Test: Arrays & File I/O

ArrayExamples.java

public class ArrayExamples {
  public static void main(String[] args) {
    // create an array of integers of length 10
    int[] arr = new int[10];
    // store the size of the array in a variable
    int len = arr.length;
    // use a for-each loop to 
    for (int i: arr) {
      System.out.println(i);
      // the output will be 10 lines of 0s since we haven't modified it
      // when an array in java is declared 
      // numbers: all 0s
      // strings: all null strings
      // booleans: all false
    }
    
    System.out.println("-------------------");
    
    for (int i = len; i > 0; i--) {
      // print the array in reverse
      // we do `i-1` because array indexes start at 0 and end at 1 less than the length
      // since we are starting from the length, we must go back one index
      // and since we used `i > 0` and not `i >= 0`, the last iteration of the loop will make i equal to 1
      // so we subtract 1 to make i = 0 when used so we can access the first element in the array
      System.out.println(arr[i-1]);
      // the output will still be 10 lines of 0s since we haven't modified it
    }
    
    System.out.println("-------------------");
    
    int[] arr1 = {0, 1, 2, 3, 4};
    int[] arr2 = {0, 1, 2, 3, 4};
    boolean equals = true;
    int length1 = arr1.length;
    int length2 = arr2.length;

    for (int i = 0; i < length1; i++) {
      for (int j = 0; j < length2; j++) {
        if (arr1[i] != arr2[j]) {
          System.out.println("Not equals");
          equals = false;
          break;
        }
        // increment i here to move the pointer back
        i++;
      }
      if (!equals) {
        break;
      }
    }
    if (equals) {
      System.out.println("Equals");
    }
    // Output: Equals
    
    System.out.println("-------------------");
    

    int[] arr1 = {0, 1, 2, 3, 4};
    int[] arr2 = {0, 1, 2, 3, 5};
    boolean equals = true;
    int length1 = arr1.length;
    int length2 = arr2.length;

    for (int i = 0; i < length1; i++) {
      for (int j = 0; j < length2; j++) {
        if (arr1[i] != arr2[j]) {
          System.out.println("Not equals");
          equals = false;
          break;
        }
        // increment i here to move the pointer back
        i++;
      }
      if (!equals) {
        break;
      }
    }
    if (equals) {
      System.out.println("Equals");
    }
    // Output: Not equals

    System.out.println("-------------------");
    
    int[] array1 = {0, 1, 2, 3, 4};
    int[] array2 = {0, 1, 2, 3, 4};
    int[] array3 = addArr(array1, array2);
    for(int i: array3) {
      System.out.println(i);
    }
    
  }
  
  public static int[] addArr (int[] arr1, int[] arr2) {
    int length1 = arr1.length;
    int length2 = arr2.length;

    int[] arr3 = new int[length1 + length2];

    for (int i = 0; i < length1; i++) {
      arr3[i] = arr1[i];
    }
    for (int i = 0; i < length2; i++) {
      arr3[length1 + i] = arr2[i];
    }
    return arr3;
  }
}

File_IO_read.java

import java.io.*;

public class File_IO_read {
  public static void main(String[] args) {
    // file name
    String fileName = "read_demo.txt";
    // declare line as a string
    String line;
    try {
      // creates a file reader and buffer
      FileReader file = new FileReader(fileName);
      BufferedReader in = new BufferedReader(file);
      // reads the first line
      line = in.readLine();
      // checks if the line has content
      while (line != null) {
        // prints the content of the line
        System.out.println(line);
        // continues with the next line
        line = in.readLine();
      }
      // close the file
      in.close();
    } catch (IOException e) {
      // in case there was a problem reading the file
      System.out.println("Error");
    }
  }
}

File_IO_write.java

import java.io.*;

public class File_IO_write {
  public static void main(String[] args) {
    String fileName = "write_demo.txt";
    try {
      FileWriter file = new FileWriter(fileName); // Add a true paramater to continue writing to to file rather than overwriting it
                                                  // FileWriter file = new FileWriter(fileName, true);
      BufferedWriter out = new BufferedWriter(file);
      out.write("Hello, World!");
      out.write(">");
      out.newLine();
      out.write(":D");
      out.close();
    } catch (IOException e) {
      System.out.println("Error");
    }
  }
}

Notes.txt

Arrays:
  There aren't that many built-ins for arrays (importing `java.util.Arrays` doesn't count)
   - array.length returns the size of the array
   - to compare 2 arrays, you can use Arrays.deepEquals (you would need to import java.util.Arrays) or you can iterate over each array and compare the values
   - to add arrays, you can create a new array, and iterate over the original arrays and add them to the new array

File I/O:
  To create a file input stream:
    FileReader file = new FileReader(fileName);
  To create a file buffer reader:
    BufferedReader in = new BufferedReader(file);
  To create a file output stream:
    FileWriter file = new FileWriter(fileName); // Add a true paramater to continue writing to to file rather than overwriting it
  To create a file buffer writer:
    BufferedWriter out = new BufferedWriter(file);
  
  Note that you need to import `java.io.*`
  


Since there is a coding part:

  Naming conventions:
    1) under 256 characters
    2) only letters, digits and underscores (_)
    3) can’t start with a digit
    4) no spaces
    5) no reserved keywords (refer to quiz 1 study notes for a list of reserved keywords)


  Data types:
    boolean - stores true or false
    byte - stores whole numbers from -128 to 127
    char - stores a single character/ASCII values
    int - stores whole numbers from -2,147,483,648 to 2,147,483,647
    long - stores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
    short - stores whole numbers from -32,768 to 32,767
    float - stores fractional numbers, sufficient for storing 6 to 7 decimal digits
    double - stores fractional numbers, sufficient for storing 15 decimal digits


  Assignment Operators:
    a = b;
      - assigns b to a

    a += b; - same as a = a + b;
      - can be used to increment a number by b
      - or to concatenate strings
      - note that if you do this to 2 chars, the chars will be converted to ints (their ascii value)

    a -= b; - same as a = a - b;
    - can be used to decrement a number by b
    - note that if you do this to 2 chars, the chars will be converted to ints (their ascii value)

    a *= b; - same as a = a * b
    - can be used to multiply a number by b
    - note that if you do this to 2 chars, the chars will be converted to ints (their ascii value)

    a /= b; - same as a = a / b
    - can be used to divide a number by b
    - note that if you do this to 2 chars, the chars will be converted to ints (their ascii value)

    a %= b; - same as a = a % b
    - can be used to divide a number by b and return the remainder
    - note that if you do this to 2 chars, the chars will be converted to ints (their ascii value)


  Arithmetic Operators:
    + adds the numbers (also used to concatenate strings to strings/numbers/chars)
    - subtracts the numbers
    * multiplies the numbers
    / divides the numbers (if both numbers used are both integers, drop the decimal once done dividing)
    % divides the numbers and returns the remainder
    ++ increments the number by 1
    -- decrements the number by 1

  Logical Operators:
    || - or
    && - and
    ! - not

  Comparison Operators:
    == - equals to (does not apply to strings; however, you can use these to compare chars)
    != - not equals to (does not apply to strings; however, you can use these to compare chars)
    > - greater than (does not apply to strings; however, you can use these to compare chars)
    < - less than (does not apply to strings; however, you can use these to compare chars)
    <= - less than or equal to (note the order of the angle bracket and the equals sign) (does not apply to strings; however, you can use these to compare chars)
    >= - greater than or equal to (note the order of the angle bracket and the equals sign) (does not apply to strings; however, you can use these to compare chars)

read_demo.txt

Hello there
Second line
Third line

write_demo.txt

Hello, World!>
:D