Advanced Perl Programming

Example: Matrices

Before we embark on this example, you must know that if you really want a good efficient implementation of matrices, you should check out the PDL module (Perl Data Language) from CPAN.

To gain a better understanding of different matrix representations, we will write routines to construct these structures from a data file and to multiply two matrices. The file is formatted as follows:

MAT1
1  2  4
10 30 0

MAT2
5  6 
1  10

Each matrix has a label and some data. We use these labels to create global variables with the corresponding names (@MAT1 and @MAT2).

An array of arrays is the most intuitive representation for a matrix in Perl, since there is no direct support for two-dimensional arrays:

@matrix = (
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
); 
# Change 6, the element at row  1, column 2 to 100
$matrix[1][2] = 100;

Note that @matrix is a simple array whose elements happen to be references to anonymous arrays. Further, recall that $matrix[1][2] is a simpler way of saying $matrix[1]->[2].

Example 2.1 reads the file and creates the array of arrays structure for each matrix. Pay particular attention to the push statement (highlighted); it uses the symbolic reference facility to create variables (@{$matrix_name}) and appends a reference to a new row in every iteration. We are assured of newly allocated rows in every iteration because @row is local to that block, and when the if statement is done, its contents live on because we squirrel away a reference to the array’s value. (Recall ...