CMUQ 15-121 Loops, Strings, I/O, and Random

• 1. Java API
• 2. Incrementors/Decrementors
  • 2.1. The Basics
  • 2.2. Pre vs. Post
  • 2.3. Quick Exercise
• 3. Loops
  • 3.1. While Loops
  • 3.2. For Loops
  • 3.3. Do-While Loops
• 4. Strings
• 5. Using the Scanner for I/O
• 6. Generating Random Numbers

1. Java API

One very nice thing about Java is that is comes with a large, robust library of code that you can make use of when building your programs. This library (called the Java API) is meticulously documented at https://docs.oracle.com/javase/8/docs/api/. At that link you can find the details of every class available in the API, and usually some examples of how to use them.

The Java API will become your very close friend during this class, and you will become very familiar with it.

2. Incrementors/Decrementors

// Increment i by 1
i += 1

// Decrement i by 1
i -= 1

public class IncrementorDemo {

    public static void main(String[] args) {     
        int i = 10;
            
        // Two different ways to increment i by one
        i++;
        ++i;
        
        // Two different ways to decrement i by one
        i--;
        --i;
    }
}

public class PrePostDemo {

    public static void main(String[] args) {
        int a = 10;
        int b = 10;

        int c = 7 + a++;
        int d = 7 + ++b;

        System.out.println("a is "+a);
        System.out.println("b is "+b);
        System.out.println("c is "+c);
        System.out.println("d is "+d);
    }

}

public class IncrementorExercise {

    public static void main(String[] args) {
        int e = 5;
        System.out.println(5 + e++ + ++e - e++ + --e - e-- );
        System.out.println(e);
    }

}

3. Loops

There are three basic types of loops in Java:For loops, while loops, and do-while loops.

public class WhileLoopDemo {

    public static void main(String[] args) {
        // Print out all the numbers from [0,10)
        
        int i = 0;
        while(i < 10) {
            System.out.println(i);
            i++;
        }
    }

}

public class ForLoopExample {

    public static void main(String[] args) {
        // Print out all the numbers from [0,10)
        
        /* 
         * The loop below can be thought of as follows:
         * 1. Before the loop runs, a new integer i is created and set to 0.
         * 2. The code inside the loop (in this case the print statement) runs once.
         * 3. The conditional check (i < 10) runs.  If the conditional returns false
         *    (meaning that i >= 10), then the loop stops.  Otherwise it continues.
         * 4. The incrementor (i++) runs and changes the value of i.
         * 5. Go back to step 2.
         */
        for(int i=0; i < 10; i++) {
            System.out.println(i);
        }
    }

}

public class DoWhileLoopDemo {

    public static void main(String[] args) {          
        /*
         *  Even though the conditional is false from the beginning, the loop
         *  always runs the first time because the conditional is checked after
         *  each loop iteration.
         */        
        int a = 5;
        do {
            System.out.print("From the do-while: ");
            System.out.println(a);
            a--;
        } while (a > 10);

        // Similar code from a normal while loop doesn't run at all
        int b = 5;
        while(b > 10) {
            System.out.print("From the while: ");
            System.out.println(b);
            b--;
        }
    }

}

4. Strings

https://docs.oracle.com/javase/8/docs/api/java/lang/String.html

Strings are not a primitive type in Java, but they are used so often that we tend to think of them that way.

• String variables store references (the address), not the actual characters of the string.
• Strings are immutable (can’t be changed).
• Useful operations:

  Method	Description
  charAt	Get a single character from a string
  compareTo	Compare two strings
  contains	See if a string contains another
  equals	Check equivalence Don't use == for strings
  length	Length of a string
  substring	Copy part of a string into a new string
  trim	Remove characters at the beginning and end of a string

5. Using the Scanner for I/O

https://docs.oracle.com/javase/8/docs/api/java/util/Scanner.html

The Scanner is a class that allows for easy input from a user. There are a few important rules when using a scanner:

• You must import java.util.Scanner
• You must declare a Scanner variable (which will reference the Scanner), create a Scanner object (use new), and bind it to standard input (System.in)
• When you print a prompt for the user, you should use print instead of println. (Can you figure out why?)
• Here are some useful methods of the Scanner:

  Method	Description
  next()	Read a String
  nextInt()	Read an Integer
  nextDouble()	Read a Double

Here is a sample program making use of the Scanner class. Try running it with various inputs and see if you can make it crash. It isn’t very reliable, is it? We’ll work on this later.

import java.util.Scanner;

public class ScannerDemo {
    public static void main(String[] args) {
        Scanner userInput = new Scanner(System.in);
		
        // Input a String
        System.out.print("Please enter a string: ");
        String inputStr = userInput.next();
		
        // Input an Integer
        System.out.print("Please enter an integer: ");
        int inputInt = userInput.nextInt();
		
        // Input a Double
        System.out.print("Please enter a double: ");
        double inputDouble = userInput.nextDouble();
		
        // Close the scanner when I'm done.
        userInput.close();
		
        System.out.println("Thanks for saying:");
        System.out.println(inputStr);
        System.out.println(inputInt);
        System.out.println(inputDouble);
    }
}

6. Generating Random Numbers

https://docs.oracle.com/javase/8/docs/api/java/util/Random.html

It is frequently very useful to be able to generate random values. This can help you with testing (you can generate random values to test your functions with) or even use them to introduce some variety into a simple game. Java provides random number generation as part of its standard library.

Take some time now to follow the link above to the API for Random. Take a look at the methods it includes that start with next. This gives you an idea of the types of random data that can easily be produced.

Consider the following sample code for an example of how to use Random:

import java.util.Random;

public class RandomDemo {

    public static void main(String[] args) {
        Random rand = new Random();
		
        System.out.println("Here are 10 random integers:");
        for(int i=0; i < 10; i++) {
            int temp = rand.nextInt();
            System.out.println(temp);
        }
		
        System.out.println("Here are 10 random integers from the range [0,5)");
        for(int i=0; i < 10; i++) {
            int temp = rand.nextInt(5);
            System.out.println(temp);
        }
		
        double randomDouble = rand.nextDouble();
        System.out.print("Here is a random double: ");
        System.out.println(randomDouble);
    }
}

It is important to note that while Random can produce seemingly random values, they are actually pseudo-random, meaning that they are mathemtically derived from an initial state. For most things you want randomness for, this is just fine. If you are doing security, this isn’t fine and you’ll need to find a better source of randomness. (This probably isn’t something you need to worry about in this class, but it is interesting to note.)