Nested Loops

To nest loops means to place one loop inside of another loop. The statement of the outer loop will be another inner loop. The following example will print a rectangular grid of stars with 4 rows and 8 columns.

for (int row = 1; row <= 4; row++){
  for (int col=1; col <= 8; col++){
    System.out.print("*");
  }
  System.out.println( );
}

Run Output:

********
********
********
********

For each occurrence of the outer row loop, the inner col loop will print 8 stars, terminated by the newline character.

The action of nested loops can be analyzed using a chart:

Suppose we wanted to write a method that prints out the following 7-line pattern of stars:

*******
 ******
  *****
   ****
    ***
     **
      *

Here is an analysis of the problem, line-by-line.

For a picture of N lines, each line L will have (L-1) spaces and (N-L+1) stars.

Here is a pseudocode version of the method.

A method to print a pattern of stars:

Print N lines of stars, each Line L consists of (L-1) spaces (N-L+1) stars a line feed

Here is the code version of the method.

void picture (int n){
  int line, spaces, stars, loop;

  for (line = 1; line <= n; line++){
    spaces = line - 1;
    for (loop = 1; loop <= spaces; loop++){
      System.out.print (" "); // print a blank space
    }
    stars = n - line + 1;
    for (loop = 1; loop <= stars; loop++){
      System.out.print ("*");
    }
    System.out.println();
  }
}