lecture 4

More on Operators and Assignments
PLUS: Reading Input from the Keyboard!!

If you can't write it down in English, you can't code it.
                     -- Peter Halpern

If you lie to the computer, it will get you.
                    -- Peter Farrar

One person's constant is another person's variable.
                   -- Alan Perlis

But First... More on Operators

Operator precedence - highest to lowest

Parentheses

* / % <--- performed in order from left to right

+ - <--- performed in order from left to right

Note: Recall that + is an operator that adds 2 numbers, but it also is the string concatenation operator when one or both operands are Strings.

Examples:
int result = 2 * ((16 - 4) / 2); // result is set to 12
String s = 4 + "6" + 2; // s is set to "462"
result = 13 % 2 - 1; // result is set to 0
result = 13 % 2 * 4; // result is set to 4

Increment and Decrement Operators

The increment operator ++ adds 1 to a number

result ++; // The value stored in result is increased by 1

The decrement operator -- subtracts 1 from a number

result --; // value stored in result is decreased by 1

These operators can come before or after the variable/value:

++count;

--count;

Example:
int count = 1;
count ++;
System.out.println(count);
-- count;
System.out.println(count);

Question: What is the output?

Example: Try out the following code:
int count = 1;
int n = 2 + count++;

And then try:
int count = 1;
int n = 2 + ++count;

Assignment Operators

Combine assignment with an operation

Ex:
count += 5;
       is equivalent to
count = count + 5;

Ex:
int result = 2;
result *= 4;   // result is 8

Ex:
int count1 = 3;
int count2 = 6;
int product *= count1 + count2;   // what value is stored in product?
       is equivalent to
int product = product * (count1 + count2);

Ex:
String s = "hello";
s += " world";             // s is "hello world"
        is equivalent to
s = s + " world";

Input from the Keyboard

Read data from the user during program execution

The Scanner class is in the java.util package

To read input from the user (via the keyboard), we must create a Scanner object associated with the keyboard.

In Java, we create objects using the new operator.

Scanner scan = new Scanner(System.in);

This Scanner object, scan, is created by the new operator and a call to a constructor, which sets up an object.

System.in represents the standard input stream (the keyboard)

Whitespace (spaces, tabs, newlines) separate the input elements, called tokens, from each other.

Useful Scanner methods:

String next() - returns the next String entered by the user

String nextLine() = returns all input remaining on the current line

double nextDouble() - returns the next input (token). If the next input is not of type double, an exception will be thrown.

Exceptions are Java's way of indicating that an error has occurred during program execution. More on this later.

int nextInt() - returns the next input token as an int (or throws an exception if next token is not an int).

Example:
Scanner scan = new Scanner(System.in);

// Prompt the user to enter an integer
System.out.print("Please enter an integer: ");

// Read and print the integer
int value = scan.nextInt();
System.out.println(value);

Example: Write a program that asks the user to enter a line of text, and then prints the first word in the line to the screen.

Example: Write a program that asks the user to enter a line of text, and then prints the line to the screen.

Example: Write a program that asks the user to enter 3 (integer) exam scores, and then prints the average.