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Archive for the ‘IB Computer Science 2018’ Category

Inserting Multiple Rows into a SQLite Database from Java

November 26, 2017 Leave a comment
package sqlitetest;
import java.sql.*;

public class SqliteTest {

    public static void main(String args[]) {
        Connection connection = null;

        try {
            /* The next line will give a ClassNotFound error unless the
               sqlite jar is in the classpath. To add it in Netbeans, 
               right-click on the project, choose Properties, Libraries 
               and add the path to the jar file. On Linux I saved it in 
               /usr/local/sqlite/sqlite-jdbc-3.20.0.jar, but this will be
               different if you are using Mac or Windows.
            */
            Class.forName("org.sqlite.JDBC");
            connection = DriverManager.getConnection("jdbc:sqlite:test.db");

            // The database must already have a country table with these fields.
            // In my table there is also a CountryId primary key field, but the
            // database assigns values to that field automatically.
            String sql = "insert into Country (CountryName, Population) values (?, ?)";
            PreparedStatement ps = connection.prepareStatement(sql);


            // In larger applications you would probably loop through a list
            // of country objects. This is just a simple demo.

            ps.setString(1, "United States of America");
            ps.setString(2, "320000000");
            ps.addBatch();

            ps.setString(1, "China");
            ps.setString(2, "1400000000");
            ps.addBatch();

            ps.executeBatch();
            connection.close();

        } catch (Exception e) {
            System.err.println(e.getClass().getName() + ": " + e.getMessage());
            System.exit(0);
        }
        System.out.println("Insert successful");
    }
}
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Simple chat server in Python

November 24, 2017 Leave a comment

Add the following code to server.py and run.

# server.py
#!/usr/bin/python

import socket

sock = socket.socket()
host = 'localhost' # Change this to "" to receive connections from any host
port = 12221  # Any unreserved port but must be same as client
sock.bind((host, port))

sock.listen(5)
conn = None

while True:
   if conn is None:
       # Halts
       print ('[Waiting for connection...]')
       conn, addr = sock.accept()
       print('Got connection from', addr)
   else:
       # Halts
       print ( '[Waiting for response...]')
       print (conn.recv(1024))
       msg = input("Enter something to this client: ")
       conn.send(msg.encode('UTF-8'))

Add the following code to client.py.

# client.py
#!/usr/bin/python

import socket

sock = socket.socket()
host = 'localhost' # Change this to remote ip as necessary
port = 12221 # Any unreserved port but must be same as server

sock.connect((host, port))
print ('Connected to', host)

while True:
msg = input("Enter something for the server: ")
sock.send(msg.encode('UTF-8'))
# Halts
print ('[Waiting for response...]')
print (sock.recv(1024))

Code is STUFF

November 9, 2017 Leave a comment

In English we distinguish between count nouns and non-count or mass nouns. Consider what feels wrong about these sentences:

  • My glass is full of waters.
  • I’ve got sands in my shoes.
  • Could I borrow a money?
  • I want to buy a furniture.

The reason these are wrong is that water, sand, money and furniture are examples of mass nouns. They are STUFF. You cannot pluralize stuff and you cannot refer to “a stuff”.

Why am I telling you this? Because “code”, as in lines of computer code, is also a mass noun. Code is STUFF. You cannot refer to “codes” and you cannot write “a code”.

Please try to remember this!

Course Information for All Courses 2017-2018

August 15, 2017 Leave a comment

Java Serialization Example

April 27, 2017 Leave a comment

Java provides a method of saving objects to file, called Serialization. In order for an object to be serializable it must implement the java.io.Serializable interface and you should set a version ID as well as shown below.

class Student implements java.io.Serializable {

   String name = "";
   Integer age = 0;
   Double gpa = 0.0;

   private static final long serialVersionUID = 1L;
}

You can save the object to file like this:

        try {
            FileOutputStream fileOut = new FileOutputStream(path);
            ObjectOutputStream out = new ObjectOutputStream(fileOut);
            out.writeObject(obj);
        } catch (IOException i) {
            i.printStackTrace();
        }

And you can load it like this:

        try {
            FileInputStream fileIn = new FileInputStream(path);
            ObjectInputStream in = new ObjectInputStream(fileIn);
            obj = (Student)in.readObject();
        } catch (FileNotFoundException fnf) {
            System.out.println("No serialized file found at " + path + ". Creating new blank object.");
            obj = new Student();
        }
         catch (Exception e) {
            e.printStackTrace();
        }

Donate to Wikipedia

January 10, 2017 Leave a comment

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Nested loops: looking for duplicates in arrays

December 1, 2016 Leave a comment

A and B are arrays of integers. Construct code to output all of the values that are in both A and B.

With questions like this, imagine you had to do it manually. Imagine you have two long lists of numbers on paper. How would you find all the values that occur in both lists?

You would probably look at the first number from list A and then scan all the way down list B to see if it’s there. If it was you’d make a note of it. Then you would get the next number from list A and do the same thing.

A computer algorithm will do exactly the same thing.

In pseudocode this is:

loop I from 0 to A.length - 1
  loop J from 0 to B.length - 1
    if A[I] = B[J] then
      output A[I]
    end if
  end loop
end loop

In Java it is:

for (int i = 0; i < a.length; i++) {
  for (int j = 0; j < b.length; j++) {
    if (a[i] == b[j]) {
      System.out.println(a[i]);
    }
  }
}