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Lecture 7 - CS159 –Arrays+Scanner

Due this week:

Review your Exam 1 grade�Fri 11pm Lab6�Sun HW 3

Tue Reading Week 6

Fri Quiz 6 / Lab 7

Sun HW 4

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An Array in memory

An array is a contiguous block of memory that can be used to hold a fixed number of homogeneous elements.

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Arrays in Java

Declaration using the [] Modifier:

Syntax: base_type[] array_name;
Example: int[] income;

Accessing Elements using the [] Operator:

Syntax: array_name[index]
Example: income[5] = 250000;
Example: total = income[3];

A Reminder About Indexes:

0-based (i.e., the "first" element has an index of 0)

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When elements are known up front

One can declare an array and assign all of the elements to it using an "array literal" (formally, an array initializer) as follows:
char[] course = {'C','S','1','3','9'};
int[] age = {18, 21, 65};

One can then assign elements or access elements using the [] operator.

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Elements unknown up front

One can declare an array and assign default values to all of the elements to it as follows:

char[] course;
int[] age;
course = new char[5]; \\ Each element is initialized to '\0'
age = new int[3]; \\ Each element is initialized to 0

One can then assign elements or access elements using the [] operator.

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Elements of Arrays as parameters

Actual Parameters:

Syntax: array_name[index]
Example: x = payment(income[0]);

Formal Parameters:

Can't be array elements

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Arrays as parameters

Formal Parameters:

Syntax: (base_type[] array_name)
Example: payment(int[] income)
Example: main(String[] args)

Actual Parameters:

Syntax: (array_name)
Example: y = payment(taxableIncome);

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Returning an Array

Syntax:

base_type[] method_name([formal_parameter]...)

Example:

public int[] schedule(){...}

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Attributes and methods

Attributes:

length contains the number of elements (and is initialized when memory is allocated)

Methods:

clone() makes a (shallow) copy

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Java vs Python

The Functionality:

In Python, a List is used to provide this kind of functionality

"Literals":

In Python, List literals are written using [ and ] (not { and })

Length:

In Python, the length of a List can be obtained using the len()function

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Java vs Python more

Special Indexes:

In Python, the "index" -1 can be used to work with the last element of the List
In Python, a range of indexes can be specified by including a : in the "index" (e.g., data[2:5])

Presence:

In Python, the in operator can be used to determine if a List contains a particular element

Size:

In Python, the number of elements in a List can be changed using the append(), insert(), extend(), and remove() methods

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PI question Runetone

https://canvas.jmu.edu/courses/2071161/assignments/19819205

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Streams

What Are They:

A sequence of bytes with indeterminate length that ends with an END-OF-STREAM character (Ctrl-D in Unix and Ctrl-Z in Windows)

Why They Are Used:

The way you work with a stream is independent of its channel (e.g., keyboard, console, network connection, storage devices)

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Standard Streams

What Are They?

Streams that are associated with (what used to be) the standard channels for input and output

What Does This Mean Now?

The terminal window (sometimes called the console) has standard streams associated with it
Most IDEs have standard streams associated with one or more windows/panes/views

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When are they created

When Are They Created?

Before main() is called

What Are They Named?

System.in
System.out
System.err

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Encapsulation of Std Streams

Standard Output/Error:

System.out and System.err are PrintStream

Standard Input:

System.in is an InputStream

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PrintStream

Unformatted Output:

PrintStream.print(java.lang.String)prints the String(i.e., sends it to the stream
PrintStream.println(java.lang.String)prints the Stringfollowed by the line separator String (which is "\n" in Linux, "\r" in OSX and "\r\n" in MSW)��An Important Point:� Text sent to an output stream may be buffered� To force text through the buffer use PrintStream.flush()� This is particularly important to be aware of when
debugging

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Formatting PrintStream

Format Strings:

Encapsulated in the Formatter:�Two Approaches to Using Format Strings:
Use the String.format(java.lang.String, java.lang.Object...)method to create a formatted Stringand then call print() or println()
Use the PrintStream.printf(java.lang.String, java.lang.Object...)convenience method to perform both steps in one call

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Format String

Components:

String literals
Format specifiers

Format Specifiers:

Syntax: %[flags][width][.precision]conversion

Conversions:

'b' for a boolean
'c' for a char
'd' for an integer
'e' for a real number in scientific notation
'f' for a real number
's' for a String

Flags:

