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Chapter 3 ■ Digital Morphology 109

Variables may be declared to be of any of these three types by stating the
type name followed by a list of variables having that type. STRING constants
areallowedin somecases, but there are no string variables.
The executable part of a MAX program is a sequence of statements enclosed
by a begin statement at the beginning and an end statement at the end.
A semicolon (;) separates each statement from the next in a statement sequence,
except before an end statement. The preceding program has only one kind of
statement: a DO statement. This is simply the word ‘‘do’’ followed by any legal
expression, and permits the expression to be evaluated without assigning the
value to anything.
The only operators seen above are << (input) and >> (output), in this case
applied to images. The expression a<<“a“ reads an image in PBM file format
from the file named “a“ into the image variable a; if the string constant is the
empty string ““, then standard input is used as the input file, and if the string is
“$1“, then the first command line argument is copied into the string, allowing
a program to open dynamically specified file. In the preceding program, two
images are read into variables. These are immediately written again under
different names: the output operator >> works in the same way as the input
operator, creating a PBM image file from the specified image.
MAX has six different types of statements, designed to allow a great
deal of flexibility in what kinds of morphological operations can be easily
implemented. In summary, the legal statements are as follows:
if ( expression ) then statement

If the expression evaluates to a non-zero integer (TRUE), the statement
that follows will be executed. The statement can be a sequence of statements
enclosed by begin–end.

if ( expression ) then statement1 else statement2
This is another form of the if statement, but if the expression is 0 (FALSE),
statement2 is executed.

loop ... end
Repeat a sequence of code. When the end is reached, execution resumes
from the statement following the loop statement. Statements within the loop
must be separated by semicolons.

exit N when expression
Exit from a loop if the expression evaluates to a non-zero integer. If N is
omitted, the exit branches to the statement following the end of the nearest
enclosing loop. If N=2, we escape from the nearest 2 nested loops, and so on.

do expression