CS100A, Fall 1997, Lecture 9 1

CS100A, Fall 1997Lecture 9, Tuesday, 30 September

Input/Output & Program Schema

System.in,

class Text,

Some basic data processing,

&

Rules of thumb for developing programs.

Schema notes and rules of thumb based on CS100 notes by Tim Teitelbaum, Fall 1996.

CS100A, Fall 1997, Lecture 9 2

Input/OutputTransmitting information between the

computer and the outside world.

MicroprocessorMemory

a

b

17

b = in.readInt();

1734

a = 2*b;

34

System.out.println(a);

CS100A, Fall 1997, Lecture 9 3

Basic Input Classes and Operations

A simple program to read two numbers and print their sum (beginning).

import java.io.*;

public static void main(String args[])

{int a, b;

// create System.in window & initialize

// Text variable in to read from it.

SystemInput sysIn = new SystemInput();

Text in = new Text(System.in);

. . .

Notes: Class SystemInput creates a small text input window. Once initialized, variable sysIn may be ignored.

Class Text provides simple input methods for basic data types.

CS100A, Fall 1997, Lecture 9 4

A simple program to read two numbers and print their sum (continued).

// Read two numbers and print their sum

System.out.print

("Please enter an integer: ");

System.out.flush( );

a = in.readInt( );

System.out.print

("Please enter another integer: ");

System.out.flush( );

b = in.readInt( );

System.out.println(a + " + " + b +

" = " + (a+b));

}

CS100A, Fall 1997, Lecture 9 5

System.in and Class Text

The standard Java variable System.in represents the keyboard, but is very limited. The only available methods read either a single character or an entire line.

Class Text is included in the CUCS Java stationery. It extends input classes like System.in to allow simple input of variables of type int, double, char, and String.

The initialization

Text in = new Text(System.in);

creates a variable, in, of class Text and connects it to System.in.

CS100A, Fall 1997, Lecture 9 6

Methods in Class Text

Most methods in class Text skip over any “whitespace” (blanks, tabs, and newlines), then read the next non-blank chunk of input and return it as a value of the specified type. Available methods:

readInt( ) — read int value

readDouble( ) — read double (floating pt)

readString( ) — read non-blank string

An additional method readLine( ) discards the rest of the input on the current line and returns the entire next line as a single string.

See the comments at the beginning of Text.java for more detailed documentation.

CS100A, Fall 1997, Lecture 9 7

A Data Processing Problem

• Input data: Zero or more exam grades in the range 0 to 100, followed by a –1.

• Sample input data: 90 85 93 40 89 -1

• Empty input sequence: –1

• General task: Read grades and print some information about them.

• Possible tasks:

– Print the grades

– Print the number of grades

– Print the average grade

– Print the highest grade

– Print the grades sorted descending

– Combinations of the above

CS100A, Fall 1997, Lecture 9 8

Example: Print the Grades

public static void main(String args[])

{SystemInput sysIn = new SystemInput();

Text in = new Text(System.in);

int grade; // current grade

// Read grades until -1 and print them

grade = in.readInt( );

while (grade != -1) {

System.out.println(grade);

grade = in.readInt( );

}

}

CS100A, Fall 1997, Lecture 9 9

Example: Print the Number of Grades

public static void main(String args[])

{SystemInput sysIn = new SystemInput();

Text in = new Text(System.in);

int grade; // current grade

int nGrades; // # grades read so far

// Read grades until -1 and print how many

grade = in.readInt( );

nGrades = 0;

while (grade != -1) {

nGrades = nGrades + 1;

grade = in.readInt( );

}

System.out.println(“# grades = ” +

nGrades);

}

CS100A, Fall 1997, Lecture 9 10

Program Schema

A program scheme is a pattern of code that has general utility. Learn it, then use it.

Example:

public static void main(String args[])

{(SystemInput and Text deleted)

// “Process” list of input values up to,

// but not including, a stopping value

int variable;

declarations;

variable = in.readInt( );

while (variable != stoppingValue) {

variable = in.readInt( );

}

}

CS100A, Fall 1997, Lecture 9 11

Placeholders in the Program Schema

• variable, declarations, stoppingValue, , , and are placeholders.

• variable contains the most recently read input value. It should be replaced with a name meaningful for the given problem.

• stoppingValue is the value that signals the end of the input.

• is the initialization: statements that initialize any necessary variables before any input value is “processed”.

• is the “processing” step for each input value (but not for stoppingValue).

• is the finalization: any necessary operations after all input values have been processed.

CS100A, Fall 1997, Lecture 9 12

Program scheme instantiated for grading

public static void main(String args[])

{(SystemInput and Text deleted)

int grade; // current grade

declarations;

grade = in.readInt( );

while (grade != -1) {

grade = in.readInt( );

}

}

CS100A, Fall 1997, Lecture 9 13

Example: Print the Number of Grades

public static void main(String args[])

{(SystemInput and Text omitted)

int grade; // current grade

:

grade = in.readInt( );

while (grade != -1) {

:

grade = in.readInt( );

}

:

}

CS100A, Fall 1997, Lecture 9 14

Example: Print the Average Grade

public static void main(String args[])

{(SystemInput and Text omitted)

int grade; // current grade

:

grade = in.readInt( );

while (grade != -1) {

:

grade = in.readInt( );

}

:

}

CS100A, Fall 1997, Lecture 9 15

Example: Print the Average Grade

(Boundary Conditions)

public static void main(String args[])

{(SystemInput and Text omitted)

int grade; // current grade

:

grade = in.readInt( );

while (grade != -1) {

:

grade = in.readInt( );

}

:

}

CS100A, Fall 1997, Lecture 9 16

Programming Rules of Thumb

• Learn program schemes (patterns) of general utility.

• If you know a relevant program scheme for the problem at hand, use it.

• Work the problem at hand to gain insight into its solution. Ask yourself “what am I doing?”

• Declare a variable for each piece of information (state) you keep track of while working the problem by hand. Write a comment that precisely describes the contents of each variable.

• Remember to take care of boundary conditions.

• Check your program by hand tracing it on simple test data.