For loops

A for loop has a slightly more complicated syntax than while or do while loop as they take three parts to initialize. Have a look at the code first then we will break it apart.

for(int i = 0; i < 10;  i++){

   // Something that needs to happen 10 times goes here

}

The apparently obscure form of the for loop is clearer when put like this:

for(declaration and initialization; condition; 
change after each pass through the loop).

To clarify further we have:

  • Declaration and initialization: We create a new int variable i and initialize it to zero.
  • Condition: Just like the other loops, it refers to the condition that must evaluate to true for the loop to continue.
  • Change after each pass through the loop: In the example i++ means that 1 is added/incremented to i on each pass. We could also use i-- to reduce/decrement i each pass. 
    for(int i = 10; i > 0;  i--){
   // countdown
}
// blast off i = 0

Note

Note that break can also be used in for loops.

The for loop essentially takes control of initialization, condition evaluation and the control variable on itself.

Nested loops

We can also nest loops within loops. For example, take a look at this next code.

int width = 20;
int height = 10;
for(int i = 0; i < width; i++){
   for(int j = 0; j < height; j++){
      // This code executes 200 times
   }
}

The previous code will loop through every value of j (0 through 9) for each and every value of i (0 through 19).

The question is why would we do this? Nesting loops is useful when we want to perform a repetitive action that needs the changing values of two or more variables. For example, we could write code like the above to loop through a game board that is twenty spaces wide and ten spaces high.

Also note that loops of any type can be nested with any other loop. We will do this in most of the projects in this book.

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