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Warm up!!

Take attendance in 420. Warm up!!. public class MoreFunctionFun { public static void main(String[] args) { int a=2, b=3, c=4; c = fun(a, b); b = fun(a, c); System.out.println( "a=" +a); System.out.println( "b=" +b); System.out.println( "c=" +c); }

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Warm up!!

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  1. Take attendance in 420 Warm up!! publicclassMoreFunctionFun{publicstaticvoidmain(String[]args){inta=2,b=3,c=4; c=fun(a,b); b=fun(a,c); System.out.println("a="+a); System.out.println("b="+b); System.out.println("c="+c); } publicstaticintfun(intn1,intn2){ n1++; n2+=n1;returnn1*n2; }} a=2 b=63 c=18

  2. More Fun With Functions! AP Computer Science

  3. Lesson 1: functions are dumb • Functions only know what main “tells” them. • Main passes information to a function through parameters. • The function ONLY knows what the parameters tell it.

  4. The tax function doesn’t know TAX_RATE!!It ONLY knows amount Try this… publicstaticvoidmain(String[]args){finaldoubleTAX_RATE=0.06;KeyboardReaderreader=new KeyboardReader();doublesubtotal,total; subtotal=reader.readDouble("Enter Purchase Amount:"); total=subtotal+tax(subtotal);} publicstaticdoubletax(doubleamount){returnamount*TAX_RATE;}

  5. publicstaticvoidmain(String[]args){finaldoubleTAX_RATE=0.06;KeyboardReaderreader=new KeyboardReader();doublesubtotal,total; subtotal=reader.readDouble("Enter Purchase Amount:"); total=subtotal+tax(subtotal, TAX_RATE);} publicstaticdoubletax(doubleamount, double tr){returnamount*tr;}

  6. Lesson 2:functions are really dumb • Functions assume that they have been passed parameters in a certain order. • This order is defined by the function header. • Even though variable names may make it seem sensible, the function ONLY cares about the order.

  7. publicstaticvoidmain(String[]args){intleg1=3,leg2=4,hyp=5;if(pythag(hyp,leg1,leg2))System.out.println(“Right Triangle!!");elseSystem.out.println("NOT right triangle"); if(pythag(leg1,leg2,hyp))System.out.println(“Right Triangle!!");elseSystem.out.println("NOT right triangle");} publicstaticbooleanpythag(inta,intb,intc){return((a*a+b*b)==c*c);} Output:NOT right triangleRight Triangle!!

  8. Lesson 3:main doesn’t trust functions • When a function is called, it only gets a copy of the parameters’ values. • The function CANNOT change any of main’s variables. • The only change possible is if main assigns the function’s returned value into a variable.

  9. a b c 5 15 25 publicstaticvoid main(String[] args){int a=5, b=15, c=25; change(a, b); b=change(a, b); c = change(a, b);} publicstaticint change(int n1, int n2){ n1 *= 100; n1 += n2;return (n2-1);}

  10. publicstaticvoid main(String[] args){int a=5, b=15, c=25; change(a, b); b=change(a, b); c = change(a, b);} publicstaticint change(int n1, int n2){ n1 *= 100; n1 += n2;return (n2-1);} n2 n1 15 5 a b c 5 15 25

  11. publicstaticvoid main(String[] args){int a=5, b=15, c=25; change(a, b); b=change(a, b); c = change(a, b);} publicstaticint change(int n1, int n2){ n1 *= 100; n1 += n2;return (n2-1);} 14 n2 n1 5 15 500 515 a b c 5 15 25

  12. publicstaticvoid main(String[] args){int a=5, b=15, c=25; change(a, b); b=change(a, b); c = change(a, b);} publicstaticint change(int n1, int n2){ n1 *= 100; n1 += n2;return (n2-1);} n2 n1 5 15 a b c 5 15 25

  13. publicstaticvoid main(String[] args){int a=5, b=15, c=25; change(a, b); b=change(a, b); c = change(a, b);} publicstaticint change(int n1, int n2){ n1 *= 100; n1 += n2;return (n2-1);} 14 n2 n2 n1 15 15 500 5 515 a b c 5 15 25

  14. publicstaticvoid main(String[] args){int a=5, b=15, c=25; change(a, b); b=change(a, b); c = change(a, b);} publicstaticint change(int n1, int n2){ n1 *= 100; n1 += n2;return (n2-1);} n2 n1 14 5 a b c 5 14 25

  15. publicstaticvoid main(String[] args){int a=5, b=15, c=25; change(a, b); b=change(a, b); c = change(a, b);} publicstaticint change(int n1, int n2){ n1 *= 100; n1 += n2;return (n2-1);} 13 n2 n2 n1 500 5 514 14 14 a b c 5 14 25

  16. Function Overloading:Same name, different parameters • Every function has a namepublic static int area(int l, int w) • Your program can have 2 (or more) functions with the same name if they have a different number and/ortype of parameters.

  17. publicclass Overloading {publicstaticvoid main(String[] args) {System.out.print ("3x5 Rectangle has area: ");System.out.println(area(3,5));System.out.print ("Circle w/radius 5 has area: ");} publicstaticdouble area(int l, int w){return l*w; } }//end class Now write an “area” function for circles and call it from the main.

  18. publicclass Overloading {publicstaticvoid main(String[] args) {System.out.print ("3x5 Rectangle has area: ");System.out.println(area(3,5));System.out.print ("Circle w/radius 5 has area: ");System.out.println(area(5));} publicstaticdouble area(int l, int w){return l*w; }publicstaticdouble area(int r){returnMath.PI*r*r; }}//end class Now write an “area” function for squares and call it from the main. Output:3x5 Rectangle has area: 15.0Circle w/radius 5 has area: 78.53981633974

  19. The computer won’t be able to tell which one you are calling!! Don’t Confuse the Computer! • Overloaded functions MUST have a different number and/or type of parameters. • Having a different parameter name is not enough!! publicstaticdoublearea(intr){returnMath.PI*r*r;} publicstaticdoublearea(ints){returns*s;}

  20. Preconditions & Postconditions • When you write functions, they should be accompanied by special comments: • Preconditions: What needs to be true when the function is called. • What do the parameters represent? • What values are legal for the parameters? • Postconditions: What will be true when the function returns. • What type of value is returned? • What does it represent?

  21. Preconditions & Postconditions • These comments provide a guarantee: • If the preconditions are met, then the postconditions will be true. • If the preconditions are not met, then there is no guarantee the function will work.

  22. Example: /* * Preconditions:l & w are nonnegative integers * representing the length & width of a rectangle * Postconditions:the area of the rectangle is returned */ publicstaticdoublearea(intl,intw){returnl*w;}/* * Preconditions:r is a nonnegative integer * representing the radius of a circle * Postconditions:the area of the circle is returned */publicstaticdouble area(int r){returnMath.PI*r*r;}

  23. Pizza per Inch:Some Practice • Now write a program that helps someone make pizza purchasing decisions. • The program should: • Ask the user for the price and diameter of a large pizza • Ask the user for the price and diameter of a medium pizza • Ask the user for the price and diameter of a small pizza • Use a function to calculate the unit price (price per square inch) for each. • Inform the user of the best deal.

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