THE NAME OPERATOR OF SNOBOL

THE NAME OPERATOR OF SNOBOL

The unary name operator provides the address or location in memory where a variable is stored. Its graphic symbol is the period (.). We'll introduce it here through an example.

Consider the indirect reference operator mentioned earlier. Suppose we want to use a variable to point to different elements of an array or table. If we try the following, we immediately discover a problem:

? A = ARRAY('10,10')

? A[4,2] = 'DOG'

? V = 'A[4,2]'

? OUTPUT = $V

Success

The indirect reference operator treats the string 'A[4,2]' as a variable name, rather than an array element. Remember, any character sequence can be used indirectly to create a variable. SNOBOL4 creates a variable called A[4,2] that has absolutely no connection with array A. The fact that this character sequence happens to look like an array reference to us is purely coincidental from SNOBOL4's point of view.

To make this work, the name operator is applied to A[4,2] to obtain the address of that array element. The address can be stored in variable V, and referenced with the indirect operator:

? V = .A[4,2]

? OUTPUT = $V

DOG

The name operator provides a general method for specifying the name of an object. Both of these statements are correct for specifying the first argument to the INPUT function:

INPUT('INFILE', 1, , 'CAPITAL.DAT')

INPUT(.INFILE, 1, , 'CAPITAL.DAT')

Either form, 'INFILE' or .INFILE, tells the INPUT function the name of the variable to be input associated. However, using the name operator allows us to associate a file with an array or table element: INPUT('A[4,2]', 1, , 'CAPITAL.DAT') (incorrect)

INPUT(.A[4,2], 1, , 'CAPITAL.DAT')

Note that alternate use of the indirect reference and name operators "cancel" one another, so

? OUTPUT = $(.($(.A[4,2])))

DOG