'-' for left-justified
'+' to always include the sign
' ' to use a space for the sign of a positive number
'0' to pad with zeros
',' to use grouping separators
'(' to put negative numbers in parentheses

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Example

import java.io.PrintStream;
public class PrintfExample
{
public static void main(String[] args)
{
int age, weight;
double monday, tuesday, wednesday;
age = 27;
System.out.printf("%d\n", age);
System.out.printf("%10d\n", age);
weight = 155;
System.out.printf("Prof. B. is %d years old\n", age);
System.out.printf("Age: %d\tWeight: %d\n", age, weight);
monday = 35.8;
tuesday = 0.0;
wednesday = -10.2;
System.out.println("\nWith No Flags, Width or Precision");
System.out.printf("%f\n", monday);
System.out.printf("%f\n", tuesday);
System.out.printf("%f\n", wednesday);
System.out.println("\nWith No Flags");
System.out.printf("%5.1f\n", monday);
System.out.printf("%5.1f\n", tuesday);
System.out.printf("%5.1f\n", wednesday);
System.out.println("\nWith + Flags");
System.out.printf("%+5.1f\n", monday);
System.out.printf("%+5.1f\n", tuesday);
System.out.printf("%+5.1f\n", wednesday);
}
}

import java.io.PrintStream;
public class PrintfExample
{
public static void main(String[] args)
{
int age, weight;
double monday, tuesday, wednesday;
age = 27;
System.out.printf("%d\n", age);
System.out.printf("%10d\n", age);
weight = 155;
System.out.printf("Prof. B. is %d years old\n", age);
System.out.printf("Age: %d\tWeight: %d\n", age, weight);
monday = 35.8;
tuesday = 0.0;
wednesday = -10.2;
System.out.println("\nWith No Flags, Width or Precision");
System.out.printf("%f\n", monday);
System.out.printf("%f\n", tuesday);
System.out.printf("%f\n", wednesday);
System.out.println("\nWith No Flags");
System.out.printf("%5.1f\n", monday);
System.out.printf("%5.1f\n", tuesday);
System.out.printf("%5.1f\n", wednesday);
System.out.println("\nWith + Flags");
System.out.printf("%+5.1f\n", monday);
System.out.printf("%+5.1f\n", tuesday);
System.out.printf("%+5.1f\n", wednesday);
}
}

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InputStream

Raw Input:

The read() methods can be used to read one or more bytes

An Observation:

We don't want to have to work with the binary representation of char, double, or int

A Better Approach:

"Wrap" the InputStream in a Scanner object and use its next___() methods

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Using Scanner object

Important Constructors:

Scanner(java.io.InputStream)

Some Relevant Methods:

Scanner.useDelimiter(java.lang.String) �Scanner.hasNextDouble(), Scanner.hasNextInt, Scanner.hasNextLine()�

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Scanner Example

import java.util.Scanner;
public class ScannerExample
{
public static void main(String[] args)
{
int age, weight;
double salary;
Scanner keyboard;
String line, name;
String[] parts;
keyboard = new Scanner(System.in);
System.out.print("Name: ");
name = keyboard.nextLine();
System.out.print("Salary: ");
salary = keyboard.nextDouble();
keyboard.nextLine(); // Consume the end-of-line character
System.out.print("Phone number (###-###-####): ");
line = keyboard.nextLine();
System.out.print("Age Weight (# #): ");
age = keyboard.nextInt();
weight = keyboard.nextInt();
keyboard.close();
}
}

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Issue when using Scanner

import java.util.Scanner;
public class ScannerInconvenienceExample
{
public static void main(String[] args)
{
int age;
Scanner keyboard;
String name;
keyboard = new Scanner(System.in);
// If you run this program you won't be able to enter your
// name because the return you enter after your age will
// be considered the "next line"
System.out.print("Enter your age: ");
age = keyboard.nextInt();
System.out.print("Enter your name: ");
name = keyboard.nextLine();
keyboard.close();
}
}

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Scanner issue/ “bug”

The Issue:

The methods that read "up to" a delimiter do not consume newline characters

A "Work Around":

Invoke nextLine() immediately after invoking one of these methods

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New approach

A Recent Addition:

A Console object can be retrieved using the System.console() method and it has both readLine() and printf() methods

Problems with this Approach:

The console() method returns null in IDEs
The streams can't be re-directed within a program making it inappropriate for many testing environments

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Acknowledgements

Parts of this activity are based on materials developed by David Bernstein.

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