For loops

To date we have used loops to repeatedly run code until some condition is met. Sometimes however, we know exactly how many times we would like a loop to run. A while loop to print the numbers from 0 to 10 would appear as follows:

int counter = 0;         //initialisation statement
while (counter <= 10) {  //continue condition
    System.out.println(counter);
    counter++;           //code executed post iteration
}
One of the negatiaves of using this format is that the initialisation statement (in this case code to keep count), the condition, and the code executed after iteration, are seperated from each other.

A different type of loop, a for loop, provides a more concise (though with syntax that is slightly more tricky to learn) way of doing things. The structure of a for loop is as follows:

for (<initialisation statement>; <continue condition>; <increment statement>) {
    //do something
}
If we wanted to convert our original while statement into a for statement, the resulting code would look like this:
for (int counter = 0; counter <= 10; counter++) {
    System.out.println(counter);
}
This code is now neater, and it has the additional benefit that the counter variable is only available inside the for loop, this avoids confusion, and means you can use the same variable name for another loop later in the same code

Using i for the counter

It's worth being aware that it is very common practise to use the letter 'i' as the variable name for a counter, thus code like this is very common:

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

More loops

In the above example, we started our loop at 0 and incremented the counter by 1 every iteration, we are no means limited to doing this, the example below shows a loop that starts at 20, and decreases by 2 until just before 10:

for (int i = 20; i > 10; i -= 2) {
    System.out.println(i); //prints 20, 18, 16, 14, 12
}

Task

For each of the following sequences, create a command line programme, and use the For loop syntax generator to get the code for a loop, add the relevent code as neccesary inside the loop, check that the result is as expected:

  1. Starting at 23 going up 4 at a time, print numbers upto and including 123.
  2. Starting at 44 going up 4 at a time, print numbers upto and including 65 (if applicable)
  3. Starting at -20, going up 10 at a time, print the numbers up to, but not including 100. Name the counter x
  4. Starting at -55, going down 1 at at a time, print the numbers down to and including -77. Name the counter i
  5. Starting at 5, going up 5 at a time, print the numbers up to and including 3

Task 2

Without using the syntax generator, complete the following tasks in the same format, but this time write the code yourself, using an example on the page above as reference if needed. Check each loop behaves as expected and then delete all the loop code before starting the next task

  1. Starting at 3, going up 1 at a time, print the numbers up to and including 7
  2. Starting at -5, going up 1 at a time, print the numbers up to but not including 7. Name the counter counter
  3. Starting at -36, going up 9 at a time, print the numbers up to, but not including 56 (if applicable). Name the counter i
  4. Starting at -5, going down 1 at at a time, print the numbers down to and including -17
  5. Starting at 11, going up 2 at a time, print the numbers up to and including 8

Task 3

  1. Use a for loop to calculate the factorial of 10 (this is 10 x 9 x 8 x 7 etc. all the way to 1). For reference, the answer is 3,628,800
  2. Use a for loop to print out the various hours in the day in the following format:
    0:00am
    1:00am
    2:00am
    3:00am
    ...
    11:00am
    12:00am
    1:00pm
    ...
    11:00pm
    

Advanced task

Using two loops (one inside the other), print out all the minutes of a day using the 24 hour clock. The start of the output would be as follows:

0:00
0:01
0:02
...