CS100 Lecture 10 1

Announcements

• Assignment P2 is due on Thursday

• Assignment P3 is handed out today

• Prelim on Monday the 19th. Coming soooooooooon.

CS100 Lecture 10 2

Today’s Topics

• Review

• Arrays– Allocation/Deallocation– Subscripting– Use of arrays to store large collections of data– Several examples

CS100 Lecture 10 3

Review of iteration

• For loops– for(j = 0; j < NUM; j++) System.out.println(j);– What do you think for(;;) does, for example?

• Invariants

• break and continue statements

CS100 Lecture 10 4

Hypothetical Problem• Input: zero or more grades from 0 to 100 preceded

by the number of grades5 90 85 40 89 12

• Task: read grades and compile information about them– print them in reverse order– print them in increasing order– print a histogram

• So, need to read in all the values before processing, need as many variables as grades. . .

• How to do this using the Java we know so far?

CS100 Lecture 10 5

Use arrays instead!

• An array is an ordered list of values.

• Each value is stored at a position in the array

• The number referring to the position is called an index

• In Java, array indices begin at 0

40

85

89

12

90

g

0

1

2

3

4

CS100 Lecture 10 6

Deconstruction of this array

• g[0] is 90

• g[1] is 85

• g[2] is 40

• g[3] is 89

• g[4] is 12

• g.length is the number of elements in the array, g.length is 5

• In “g[4]”, 4 is the index or subscript

CS100 Lecture 10 7

Another example

• h.length is the number of array elements in array h -- here, h.length is 323

-32

54

-101

82

1

h

0

1

2

3

4

. . .

93322

CS100 Lecture 10 8

Some notation

segment number of values in it empty when

g[h..i-1] i-h h = i

g[i..j] j+1-i j = i-1

g[j+1..k-1] k-(j+1) j = k

h i j k

g

CS100 Lecture 10 9

Declaring an array variable• int[] g; g

• float[] averages; averages

• Employee[] employees; employees

• Declaring a variable does not “allocate” or create the array of elements; it only declares a variable that can contain a reference to an array of elements.

• An array is much like a class in that an array variable contains a reference to an array.

null

null

null

CS100 Lecture 10 10

How to get the space?

• g = new int[5]; g

• Employee[] e = new Employee[206]; e

0 1 2 3 4

null null null null null ... null null

0 1 2 3 4 … 204 205

CS100 Lecture 10 11

More on declaration and accessing

• What happens in the statemente[3] = new Employee(“Millett”, 1999);

• Given allocated array e, we can reference e[0], e[1] …., but we can also use expressions as array indices: e[2*b], e[i], etc.

CS100 Lecture 10 12

Using array elements

• Suppose g is an array of integers

• Then, g[i] where i is in range can be used just as an integer variable– g[i] = 3; a = g[i]*2; c.setX(g[i]);– System.out.println(“value is ” + g[i]);

• Note that arrays are objects -- therefore have the same ‘call by value’ conventions

• Java does bounds checking -- throws exception if out of bounds

CS100 Lecture 10 13

Example program with arrays// Read in a list of integer grades, preceded by the

// number of grades, and print in them in reverse order

int n= Integer.parseInt(stdin.readLine()); // number of grades

int[ ] g= new int [n]; // g[0..n-1] are the grades

// Read in the grades

int i= 0;

while (i != g.length) {

g[i]= Integer.parseInt(stdin.readLine()); i= i+1; }

// Print grades in reverse order

int k= n;

while (k > 0) {

System.out.println(g[k - 1]); k = k-1; } }

CS100 Lecture 10 14

Rewrite previous with for loopsint n= Integer.parseInt(stdin.readLine()); // number of grades

int[ ] g= new int [n]; // g[0..n-1] are the grades

// Read in the grades

for(int i = 0; i < g.length; i++) {

g[i]= Integer.parseInt(stdin.readLine());}

// Print grades in reverse order

for (int k = n; k > 0; k--) {

System.out.println(g[k - 1]); }

CS100 Lecture 10 15

Histogram example -- explanationProgram scheme to print “histogram” of grades

// Read in a list of grades in the range 0..100, preceded

// by the number of grades, and

// print out how many times each grade appears

int n= in.readInt(); // number of grades

int[ ] f= new int [101]; // f[i] will be the no. of times grade i appears

// Initialize frequencies f[0..100] to 0.

// Read in the grades and make up array f.

// Print the grades and their frequencies (print only the grades in 0..10

// that appeared at least once in the input)

}

CS100 Lecture 10 16

// Read in a list of grades in the range 0..100, preceded by the number

// of grades, and print out how many times each grade appears

int n= in.readInt(); // number of grades

int[ ] f= new int [101]; // f[i] will be the no. of times grade i appears

// Initialize frequencies f[k] to 0.

int k= 0;

while (k != f.length)

{f[k]= 0; k= k+1;}

CS100 Lecture 10 17

// Read in the grades and make up array f

int i= 0;

// Inv: i grades have been read in and

// each f[k], for 0<=k<=100, contains

// the no. of times grade k was read in

while (i != f.length) {

int grade= Integer.parseInt(stdin.readLine() );

f[grade]= f[grade] + 1;

i= i+1;

}

CS100 Lecture 10 18

// Print the grades and their frequency (print only the

// grades in 0..100 that appeared at least once in the

// input)

int i= 0;

// Inv: the grades 0..i-1 that occurred at least

// once have been printed

while (i != f.length) {

if (f[i] != 0)

System.out.println(“grade: ” + i + “ frequency: ” + f[i]);

i= i+1;

}

CS100 Lecture 10 19

Palindromes• A palindrome is a word that reads the same backwards and forwards.

– The empty string

– A

– AA

– ABBA

– NOON

• The following are palindromes if blanks and punctuation are ignored

– able was I ere I saw elba

– a man a plan a canal panama

CS100 Lecture 10 20

Is a given array a palindrome? // Return the value of the statement “array b is a palindrome”

static public bool isPalindrome(char[ ] b) {

int i= 0; int j= b.length;

// Invariant: b is a palindrome iff b[i..j-1] is. In other

// words, b[j..length-1] is the reverse of b[0..i-1]

while ( j - i >1) {

j= j-1;

if (b[i] != b[j])

return false;

i= i+1; }

// {b[i..j-1] has 0 or 1 elements, so it’s a palindrome}

return true; }

CS100 Lecture 10 21

Odd Syntax Thing

• int[] grades; is equivalent to int grades[];

• Be careful however:int a, b, c;int[] x, y, z;int r[], s, t[];

• Best to associate array brackets with the type not the variable name

CS100 Lecture 10 22

Initializer Lists

• You can initialize the elements of an array just as you can initialize a variable

• Exampleint[] grades = {88, 92, 65, 77, 33};

• grades is now an array of 5 integers with values as shown

• Such an initializer list can only be used when the array is first declared

Note curlybraces

CS100 Lecture 10 23

Arrays of objects• Remember: Java allows arrays of more than ints

• Suppose you would like an array of strings:String[] words = new String[25];

• We could declare an array of employees, as we saw previously:Employee[] workers = new Employee[25];workers[3] = new Employee(“Millett”, 1999);workers[3].setSalary(50000);

CS100 Lecture 10 24

Arrays as parameters• Similar to when passing objects as parameters

• Declaration: z(int[] list)Call: z(a) where a is declared int[] z can change elements of the array, but not array reference itself

• Can also pass elements of the array as parametersDeclaration: q(int x)Call: q(a[3])