# Java exercises

:::info

All this programs are one class only called Main so they can be executed in w3schools tryit editor:

https://www.w3schools.com/java/tryjava.asp?filename=demo_helloworld
:::

The base of these programs is this:

https://www3.ntu.edu.sg/home/ehchua/programming/java/J2_Basics.html

## Exercise about dataflow (if/else)

Write the output of this code:

double temperature = 80.1;
if (temperature > 80) {
   System.out.println("Too Hot!");
} else {
   System.out.println("Too Cold!");
}
System.out.println("yummy!");

Solution
:::spoiler
Too Hot!
yummy!
:::

Write the output of this code:

int markers = 30;
if (markers < 30) {
   System.out.println("We have to buy a lot more markers!");
   markers += 20;
} else if (markers <= 40) {
   System.out.println("We have to buy some more markers!");
   markers += 5;
}
else {
    System.out.println("I have enough markers");
}
System.out.println("I have these markers:" + markers);

Solution
:::spoiler
We have to buy some more markers!
I have these markers:35
:::

## iterations (for and while loops)

One other concept that we should have clear is loops

### Variations of for loop

LOOP 1
Write the output of this code:

int total = 0
for (int x = 0; x<10; x++) {
    total++;
}
System.out.println(total);

Solution
:::spoiler
10
(number of loops done)
:::

LOOP 2
Write the output of this code:

int total = 0
for (int x = 0; x<6; x++) {
    total=total+x;
}
System.out.println(total);

Solution
:::spoiler
15
(0+1+2+3+4+5)
:::

LOOP 3
Write the output of this code:

int total = 0
for (int x = 1; x<10; x= x+2) {
    total=total+x;
}
System.out.println(total);

Solution
:::spoiler
25
(1+3+5+7+9)
:::

LOOP 4
Write the output of this code:

int total = 0
for (int x = 1; x<10; x= x+2) {
    total++;
}
System.out.println(total);

Solution
:::spoiler
5
(five times the loop has been executed)
:::

LOOP 5
Write the output of this code:

int total = 0
for (int x = 1; x<8; x= x+2) {
    System.out.println(total);
}

Solution
:::spoiler
0
0
0
0
(four times the loop has been executed)
:::

LOOP 6

Now

Construct a code that sums all the numbers from 1 to 1000.

Construct a code that outputs a count from 10 to 0 and then prints "lift off!"

Construct a code that outputs the sum of all numbers divisible by 23 from 0 to 10000

## Other example codes

Here we have a program that adds all the numbers from 1 to 1000 that are divisible by 3, 5 and 7

public class Main {
  public static void main(String[] args) {
     final int lowerLimit = 1;
      final int upperLimit = 1000;
      int sum  = 0;    // For accumulating odd numbers, init to 0
 
      int number = lowerLimit;
      while (number <= upperLimit) {
      // divisible by 3, 5 or 7, but NOT by 15, 21, 35 or 105.
		if ( (number % 3 == 0) || number % 5 == 0  || number % 7 == 0 )   {
         if ( number % 15 != 0 && number % 21 != 0 && number % 35 != 0 && number % 105 != 0)
         {
         	sum += number;
         }
        }
        number++;  // Next number
      } 
      // Print the result
      System.out.println("The sum of these numbers from " + lowerLimit + " to " + upperLimit + " is " + sum);
 
   }
}

public class Main {
  public static void main(String[] args) {
     final int limiteInferior = 999;
      final int limiteSuperior = 2010;
      int counter  = 0;    
 
      int number = limiteInferior;
      while (number <= limiteSuperior) {
		if ( number % 4 == 0) 
         {
         	counter++; 
			System.out.println(number + "AD");
         }
        number++;  // Next number
      } 
      // Print the result
      System.out.println("Total number of leap years " + counter);
 
   }
}