method is small piece of a program designed to achieve some specific task and usually returns some piece of information. A method is invoked by a method call. To call a method, you provide its name and the correct arguments that are necessary for the method to execute. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

23. methods Here’s a good analogy of a worker and boss to describe methods and their callers. A boss (the caller) asks a worker (the method that is called) to perform a task (the code in the method) and report (return results) back when the task is done. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

24. methods All methods are defined within some class. we have seen the main method defined in the class of every application so far. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

25. other methods we’ve used We have used other methods available in the Java API, methods like parseInt and parseFloat and println are available to us to use in their respective classes Integer, Float and System.out The methods in the Math class are other examples. These methods were defined for us to do a specific task. We call them when we need them. e.g. in the case of Integer.parseInt --- it’s task is to convert a String into an int. It takes a String as an argument (parameter) and returns a value of type int. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

26. other methods we’ve used e.g. in the case of Integer.parseInt --- it’s task is to convert a String into an int. It takes a String as an argument (parameter) and returns a value of type int. example of a call to this method: some_int = Integer.parseInt( some_str ); In this example, some_str is the argument that is being passed in to the parseInt method and some_int is the variable that will get set to the value returned by the method. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

27. methods We can create our own methods that we can call to perform specific tasks. Let’s say we’re writing a program to handle employee’s salaries. We might need to compute a salary after a raise. This example method would need to have access to the current salary and the raise percentage. It would then calculate the salary after the raise and return this value. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

28. example of a programmer-defined method we might name this method salary_after_raise we need to take in two values, one for the current salary and one for the raise percentage. What primitive type might these be? we also need to return the salary that is computed. What primitive type might this returned value be? Michael Eckmann - Skidmore College - CS 106 - Fall 2007

29. example of a programmer-defined method so, this method could look like: public static double salary_after_raise(double curr_sal, double raise_pct ) { double new_sal; new_sal = curr_sal * ( 1 + raise_pct / 100 ); return new_sal; } Michael Eckmann - Skidmore College - CS 106 - Fall 2007

30. example of a programmer-defined method to call this method from some other method within the same class do the following: // example variable declarations... double new_salary; double old_sal = 35000; double raise = 4.5; // our call to the salary_after_raise method new_salary = salary_after_raise( old_sal, raise ); Michael Eckmann - Skidmore College - CS 106 - Fall 2007

31. example of a programmer-defined method Let’s look at a complete program that contains this method and calls it several times. Michael Eckmann - Skidmore College - CS 106 - Fall 2007

32. example of a programmer-defined method When certain code can be used in several places in a program, you may want to create a method containing that code. Then, wherever that code would have been, a simple method call appears. Programmer defined methods also aid in the ability to understand a program (readability) and make changes (maintainability). Michael Eckmann - Skidmore College - CS 106 - Fall 2007

33. when does a method end it’s execution? when it hits a return; statement when it hits a return some_expression; statement or, when it hits the right curly brace of the method whichever comes first. when the method ends its execution, the program execution continues back at the method call (from where the method was called)

34. more about methods methods that do not return a value are of type void (like the main method.) Methods can have 0 or more parameters separated by commas. Those with 0 parameters are defined with nothing between the parentheses to call a method with no parameters, you must still use the parentheses but with nothing between them Note that the return type of a method can be different than any or all of the types of parameters that get passed in.

35. example method that returns nothing and has no parameters public static void print_error_msg() { System.out.println(“Invalid entry.”); System.out.println(“You must reenter.”); } how to call this method: note that there are no arguments passed in nor is there a variable to which to assign the returned value (because it doesn’t return a value.) print_error_msg();

36. example method that returns nothing but has one parameter public static void print_msg(String the_msg) { System.out.println(“The message is: ” + the_msg); // note: no return statement since nothing to return // The return type of this method is void. } how to call this method: print_msg(“Good afternoon gentlemen. I am a HAL 9000 computer.”);

37. example method that returns a String but has no parameters public static String random_string() { int num = (int) (Math.random() * 3); if (num == 0) return “Hello”; if (num == 1) return “Goodbye”; if (num == 2) return “$#@&!*”; } how to call this method: System.out.println(“the random string is: “ + random_string());

38. reasons to create and use methods separation and modularization of ideas is helpful when writing large programs because smaller parts of a problem are easier to solve the individual methods can be tested and confirmed correct to reduce debugging woes methods lend themselves to software reuse less code repetition more readable, better designed code