II. Notes on Data Structuring * - Cornell University

NOTES ON DATA STRUCTURING 135
In some cases, it is more efficient to avoid the copying of an item which is
involved in the from operation. These cases may be dealt with by merely
omitting the left hand variable, e.g.
from x
back from x.
In this case, access to the items of the sequence will usually be made by the
selectors x. first and/or x. last.
It is very common to wish to scan all the items of a sequence in succession;
a suitable notation for this is modelled on the for statement:
for v in x do S;
If x is empty, the statement is omitted. Otherwise the variable v (regarded
as local to S) takes in succession the values of all items from the sequence
x, and S is executed once for each value. In this construction neither x nor v
should be updated within S.
A similar construction can be used for defining a sequence as an item-by-
item transformation E(v) of items v in sequence s.
for v in s take E(v).
In deciding a representation for a sequence, it is most important to know
which of the selective updating operations are going to be carried out upon it.
(1) If the only operation is from, the sequence is known as an input
sequence; obviously in order to have any value at all, an input sequence
must be initialised to some value existing in the outer environment in which
it is declared. The association of a sequence local to a program with some
file existing more or less permanently on backing store is often known as
"opening" the file for input, and we assume that this operation is invoked
implicitly on declaration of a local input sequence. The reverse operation of
"closing" the file is invoked implicitly on exit from the block to which
the sequence is local.
(2) If the only operation is writing to the file, the sequence is known as an
output sequence. An output sequence may be initialised from the environment
in the same way as an input sequence; or more commonly, it may take an
empty initial value. In either case, in order to serve any useful purpose, the
final value of the sequence on exit from the block must be assigned to some
variable existing in the outer environment in which the sequence is declared.
The identity of this outer variable should be declared together with the
sequence; if this outer variable is held more or less permanently on backing
store, it is known as an output file; and the rules for implicit invocation of
opening and closing of the file on entry and exit to the block are similar to
those for input files.