1. CS 106 Introduction to Computer Science I 02 / 23 / 2007 Instructor: Michael Eckmann

2. Michael Eckmann - Skidmore College - CS 106 - Spring 2007 Today’s Topics questions, comments? solution to lab problem to determine highest and lowest temperature in an array random method in the Math class programmer defined methods

3. 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?

4. 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?

5. 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.

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

7. 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.

8. 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;

9. 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?

10. 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() ?

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

12. what is a method A method is a 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 - Spring 2007

13. methods Here’s a good analogy of a boss and worker 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 - Spring 2007

14. methods methods are defined within a class. we have seen the main method defined in the class of every application so far. Michael Eckmann - Skidmore College - CS 106 - Spring 2007

15. 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 These methods are 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 and returns a value of type int. Michael Eckmann - Skidmore College - CS 106 - Spring 2007

16. 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 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 the value returned by the method. Michael Eckmann - Skidmore College - CS 106 - Spring 2007

17. 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 - Spring 2007

18. example of a programmer-defined method we might name this method salary_after_raise we need to take in two arguments, one for the current salary and one for the raise percentage. What primitive type might these arguments 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 - Spring 2007

19. example of a programmer-defined method so, this method could look like: public 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 - Spring 2007

20. 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_sal; double old_sal = 300; double raise = 4.5; // our call to the salary_after_raise method new_sal = salary_after_raise( old_sal, raise ); Michael Eckmann - Skidmore College - CS 106 - Spring 2007

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

22. example of a programmer-defined method In our example, the method was called from only one place but in real applications, certain methods will need to be called in many different parts of the program. 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. They also aid in the ability to understand a program (readability) and make changes. Michael Eckmann - Skidmore College - CS 106 - Spring 2007

23. 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 at the method call

24. more about methods methods that do not return a value are of type void (like the main method.) methods are not required to have any parameters; these 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.

25. example method that returns nothing and has no parameters public 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 nor is there a variable to which to assign the returned value (because it doesn’t return a value.) print_error_msg();

26. example method that returns nothing but has one parameter public 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.”); // name the film.

27. other 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 reusability less code repetition more readable, better designed code