A Bad Idea Monday, April 16, :23 AM

Transcription

1 Writing Functions Page 1 A Bad Idea 10:23 AM These notes are going to introduce how to write a function using an idea that may completely mislead the weak minded student. All students must be particularly careful to NOTICE all of the warnings that appear in the lesson.

2 Writing Functions Page 2 A Problem 10:25 AM You are asked to write a Java function which is given an integer array as its parameter and which will compute the sum of the elements of that array and then return that sum. The BAD IDEA is that we are going to solve ANOTHER PROBLEM INSTEAD The DANGER is that you will confuse the two problems: (1) (2) Original problem: WRITE a FUNCTION Alternate problem: write a user program

3 Writing Functions Page 3 Alternate Problem 10:28 AM Here is the ALTERNATE problem: Write a program which will input an integer array from the user in brackets notation and will compute the sum of its elements and print out that sum.

4 Writing Functions Page 4 Comparison 10:29 AM Here is a comparison of the two problems - highlighting how they are different and how they are the same: ORIGINAL ALTERNATE You are asked to write a Java function which is given an integer array as its parameter Write a program which will input an integer array from the user in brackets notation and which will compute the sum of the elements of that array and then return that sum. and will compute the sum of its elements and print out that sum.

5 Writing Functions Page 5 More Comparison 10:38 AM Here are the essential differences again ORIGINAL ALTERNATE function program Given parameter User input return output

6 Writing Functions Page 6 Alternate Solution 10:41 AM By now (in this course) you should have no problem writing the ALTERNATE program as described (using JavaBlocks) - it is just a for-in loop with a simple summation in it. static Console cin = new Console(); public static void main(string[] args) { int[] data = cin.nextintarray(); int total = 0; for(int item : data) { out.println("total="+total);

7 Writing Functions Page 7 Alternate Subdivided 10:45 AM Here is the same alternate program broken into parts: static Console cin = new Console(); public static void main(string[] args) { Program stuff int[] data = cin.nextintarray(); User input int total = 0; for(int item : data) { Actual computation work out.println("total="+total); User output Program stuff

8 Writing Functions Page 8 Transformation 10:49 AM We will TRANSFORM the user I/O program into a function through a series of moves First we remove the programming stuff: int[] data = cin.nextintarray(); int total = 0; for(int item : data) { out.println("total="+total);

9 Writing Functions Page 9 Use Function Heading 10:50 AM Next we replace the User input part with a function heading - also making up a name addemup for our function. int addemup(int[] data) { int[] data = cin.nextintarray(); int total = 0; for(int item : data) { out.println("total="+total);

10 Writing Functions Page 10 Use return 1:36 PM Next we replace the user output part by a return command: int addemup(int[] data) { int total = 0; for(int item : data) { return total; out.println("total="+total);

11 Writing Functions Page 11 Close the function 1:39 PM Finally we add the close brace defining the end of the function int addemup(int[] data) { int total = 0; for(int item : data) { return total;

12 Writing Functions Page 12 Packed Back Up 1:41 PM The same thing without all the blank lines showing the structure int addemup(int[] data) { int total = 0; for(int item : data) { return total;

13 Writing Functions Page 13 WARNING 1:42 PM WARNING A function does NOT imply Input/Output Here is a program that fails - and would not do input/output in any case. class BadIdea { static int addemup(int[] data) { int total = 0; for(int item : data) { Total = total + item return total public static void main(string[] args) { int[] data; int sum = addemup(data);

14 Writing Functions Page 14 Function and IO 1:47 PM Here is what the original PROGRAM did - but using a function to compute the total. class OkayIdea { static Console cin = new Console(); static int addemup(int[] data) { int total = 0; for(int item : data) { return total public static void main(string[] args) { int[] data = cin.nextintarray(); Note - all the function does is move the computational part out of the main routine into a separate place int sum = addemup(data); println("total="+sum); The programmer must still put in commands for the user input and output

15 Writing Functions Page 15 Why Bother 1:53 PM So why bother with a function at all - seems like unnecessary work. BECAUSE: 1 Breaking up a program into smaller parts is a GOOD THING (trust 100,000 programmers) 2 We can do a more complicated program without much more work.

16 Writing Functions Page 16 Program TWO 1:55 PM Now suppose we want a program to read in a bracketed array of integers and print its sum AND THEN to read in a second bracketed array of integers and print its average. The solution below uses the function that has been designed. class GoodIdea { static Console cin = new Console(); static int addemup(int[] data) { int total = 0; for(int item : data) { return total; public static void main(string[] args) { int data[] = cin.nextintarray(); println("total="+addemup(data); int more[] = cin.nextintarray(); println("avg="+addemup(more)/more.length);

17 Writing Functions Page 17 Program Three 1:59 PM Another way to do program Two class BetterIdea { static Console cin = new Console(); static int addemup(int[] data) { int total = 0; for(int item : data) { return total; static void work(int[] data,string legend,boolean avg) { int ans = addemup(data); if(avg) ans = ans/data.length; println(legend+ans); public static void main(string[] args) { int[] data = cin.nextintarray(); work(data,"total=",false); int[] more = cin.nextintarray(); work(more,"avg=",true); NOTE: Although a function does NOT AUTOMATICALLY do IO, it can do so if desired.

18 Writing Functions Page 18 Program Four 2:03 PM Yet Another way to do the program class GoodIdea { static Console cin = new Console(); static int addemup(int[] data) { int total = 0; for(int item : data) { return total static void allwork(string legend,boolean avg) { int[] data = cin.nextintarray(); // NOW LOCAL int ans = addemup(data); if(avg) ans = ans/data.length; println(legend+ans); public static void main(string[] args) { allwork("total=",false); allwork("avg=",true); // NO ARRAYS HERE Knowing how to divide up the program's work among a variety of functions is an ART - best acquired by lots of trial and error and practice.