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CS1101: Programming Methodology http://www.comp.nus.edu.sg/~cs1101x/

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We will cover the other sections some other time. Arrays. In computer science, we deal with very large ... digit2, digit1, step1, step2, step3; char checkCode; ... – PowerPoint PPT presentation

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Title: CS1101: Programming Methodology http://www.comp.nus.edu.sg/~cs1101x/


1
CS1101 Programming Methodology
http//www.comp.nus.edu.sg/cs1101x/
Aaron Tan
2
This is Week 6
  • Last week
  • Week 4s Exercise 5 (Prime number)
  • A mini programming test!
  • Chapter 5 Using Pre-Built Methods
  • Other classes Random, DecimalFormat
  • This week
  • Chapter 10 Arrays
  • Only sections 10.1 to 10.6
  • We will cover the other sections some other time.

3
Arrays
  • In computer science, we deal with very large
    amount of data.
  • Eg 3000 integers, 365 days, 1 million real
    numbers.
  • Do you want to create so many variables?
  • If the data are homogeneous (of the same type),
    we can group them into a single collection.
  • Array is an indexed collection of homogeneous
    data.
  • Lets get to Chapter 10 now!

4
Array Declaration Syntax
  • Array declaration syntax
  • ltelement-typegt ltarray-variablegt
  • Example double values
  • Alternative syntax
  • ltelement-typegt ltarray-variablegt
  • Example double values
  • I prefer the first one, its more readable and
    meaningful. The second form is more commonly used
    by C/C programmers.

5
Classic Array Problems
  • Sum the values in an array.
  • Find the maximum (or minimum) value in an array.
  • Search for a value in an array.
  • Sort the values in an array.

6
Loading an Array
  • Before we solve a problem involving array, we
    need to first load values into the array!
  • If you know the values before-hand, use array
    element initialization
  • Eg int numbers 3, 7, -12, 8, 7
  • Slides 12 and 14 of Chapter 10
  • If not, you need to read the values from the user
  • Use a loop to read in the values
  • Slides 6-7, and 15-16 of Chapter 10
  • We will learn how to read data from a file some
    other time.

7
Exercise 1 Summing an Array
  • Write a program SumArray.java to compute the sum
    of all the values in an array containing double
    values. Display the sum in 3 decimal places.
  • Lets do it into 2 phases load the array with
    values first, then compute the sum. (Instead of
    accumulating the sum as we load the array.)

Size of array 10 Enter 10 values 5.1 16 3.2 1.8
-4 12.3 8 3.3 -2 9.1 The sum is 52.800
  • Download SumArray.java from course website,
    Resources, Lectures page.

8
Exercise 2 Finding maximum value
  • Write a program FindMax.java to find the largest
    value in an integer array. (Assume there is at
    least one element in the array.)

Size of array 5 Enter 5 values 10 -20 43 79 8
The largest value is 79
  • Take home exercise What if you want to report
    the index of the largest value, instead of the
    value itself? (This problem is not well-defined!
    Why?)

Size of array 5 Enter 5 values 10 -20 43 79 8
The largest value is at index 3
9
Common Mistake Index Out of Range
  • Beware of ArrayIndexOutOfBoundsException.

public static void main(String args)
int numbers new int10 . . . for
(int i1 iltnumbers.length i)
System.out.println(numbersi)
10
Modular Programming (1/5)
  • As our problems get more complex, the main()
    method might get too long.
  • It is advisable to split the problem into smaller
    sub-problems, and to write appropriate methods
    for the sub-problems.
  • In general a problem is solved in 3 steps input
    ? computation ? output.
  • It is customary to write a separate method to
    perform the computation step. (If the computation
    is complex, it should be split further into
    smaller steps and each step performed by a
    method.)

