MSI 692: Special Topics
in Information Technology
Lecture 4: Strings & Arrays
Sanjay Goel
University at Albany, SUNY
Fall 2004
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Outline for the Class
Arrays
• Recap
• Strings
• Arrays
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Creation and Access
Copying & assigning arrays
Storage & Indexing
Multidimensional Array
• Examples
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Strings
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Objects and Classes
Standard Classes: String
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String class represents character strings
All string literals in Java programs are implemented as
instances of this class, e.g. “abc”
String class includes methods for
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Examining individual characters of the sequence
Comparing strings
Searching strings
Extracting substrings
Creating a copy of a string with upper or lower case
translation.
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Objects and Classes
String Methods
• String class has following methods
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boolean equals(Object anObject) Compares this string with another
int length() Gets length of string
char charAt(int index) Returns char at index pos. in string
int compareTo(String str) Returns an integer based on lexigographic order
int indexOf(in ch) Gets position of character in string (-1 if not present)
int indexOf(String str) Gets position of first letter of str in the string
String concat(String str) Concatenates two strings and returns
String toLowerCase() Converts to lower case
String toUpperCase() Converts to upper case
char[] toCharArray() Returns character array
static String valueof(type prim) Converts primitive to string.
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Objects and Classes
Standard Classes: Palindrome
static boolean isPalindrome(String s) {
int left = 0;
int right = s.length() – 1;
while (left < right) {
if (s.charAt(left) != s.charAt(right))
return false;
left++;
right--;
}
return true;
}
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Creating Classes
The equals Method
• Definition
– Returns true if the strings are the same and false if they differ.
– As soon as a character in one string is not equal to the corresponding
character in the second string, the comparison is terminated and the
condition returns to false.
• Syntax
• Example
– (strName.equals(strInput))
(strName.equals(“Bill”))
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Creating Classes
The equalsIgnoreCase Method
• Definition
– Is similar to the equals method but does not differentiate between
uppercase and lowercase.
• Syntax
• Example
– String strName = new String(“Joan”) ;
String strName2 = new String(“JOAN”) ;
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Creating Classes
The compareTo Method
• Definition
– Used to determine less than or greater than.
– Returns an integer with one of three possible values.
• Syntax
• Example
– String strWord = new String(“Hope”) ;
String strWord2 = new String(“Hopeless”) ;
//Compare the strings
if (strWord.compareTo(strWord2 < 0)
//Display a message - - What will it be?
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Arrays
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Arrays
Introduction
• When do we need arrays?
– When we have large data sets of the same type
– e.g. test scores of the students or prime numbers between 0 and 100.
– It is cumbersome to define a separate variable for each value.
– Allows you to define one single variable and just put multiple values in
• An array in Java is a container that holds a related group of values
of the same type.
– The elements in an array are ordered are numbered 0 through n-1
– where n is the number of individual elements in the array.
•
The position of an element in an array is called index of the
array
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Arrays
Storage in Memory
• Before we even start let us understand how memory is
organized and how data is stored in memory.
– Each location in memory has a unique address.
• Data:
0
1
2
3
4
5
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Arrays
Memory Pointers
• This means that the memory location where the array
points does not contain array data rather it contains a
reference or a pointer to the actual array data.
– When you declare an array you just create a pointer which
points to nothing.
– To find the address in memory of a specific element, the
computer can add the element’s index to the address of the
location of the first element.
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Arrays
Array Declaration and Memory Allocation
• This is a two step process:
• Array Declaration
– You declare array variables just like any other variable by specifying the
type followed by the name of the variable.
– variableType [] variableName;
– int [] myArray;
• Memory Allocation
– To actually create an array you have to use new.
• arrayName = new arrayType[length]
– length is an integer expression
– e.g. myArray = new int[10]
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Arrays
Array Length
• You can do both things in a single step
– arrayType arrayName = new arrayType[length]
– Once an array is created it has a fixed size.
– An array can be reassigned to point to a different array that
has a different size
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Arrays
Array Indexing
• Array Indexing
– Once you declare an array. Each element can be accessed by
using an index variable of integer type. The indexing is from 0
to n-1 for an array of length n.
– arrayName[expr]
• This expression is any integer expression
– ith element of the array is represented as arrayName[i-1]
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Arrays
Assigning Arrays
int [] a = new int[10];
a = new int[20];
or
int [] a1 = {10, 20, 30}
int [] a2 = {1, 2, 3, 4, 5}
a1 = a2; // Now both a1 and a2 are pointing to same variable
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Arrays
Length of an Array
• The length of the array is stored in a variable called
length and can be accessed as:
arrayName.length
e.g. a.length;
for (int i = 0; i < a.length. i++) {
}
• If you index past the end of the array you get an
IndexOutOfBoundsException error.
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Arrays
Passing arrays to methods
•
Java passes parameters by value.
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Arrays are reference types i.e. they store the address of the array
location
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So when we pass arrays as arguments a copy of the reference
value (address) is passed to the method.
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Two Scenarios
1.
The contents of the array are modified
a.
The main program sees the changes to the array
 show using stack
2.
The array is assigned to another array in the method
a.
No change happens in the calling method
 show using stack
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Arrays
Copying Arrays
• You need to copy arrays element by element rather than just assigning one
array to another. By just assigning one array name to another you are just
copying pointers without copying the array.
• If you want to create a new array from an old array you need to create a new
array and assign all the values from the old array to the new array, e.g.
static int[] duplicate(int[] a) {
int[] theCopy = new int[a.length];
for(int I = 0; I < a.length; I++) {
theCopy[I] = a[I];
}
return theCopy;
}
a1 = duplicate(a2)
• Cloning: For one-dim arrays java provides a cloning mechanism
– i.e. a1 = (int[])a2.clone(); // built-in array copy
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Arrays
Types and Arrays
• Similar to integer arrays you can have arrays in other primitive
types and also classes.
• Primitive Types
– double [] d is an array of doubles
– char [] c is an array of characters
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Non-Primitive Types
– String[] args is an array of Strings
– Point[] points is an array of points
• Show the example of sieve of eratosthenes
• Show the example of character count
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Arrays
Multidimensional Array
• Just like single dimensional arrays we have multi-dimensional
arrays
int[] a1;
Row
int[][] a2;
Matrix
int[][][] a3;
3D Grid
• Declaring 2D arrays
int[][] a2 = new int[expr1][expr2];
• Initializing 2D arrays
int[][] a = {{1,2},{3,4},{5,6}};
// 3x2
int[][] b = {{1,2,3}, {4,5,6}};
// 2x3
int[][] c = {{1,2,3,4,5,6}};
// 1x3
int[][] ragged = {{1,2}, {3,4,5}, {6}} // 3 rows each with different # of elements
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Arrays
Examples
•
Sum of an array two different ways manually without using
loops
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Minimum and maximum of an array
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Sieve of Eratosthenes
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Example of Character Count
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Sorting of arrays (selection sort)
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Searching an ordered array
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Algorithm Complexity
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Sorting and Searching
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