Paying around with Pascal recently, I had forgot about how nice its sets structures are. In addition to arrays and records, sets offer an alternative built-in data structure. For those that don’t know, a set is a collection of items of the same type – it is essentially a packaged, unindexed data structure. They are inherent flexible, and useful, largely because of the syntax. Operations are provided for set union, intersection, membership, and construction. For example:
var digit : set of '0'..'9'
range : set of 1..100;
alphabet : set of 'a'..'z';
defines a set, digit, that may contain any combination of the characters ‘0’ through ‘9’. The predicate:
x in digit
tests whether or not the value of the variable x is in the set digit. Here’s a good example of how sets can be used::
operandset, operatorset: set of char;
operatorset := ['%','+','-','*','/','^'];
operandset := ['a'..'z','0'..'9'];
This leads to very efficient statements, of the form:
if x in operandset
then write(' operand ')
else if x in (operatorset + ['(',')'])
then write(' operator or parenthesis ')
else write(' illegal symbol ')
Here’s the equivalent in Fortran, which has no similar structure:
if ((x >= 'a' .and. x <= 'z') .or. &
(x >= '0' .and. x <= '9')) then
elseif (x == '%' .or. x == '+' .or. x == '-' .or. &
x == '*' .or. x == '/' .or. x == '^' .or. &
x == '(' .or. x == ')') then
write(*,*) "operator or parenthesis"
write(*,*) "illegal symbol"
Now although many languages such as C++ provide support for sets in some form or another, rarely is it part of the core language. So languages such as Fortran, or C contain no real set structure. Why do I care about this? Because there are certain circumstances when a set can help reduce syntactic complexity, as the example above shows.