1-monday.md

For Loops and While Loops

A Loop is a programming construct that can help with repetitive tasks.

There are different ways of creating loops in programming languages. We will look at the most important one

For loop

The for loop is the most used loop and one you see quite often. The syntax is a bit weird, but let's run through an example:

for (int i = 0; i < 3; i++) {
    System.out.println("This is iteration " + i);
}

The first part is the for. This just tells Java that we will be creating a for loop. Now comes three parts of the for loop:

1. int i = 0; This is the starting condition. This means that Java will create a for loop where the variable i is 0
2. i < 3; This part tells Java for how many times the for loop should run. In this example the for loop should run while i is smaller than 3
3. i++ This part tells Java what to do after each iteration. In this example the for loop tells Java to increment i by 1 for eacher iteration

Inside the brackets we write the code to run for each iteration. This code will print the following to the console

This is iteration 0
This is iteration 1
This is iteration 2

There are other kind of loops like a while or do-while loop. We might get into that later.

Scope

Scope is a concept about where variables will be exist and where they no longer will exist.

Let's say i wanted to do something like this 👇

for (int i = 0; i < 7; i++) {
    System.out.println("Hello");
  	System.out.println(i);
  	// Above this line number does not exist!
    int number = i;
  	// below this line number exists but only until the closing bracket!
}

System.out.println("The last number is: " + number); // Cannot resolve number

I make a for loop that runs from 0 - 6. Then I assign the current iteration number (i) to number

When the loop is done I print out number. But there is a problem! Java tells me that Cannot resolve number. This means cannot find number. The reason is that number only exists after it has been defined and within the curly brackets of the for loop! This is called block scope

To fix this problem we can define number before the for loop. Now number will be available to the for loop

int number = 0;
for (int i = 0; i < 7; i++) {
    number = i;
}

System.out.println("The last number is: " + number); // prints 6