CS 112 Introduction to Programming Summary of Method Definition and Invocation; printf/format Formatting Yang (Richard) Yang Computer Science Department Yale University 208A Watson, Phone:

Outline  Admin and recap  Summary of method definitions and invocation m Method overloading m printf and String.format 2

Admin  PS4 posted yesterday m There was some revision (Part 2 was changed) to PS4. Please be sure to reload the page 3

Recap: Animation … int h1 = 600, v1x = 30, v1y = 20; int h2 = 500, v2x = 40, v2y = 28; for (double t = 0; t < 10; t += FRAME_T/ ) { double x1 = v1x * t; double x2 = v2x * t; double y1 = h1 + v1y * t * t * t / 2; double y2 = h2 + v2y * t * t * t / 2; Car.drawCar(x1, y1, CAR1_SIZE ); Car.drawCar(x2, y2, CAR2_SIZE ); StdDraw.show(FRAME_T); // hold image for 30 ms StdDraw.clear(); // now clear up }

StdDraw.show(): Details  StdDraw.show(int t) m Display on screen, pause for t milliseconds, and turn on animation mode: subsequent calls to drawing methods such as line(), circle(), and square() will not be displayed on screen until the next call to show().  StdDraw.show() m Display on-screen and turn off animation mode: subsequent calls to drawing methods such as line(), circle(), and square() will be displayed on screen when called. 5

Without Animation

Animation: Use Both Screen (Video Memory) and (Double) Buffer In animation mode, drawing is in the (double) buffer Next show(t) statement copies to screen

Example: AnimationShow.java  Try running the example in Debug mode m Right click first line and toggle breakpoint m Execute in Debug mode m Execute (step over) each statement 8

Recap: Defining a Method Returning a Value public static type name ( parameters ) { statements ;... return expression ; } methodname returntype parameter list properties public static double fToC(double degreesF) { return (degreesF - 32) * 5.0 / 9.0 ; }

A Common Error  Many people incorrectly think that a return statement sends a variable's name back to the calling method. public static void main(String[] args) { fToC(60); System.out.println(“60F = " + result); } public static double fToC(double degreesF) { double result = 5.0 / 9.0 * (degreesF - 32); return result; } // ERROR: result not defined

Fixing the Common Error  Instead, returning sends the variable's value back. m The returned value must be stored into a variable or used in an expression to be useful to the caller. public static void main(String[] args) { double c = fToC(65); System.out.println(“65F = " + c + “C”); System.out.println(“Again, 65F = " + fToC(65) + “C”); } public static double fToC(double degreesF) { double result = 5.0 / 9.0 * (degreesF - 32); return result; }

Return vs Paremeter  Return is the opposite of a parameter: m Parameters send information in from the caller to the method. m Return value sends information out from a method to its caller. main Math.abs(-42) -42 Math.round(2.71)

Exercise: Revise CarLaunch  Revise CarLaunchV2 to use a method with return

Exercise Solution public static double pos(double initPos, double speed, double a, double t) { return initPos + speed * t + a * t * t / 2; }

Outline  Admin and recap  Summary of method definitions and invocation 15

16 Summary: Method Definition  Why define methods? m Denote structure, eliminate redundancy m A method with parameters solves an entire class of similar problems m A method with return gives back an answer on a question

Method “Puzzle” System.out.print( Math.round(10.3) ); // Math.round() has two definitions // definition 1 static long round(double a) // definition 2 static int round(float a) Two definitions of same method name? Many methods in Math has multiple definitions:

18 Method Definition and Invocation Rules  Definition rule: m You can define multiple methods with the same name in a class. This is called method overloading m To distinguish different overloaded methods, these methods must have different signatures The signature is the sequential list of the type of each parameter  Invocation rule: m Java compiler picks the method with the best match in signature, allowed by implicit conversion.

19 Overloaded Methods double tryMe (int x) { return x +.375; } Version 1: signature: int double tryMe (int x, double y) { return x * y; } Version 2: signature: int_double result = tryMe (25, 4.32)Invocation double tryMe (double x, int y) { return x * y; } Version 3: signature: double_int double tryMe (double x, double y) { return x * y; } Version 4: signature: double_double

20 Overloading Picks the Best Match allowed by Implicit Conversion double tryMe ( int x ) { return x + 5; } double tryMe ( double x ) { return x *.375; } double tryMe (double x, int y) { return x + y; } tryMe( 1 ); tryMe( 1.0 ); tryMe( 1.0, 2); tryMe( 1, 2); tryMe( 1.0, 2.0); Which tryMe will be called?

Overload Matching only Signature int x = Math.round(10.3); // Math.round() has two definitions // definition 1 static long round(double a) // definition 2 static int round(float a) Best match. ERROR: Type mismatch. I know 10 will fit as an int: how do I change from long to int? int x = (int)Math.round(10.3);

22 Method Invocation and Parameter Passing  Corresponding actual argument in the invocation is assigned to the corresponding formal argument public static void printNumber(int number, int count) { for (int i = 1; i <= count; i++) { System.out.print(number); } System.out.println(); } int line = 3; printNumber(line-1,5); // equiv: number = 2; count = 5;

23 Formal Arguments are Local Variables  In Java, a formal argument is a local variable of a method  The formal argument and the actual argument are different variables, with different memory locations, even if they have the same name.  When a primitive variable is passed as the actual argument to a formal argument, the value is copied m Value copying implies value semantic m Implication: modifying the parameter inside the method will not affect the variable passed in.