11
Modular Programming (2/5)
  • Download CheckNRIC.java program which we did
    before. Heres the partial code

public static void main(String args)
Scanner stdIn new Scanner(System.in)
System.out.print("Enter 7-digit NRIC number ")
int number stdIn.nextInt() int digit7,
digit6, digit5, digit4, digit3, digit2,
digit1, step1, step2, step3 char checkCode
// computation of check code - code omitted
... System.out.println("Check code "
checkCode)
12
Modular Programming (3/5)
  • Modularizing CheckNRIC.java

public static void main(String args)
Scanner stdIn new Scanner(System.in)
System.out.print("Enter 7-digit NRIC number ")
int number stdIn.nextInt() char
checkCode // computation of check code
checkCode generateCheckCode(number)
System.out.println("Check code " checkCode)

Pass number into the method generateCheckCode().
Delegate the job to the method generateCheckCode()
.
What are you expecting generateCheckCode() to
return? A character.
13
Modular Programming (4/5)
The method returns a character.
  • How does generateCheckCode() method look like?

public static char generateCheckCode(int num)
// Extract digits int digit7
num10 num / 10 int digit6 ... ...
char code ... ... return code
The method expects an integer argument.
The return statement passes the character to the
caller.
  • Download NewCheckNRIC.java and compare it with
    CheckNRIC.java.

14
Modular Programming (5/5)
  • Lets see how we can modularize our programs
    for the previous two exercises.
  • I will show you NewSumArray.java and
    NewFindMax.java.

15
Exercise 3 Coin Change
  • Download the file CoinChange.java from the course
    website, Resources, Lectures.
  • Rewrite it using an array of coin denominations
    (int coins). Name your program
    NewCoinChange.java.
  • Modularize your program by writing a method
    computeCoins().
  • What is its return type?
  • Does it have any argument? If so, what is the
    type of its argument?

16
Method main() (1/2)
  • Now that we have learnt array, lets check out
    the main() method.
  • Usual signature for main() method public static
    void main(String args)
  • args is an array of String objects
  • Consider this

public class Demo public static void
main(String args) for (int i0
iltargs.length i) System.out.println("ar
gs" i " " argsi) // end main
end Demo
17
Method main() (2/2)
  • This allows user to specify command line
    arguments when executing the program. java Demo
    10 ABC-D hello "Ice Cream"
  • Output

args0 10 args1 ABC-D args2
hello args3 Ice Cream
18
Sorting and Searching
  • I will be covering more topics in every lecture
    from now on to make up for the lost lecture on 27
    October (Deepavali).
  • Sorting
  • Searching
  • The above two topics are not included in the
    mid-term test.

19
Sorting
  • Classic computer science problem
  • Sort an array
  • Three basic (but slow) sorting algorithms
  • Selection sort
  • Bubblesort
  • Insertion sort
  • Other faster sorting algorithms (covered in
    CS1102 and other advanced modules)
  • Mergesort
  • Quicksort
  • Heapsort, etc.

20
Selection Sort (1/2)
  1. Find the smallest element in the list.
  1. Exchange the element in the first position and
    the smallest element. Now the smallest element is
    in the first position.
  1. Repeat Step 1 and 2 with the list having one less
    element (i.e., the smallest element is discarded
    from further processing).

This is the result of one pass.
21
Selection Sort (2/2)
Result AFTER one pass is completed.
2
3
7
8
22
Bubble Sort (1/2)
  • Algorithm

Assume array is arr for (int i arr.length 1
i gt 0 i--) for (int j 0 j lt i j)
if (arrj gt arrj1 swap
arrj with arrj1
  • Can you write the code?

23
Bubble Sort (2/2)
Showing the first pass
ok
The largest value 90 is at the end of the list.
24
Searching
  • Another classic computer science problem
  • Search for a value in a list of items
  • Two algorithms
  • Sequential search (also called linear search)
  • Binary search (applicable for sorted array)
    much faster

25
Announcement/Reminder (1/2)
  • Lab 2
  • Release 16 September (Tuesday), 2359hr.
  • Deadline 1 October (Wednesday), 2359hr.
  • Identical codes
  • Please do not share codes for your lab
    assignments!

26
Announcement/Reminder (2/2)
  • Consultation
  • 24 September (Wednesday), 10am 12nn.
  • I will be in PL3.
  • Mid-term test
  • 4 October, Saturday, 12noon, LT15 (for CS1101X
    students)
  • Refer to course website for more info
    http//www.comp.nus.edu.sg/cs1101x/3_ca/termtests
    .html

27
This is Week 6
  • Next week?
  • Recess! (Hooray!)
  • The week after next?
  • Chapter 6 Object-Oriented Programming (finally!)
  • Mid-term test (argh!)

28
End of file
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