Difference between revisions of "Nested loops"

Revision as of 20:25, 19 March 2007

Introduction

A nested loop is simply just more than one loop inside another. There are no special rules for using nested loops, you only need to take them one at a time. This may sound complicated but after you start using nested loops it will seem like second nature. The best way to start learning about nested loops is to look at an example using matrices.

Matrix Example

If you have never encountered a matrix before you can think of it as rows and columns of numbers. Or you can think of a matrix as simply a table of rows and columns like so:

A simple matrix with two rows and three columns.

012
345

Now suppose you wanted to print that matrix out in a program by simply using loops. An easy solution would be to use double nested for loops such as:

int row, column;
int count = 0;

for(row = 0; row < 2; row++)
{
     for(column = 0; column < 3; column++)
     {
          System.out.print(count);
          count++;
     }
     System.out.println();
}

The best way to describe what is happening here is to break this code up into the two loops.

Outer Loop

for(row = 0; row < 2; row++)
{
 ...
}

We already know that a for loop will perform the same task until its test condition is met. In this example, the loop will execute the code inside two times meaning that whatever is inside this loop will be performed twice.

Inner Loop

for(column = 0; column < 3; column++)
{
     System.out.print(count);
     count++;
}
System.out.println();

Here is a simple for loop that prints out the value of count, updates count and finishes by adding a new line once the loop's test condition is met. This loop will run three times printing out:

012

Putting it All Together

• The outer loop will perform whatever is inside two times.
• The contents of the outer loop is another loop.
• The inner loop prints out the value of count in sets of three.
• Therefore the inner loop will be executed two times with a total of 6 iterations (3 per execution).

Step-by-Step

Other Uses

An important note to remember is that you can mix and match different types of loops together to create nested loops.

Example:

while(...)
{
    for(...)
    {
       for(...)
       {
            ...
       }
    }
    while(...)  
    {
        ...
    }
}

• When you learn about multi-dimensional arrays, double for loops will come in handy (see Matrix Example).

Example Program

/**********************************
* NestedX:
* Prints out a fancy "ascii art"
* picture in the shape of an X where
* the user supplies the size of the X.
***********************************/

import javax.swing.*;

public class NestedX
{
	public static void main(String[] args)
	{
		int size = 0;
		int i, j;

		// Keep asking for input until it is greater or equal to 3.
		while(size < 3)
		{
			size = Integer.parseInt(JOptionPane.showInputDialog("Enter a number greater or equal to 3"));
		}

		for(i = 0; i < size; i++)
		{
			for (j = 0; j < size; j++)
			{
				if(i == j)
				{
					System.out.print("*");
				}
				else if(j == (size - (i + 1)))
				{
					System.out.print("*");
				}
				else
				{
					System.out.print(" ");
				}
			}
			System.out.println();
		}

		System.out.println("\n~~~End of processing~~~");
		System.exit(0);

	}// end main
}// end NestedX

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