CISC 1115 Lecture #10

CISC 1115
Introduction to Programming Using Java
Lecture #10
Techniques II

With the availability of general loop, many additional, commonly-used techniques can be introduced:

Accumulation

We saw hints of this previosly in the i++ of our for loops.

accumulation is the process of keeping track of a running sum of values —

Basic form:

int total = 0;		// initialize running sum
…
while (condition) {		// or for (i.e.., some sort of loop)
	…
	total += …;				// add to the running sum
	…
}
…

Program 10.1

Sum up the numbers from 1 to 20

public class Program_10_1 {
        public static void main(String [] args) throws Exception {
                int total = 0;

                for (int i = 1; i <= 20; i++) 
                        total += i;
                System.out.println("The total is: " + total);
        }
}

could also be used for a running product
the running sum could be conditionally updated (i.e., not necessarily increased on every loop iteration)

The accumulation could also be used as the terminating condition

Program 10.2

Prompt the user for numbers, until the total reaches or exceeds 100, and then print out the total

import java.util.*;

public class Program_10_2 {
        public static void main(String [] args) throws Exception {
                Scanner scanner = new Scanner(System.in);

                int total = 0;

                while (total < 100) {
                        System.out.print("Enter a number: ");
                        int number = scanner.nextInt();
                        total += number;
                }
                System.out.println("total: " + total);
        }
}

Keeping Count

A special case of accumulation: the running sum is increased by 1 as we keep track of the item(s) we are counting.

Program 10.3

Print out the number of integers read in from a file (read until eof).

import java.io.*;
import java.util.*;

public class Program_10_3 {
        public static void main(String [] args) throws Exception {
                Scanner scanner = new Scanner(new File("numbers_2.text"));

                int count = 0;

                while (
                scanner.hasNextInt()) {
                        int number = scanner.nextInt();
                        count++;                                                                        // Note we're not doing anything with the number
                }
                System.out.println(count + " numbers were read in");
        }
}

Searching for a Value

Given a specific value, determine whether the value appears in a sequence of numbers

Program 10.4
Prompt the user for an integer and output whether the number appears in a file of integers

Various Searching Techniques

Exit the program upon encountering the value

import java.io.*;
import java.util.*;

public class Program_10_4a {
        public static void main(String [] args) throws Exception {
                Scanner keyboard = new Scanner(System.in);
                Scanner datafile = new Scanner(new File("numbers_4.text"));

                System.out.print("What number are you looking for? ");
                int val = keyboard.nextInt();

                while (datafile.hasNextInt()) {
                        if (val == datafile.nextInt()) {
                                System.out.print("Found it!");
                                System.exit(0);
                        }
                }

                System.out.print("Not there");
                System.exit(1);
        }
}

Somewhat 'drastic' and not applicable if you want to do something after you did or did not find the value

Set boolean flag

import java.io.*;
import java.util.*;

public class Program_10_4b {
        public static void main(String [] args) throws Exception {
                Scanner keyboard = new Scanner(System.in);
                Scanner datafile = new Scanner(new File("numbers_4.text"));

                System.out.print("What number are you looking for? ");
                int val = keyboard.nextInt();

                boolean found = false;
                while (datafile.hasNextInt()) 
                        if (val == datafile.nextInt()) 
                                found = true;

                if (found) 
                        System.out.println("Found it!");
                else
                        System.out.println("Not there");

        }
}

Wasting time continuing through the file once you've found it

The two most commonly used techniques:

break logic

import java.io.*;
import java.util.*;

public class Program_10_4c {
        public static void main(String [] args) throws Exception {
                Scanner keyboard = new Scanner(System.in);
                Scanner datafile = new Scanner(new File("numbers_4.text"));

                System.out.print("What number are you looking for? ");
                int val = keyboard.nextInt();

                boolean found = false;
                while (datafile.hasNextInt()) 
                        if (val == datafile.nextInt())  {
                                found = true;
                                break;
                        }

                if (found) 
                        System.out.println("Found it!");
                else
                        System.out.println("Not there");

        }
}

Somewhat controversial
Can be difficult to follow if loop is complex and/or lengthy

Set boolean flag and incorporate it into the loop condition

import java.io.*;
import java.util.*;

public class Program_10_4d {
        public static void main(String [] args) throws Exception {
                Scanner keyboard = new Scanner(System.in);
                Scanner datafile = new Scanner(new File("numbers_4.text"));

                System.out.print("What number are you looking for? ");
                int val = keyboard.nextInt();

                boolean found = false;
                while (!found && datafile.hasNextInt()) 
                        if (val == datafile.nextInt()) 
                                found = true;

                if (found) 
                        System.out.println("Found it!");
                else
                        System.out.println("Not there");

        }
}

Other side of the coin from using break

The Classic Newbie Searching Error

while (datafile.hasNextInt()) {
	if (val == datafile.nextInt()) {
		System.out.print("Found it!");
		System.exit(0);
	}
	else {
		System.out.print("Not there");
		System.exit(1);
	}
}

Finding the Maximum/Minimum of a Sequence of Values

Given a specific value, determine whether the value appears in a sequence of numbers

Program 10.5

Find the largest value in a file of integers.

import java.io.*;
import java.util.*;

public class Program_10_5 {
        public static void main(String [] args) throws Exception {
                Scanner scanner = new Scanner(new File("values.text"));

                int max = scanner.nextInt();
                while (scanner.hasNextInt()) {
                        int num = scanner.nextInt();
                        if (num > max) max = num;
                }

                System.out.println("The maximum number read in is " + max);
        }
}

The Classic (Not-So-) Newbie Max-of-a-Squence Error

int max = -1;
while (scanner.hasNextInt()) {
	int num = scanner.nextInt();
	if (num > max) max = num;
}

`break` and `continue`

break causes an immediate exit from the loop
continue cause execution flow to proceed to the next iteration, bypassing all remaining loop body logic

Somewhat controversial as they allow for exit from the loop other than from the header (break) / unexpected flow of logic from the middle of the loop body back up to the loop header (continue)

CISC 1115 Introduction to Programming Using Java Lecture #10 Techniques II