Value Semantics int a = 100; double x = 45.12; a x A value variable stores a value of the type of the variable.

25 Value Variables int a = 100; double x = 45.12; int aa; 100 a x aa

26 Value-Variable Assignment int a = 100; double x = 45.12; int aa; aa = a; 100 a x aa An assignment of one value variable to another value variable copies the value. 100

27 Value-Variable Assignment int a = 100; double x = 45.12; int aa; aa = a; a = 200; 100 a x aa 100 Change the value of one value variable will not change the other. 200

Exercise: “Parameter Mystery” public static void strange(int x) { x = x + 1; System.out.println("1. x = " + x); } public static void main(String[] args) { int x = 23; strange(x); System.out.println("2. x = " + x);... } Output: 1. x = x = 23

Explanation: main() start 29 public static void main(String[] args) { int x = 23; strange(x); System.out.println("2. x = " + x); } 23 args x

Explanation: Invocation 30 public static void strange(int x) { x = x + 1; System.out.println("1. x = " + x); } public static void main(String[] args) { int x = 23; strange(x); System.out.println("2. x = " + x); } 23 args x x compiler declares formal argument x and copies value from the actual argument 23

Explanation: Local update 31 public static void strange(int x) { x = x + 1; System.out.println("1. x = " + x); } public static void main(String[] args) { int x = 23; strange(x); System.out.println("2. x = " + x); } 23 args x 23 x 24

Explanation: Method return 32 public static void strange(int x) { x = x + 1; System.out.println("1. x = " + x); } public static void main(String[] args) { int x = 23; strange(x); System.out.println("2. x = " + x); } 23 args x x compiler un-declares formal argument 24

Explanation: Method return 33 public static void main(String[] args) { int x = 23; strange(x); System.out.println("2. x = " + x); } 23 args x

Exercise: "Parameter Mystery” public class ParameterMystery { public static void main(String[] args) { int x = 9; int y = 2; int z = 5; mystery(z, y, x); mystery(y, x, z); } public static void mystery(int x, int y, int z) { System.out.println(z + " and " + (y - x)); }

(Offline) Practice: Loan Calculator  Design a loan program to compute the monthly amortization table of a fixed-rate loan 35 Loan.java

Rules of Fixed-Rate Loan  Assume N periods (e.g., 120 months)  For each period, borrower pays interest on the remaining owed (principal) at the fixed rate  At the end of N’s period, the remaining principal goes to 0 36

Fixed-Rate Loan Calculation Alg.  Alg. focuses on owed (principal) Owed at initiation: Owed after 1 month: Owed after 2 month: Owed after 3 month:

Mapping Loop Variable Owed after N month: apply Owed after N month: Payoff loan after N month =>

Flexible (Variable) Number of Method Arguments  In the most general case, Java allows flexible (variable) method signature, e.g.,  We will not learn how to define such methods, but will use some: printf() and format() (avoided by Java before 1.5 but available in C; typically preferred by many programmers) 39 public static type name ( param1, … param2 )

Formatting Text with printf System.out.printf(" format string ", parameters );  A format string can contain placeholders to insert parameters, e.g.,  %d integer  %f real number  %s string these placeholders are used instead of + concatenation m Example: int x = 3; int y = -17; System.out.printf("x is %d and y is %d!\n", x, y); printf does not drop to the next line unless you write \n // x is 3 and y is -17!

printf Width  % W d integer, W characters wide, right-aligned  %- W d integer, W characters wide, left-aligned  % W f real number, W characters wide, right-aligned ... for (int i = 1; i <= 3; i++) { for (int j = 1; j <= 10; j++) { System.out.printf("%4d", (i * j)); } System.out.println(); // to end the line } Output:

printf Precision  %. D f real number, rounded to D digits after decimal  % W. D f real number, W chars wide, D digits after decimal  %- W. D f real number, W wide (left-align), D after decimal double gpa = ; System.out.printf("your GPA is %.1f\n", gpa); System.out.printf("more precisely: %8.3f\n", gpa); Output: your GPA is 3.3 more precisely:

printf Formatting  Many more formatting control options supported by printf, e.g., using the comma (,) to display numbers with thousands separator 43 System.out.printf("%,d\n", 58625); System.out.printf("%,.2f\n", ); Output: 58,625 12,345,678.90

System.out.printf and String.format  String.format has the same formatting capability as printf, except that printf outputs and String.format returns: 44 System.out.printf("%,.2f\n", ); String s = String.format("%,.2f\n", ); System.out.print( s ); Output: 12,345,678.90

Exercise: F2C  Revise F2C print 2 decimal digits.