II. Notes on Data Structuring * - Cornell University

NOTES ON DATA STRUCTURING 101
subrange. In this case, conversion to a subrange involves subtraction as well
as a check, and conversion in the opposite direction involves an addition.
Apart from these conversions, enumerations and subranges in either
representation can be treated identically. Tests of ordering can be accom-
plished by normal integer instructions of the computer, and succ and pred
involve addition or subtraction of unity, followed by a test that the result is
still within range.
The case discrimination can be most efficiently carried out by a switch-
jump. For example, in ALGOL 60 the first example quoted above (3.1.(3))
would be coded:
begin switch ss: = red, yellow, blue;
end: end.
go to ss[pc + 1];
red:begin .... ; go to end end;
yellow:begin... ; go to end end;
blue:begin .... ; go to end end;
This can be efficiently represented in machine code, using an indexed
jump and a switch table, indicating the starting addresses of the portions of
code corresponding to the limbs of the discrimination.
The implementation of the for statement corresponds in an obwous way
to the for statement of ALGOL 60, with a step length of unity. The con-
ventions proposed above, which regard the counting variable as a local
constant of the loop, not only contribute to clarity of documentation, but
also assist in achieving efficiency on a computer, by taking advantage of
registers, special count and test instructions, etc.
3.3. EXAMPLE
The character set of a computer peripheral is defined by enumeration:
type character = ( .... );
The set includes the subranges
type digit = nought., nine;
type alphabet = A.. Z;
as well as individual symbols, point, equals, subten, colon, newline, space,
as well as a number of other single-character operators and punctuation
marks.
There is a variable
buffer: character
which contains the most recently input character from the peripheral. A
i=