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Chapter 6 Strings Arrays Outline 6.1 Language Support 6.1.1 The String Class 6.1.2 The Masquerade and the + Operator 6.2 String Handling 6.2.1 Overview of String Methods 6.2.2 Accessors 6.2.3 Transformers 6.2.4 Comparators 6.2.5 Numeric Strings

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slide1

Chapter 6

Strings

Arrays

slide2

Outline

6.1 Language Support 6.1.1 The String Class 6.1.2 The Masquerade and the + Operator

6.2 String Handling 6.2.1 Overview of String Methods 6.2.2 Accessors 6.2.3 Transformers 6.2.4 Comparators 6.2.5 Numeric Strings

6.3 Applications 6.3.1 Character Frequency 6.3.2 Character Substitution 6.3.3 Fixed-Size Codes 6.3.4 Variable-Size Codes

6.4 Advanced String Handling 6.4.1 The StringBuffer Class

slide3

Outline

ArraysIntroduction 1-D Arrays 2-D Arrays

slide4

6.1 Language Support

Write a fragment that creates a String that encapsulates “York”

slide5

Are Strings Special?

Write a fragment that creates a String that encapsulates “York”

String s = new String("York");

Creating strings is not different from creating any other object.

slide6

The Masquerade

Can create a String without using new:

String s = "York";

The compiler replaces the above with:

String s = new String("York");

slide7

The + Operator

Can concatenate two strings using a "fake" operator:

String s = "York" + "Lane";

The compiler replaces the above with:

String s = new String("YorkLane");

These (convenience) shortcuts make strings "look like" mutable primitive types.

slide8

More on the + Operator

How does the compiler handle x + y ?

  • If x and y are both numeric, this is the addition operator.
  • If eitherx or y is a string, this is the concatenation operator. In this case, the other operand is coerced to a string.
  • Otherwise, there is a syntax error in this expression.
slide9

6.2 String Handling

Given a String, invoke:

  • Accessors
  • Transformers
  • Comparators
  • Numeric/String Converters

Note the absence of mutators

slide10

String Methods

  • length()
  • charAt(int)
  • substring(int,int) (int)
  • indexOf(String) (String,int)
  • toString() and equals()
  • compareTo()
  • toUpperCase() and toLowerCase()
slide11

Numeric Strings

The Wrapper Classes

String s = "1020";int number = Integer.parseInt(s);

The other way (from number to string) is best handled thru the + operator (see next)

slide12

6.3 Applications

Read the four applications in sections 6.3.1-4 and note, in particular, how indexOf and substring can be used to perform pattern lookup/substitution.

Here, we will discuss three different applications but they employ the same techniques:

slide13

Applications:

  • SpaceCounterPrompt for, and read, a string from the user. Output the number of spaces in it.
  • FileSpaceCounterSimilar to the previous one but it gets its input from the file. The user is prompted to enter the filename.
  • DigitSpellerRead a string from the user and spell out the names of the digits in it, e.g. input "this6is a5 test4" leads to output: "SIX", "FIVE", and "FOUR".
slide14

6.4 Advanced String Handling

  • Efficiency calls for immutability
  • A separate class, StringBuffer, was added to handle mutation.
  • The new class has three mutators:StringBuffer append(anything)StringBuffer insert(int, anything)StringBuffer delete(int, int)
slide15

The + Operator & StringBuffer

Given two strings x and y, the compiler replaces:

String s = x + y;

with:

String s = new StringBuffer().append(x).append(y).toString();

arrays
An array is an ordered list of values

79 87 94 82 67 98 87 81 74 91

Arrays

Each value has a numeric index

The entire array

has a single name

0 1 2 3 4 5 6 7 8 9

scores

An array of size N is indexed from zero to N-1

This array holds 10 values that are indexed from 0 to 9

arrays17
A particular value in an array is referenced using the array name followed by the index in brackets

For example, the expression

scores[2]

refers to the value94(the 3rd value in the array)

That expression represents a place to store a single integer and can be used wherever an integer variable can be used

Arrays
arrays18
For example, an array element can be assigned a value, printed, or used in a calculation:

scores[2] = 89;

scores[first] = scores[first] + 2;

mean = (scores[0] + scores[1])/2;

System.out.println("Top = " + scores[5]);

Arrays
arrays19
The values held in an array are called array elements

An array stores multiple values of the same type (the element type)

The element type can be a primitive type or an object reference

Therefore, we can create an array of integers, or an array of characters, or an array ofStringobjects, etc.

In Java, the array itself is an object

Therefore the name of the array is a object reference variable, and the array itself must be instantiated

Arrays
declaring arrays
Thescoresarray could be declared as follows:

int[] scores = new int[10];

The type of the variablescoresisint[](an array of integers)

Note that the type of the array does not specify its size, but each object of that type has a specific size

The reference variablescoresis set to a new array object that can hold 10 integers

Declaring Arrays
declaring arrays21
Some examples of array declarations:

float[] prices = new float[500];

boolean[] flags;

flags = new boolean[20];

char[] codes = new char[1750];

Declaring Arrays
bounds checking
Once an array is created, it has a fixed size

An index used in an array reference must specify a valid element

That is, the index value must be in bounds (0 to N-1)

Each array object has a public constant calledlengththat stores the size of the array

It is referenced using the array name:

scores.length

Bounds Checking
alternate array syntax
The brackets of the array type can be associated with the element type or with the name of the array

Therefore the following declarations are equivalent:

float[] prices;

float prices[];

The first format generally is more readable

Alternate Array Syntax
initializer lists
An initializer list can be used to instantiate and initialize an array in one step

The values are delimited by braces and separated by commas

Examples:

int[] units = {147, 323, 89, 933, 540,

269, 97, 114, 298, 476};

char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};

Initializer Lists
initializer lists25
Note that when an initializer list is used:

thenewoperator is not used

no size value is specified

The size of the array is determined by the number of items in the initializer list

An initializer list can only be used only in the array declaration

Initializer Lists
arrays of objects
The elements of an array can be object references

The following declaration reserves space to store 25 references toStringobjects

String[] words = new String[25];

It does NOT create theStringobjects themselves

Each object stored in an array must be instantiated separately

Arrays of Objects
arrays of objects27
Objects can have arrays as instance variables

Many useful structures can be created with arrays and objects

The programmer must determine carefully an organization of data and objects that makes sense for the situation

Arrays of Objects
two dimensional arrays
A one-dimensional array stores a list of elements

A two-dimensional array can be thought of as a table of elements, with rows and columns

Two-Dimensional Arrays

one

dimension

two

dimensions

two dimensional arrays29
To be precise, a two-dimensional array in Java is an array of arrays

A two-dimensional array is declared by specifying the size of each dimension separately:

int[][] scores = new int[12][50];

A two-dimensional array element is referenced using two index values

value = scores[3][6]

The array stored in one row or column can be specified using one index

Two-Dimensional Arrays
multidimensional arrays
An array can have many dimensions

If it has more than one dimension, it is called a multidimensional array

Each dimension subdivides the previous one into the specified number of elements

Each array dimension has its ownlengthconstant

Because each dimension is an array of array references, the arrays within one dimension can be of different lengths

these are sometimes called ragged arrays

Multidimensional Arrays
command line arguments
The signature of themainmethod indicates that it takes an array ofStringobjects as a parameter

These values come from command-line arguments that are provided when the interpreter is invoked

For example, the following invocation of the interpreter passes an array of threeStringobjects intomain:

C:\> java StateEval pennsylvania texas arizona

These strings are stored at indexes 0-2 of the parameter

Command-Line Arguments