67 lines
1.8 KiB
OpenEdge ABL
67 lines
1.8 KiB
OpenEdge ABL
{ $Id$ }
|
|
|
|
program ArrayWithoutArray(input, output);
|
|
{ We simulate a (read-only) array by constructing a mapping
|
|
function map(n) which yields the n-th element.
|
|
We demonstrate its existence by first printing the length
|
|
of the array and then its contents.
|
|
This technique was first introduced by F.E.J. Kruseman-Aretz,
|
|
in the early sixties.
|
|
}
|
|
|
|
procedure Action(n: integer; function map(n: integer): integer);
|
|
{ Action is called when the construction of the virtual
|
|
array is finished. Actually, all elements now reside
|
|
on the stack.
|
|
n: the length of the array,
|
|
map: the mapping function.
|
|
}
|
|
var i: integer;
|
|
begin { show that the whole array is still there }
|
|
writeln('#elems:', n);
|
|
write('elems:');
|
|
for i:= 1 to n do
|
|
write(map(i))
|
|
end {Action};
|
|
|
|
procedure Construct(n: integer; function oldmap(n: integer): integer);
|
|
{ For each value read, Construct will store that value and
|
|
declare a new map function, composed of the old one
|
|
augmented by the new value.
|
|
It then calls itself recursively for the next value.
|
|
|
|
n: element number on this level
|
|
oldmap: map for 1 .. n-1
|
|
}
|
|
var x: integer; { the value stored at level n }
|
|
|
|
function newmap(i: integer): integer;
|
|
{ yields elements stored so far }
|
|
begin
|
|
if { the i-th element is kept on this level}
|
|
i = n
|
|
then { yield it }
|
|
newmap := x
|
|
else { try lower down the road }
|
|
newmap := oldmap(i)
|
|
end {newmap};
|
|
|
|
begin
|
|
read(x);
|
|
if { it is a valid value }
|
|
x >= 0
|
|
then { we continue reading values and constructing maps }
|
|
Construct(n + 1, newmap)
|
|
else { we stop reading and pass the info on to Action }
|
|
Action(n - 1, newmap)
|
|
end {Construct};
|
|
|
|
function EmptyMap(n: integer): integer;
|
|
begin
|
|
writeln('Illegal index', n, '; 0 yielded.');
|
|
EmptyMap := 0
|
|
end {EmptyMap};
|
|
|
|
begin
|
|
Construct(1, EmptyMap)
|
|
end.
|