Part 8
Strings

Subjects covered...

	Slicing, using TO

Given a string, a substring of it consists of some consecutive
characters from it, taken in sequence. Thus "cut" is a substring of
"cutlery", but "cute" and "cruelty" are not substrings.

There is a notation called slicing for describing substrings, and this
can be applied to arbitrary string expressions. The general form is...

	string expression (start TO finish)

...so that, for instance...

	"abcdef"(2 TO 5)

...is equal to 'bcde'.

If you omit the start, then 1 is assumed; if you omit the finish, then
the length of the string is assumed. Thus...

	"abcdef"( TO 5)      is equal to 'abcde'
	"abcdef"(2 TO )      is equal to 'bcdef'
	"abcdef"( TO )       is equal to 'abcdef'

You can also write this last one as "abcdef"().

A slightly different form misses out the TO and just has one number.

	"abcdef"(3) is equal to "abcdef"(3 TO 3) is equal to 'c'.

Although normally both start and finish must refer to existing parts
of the string, this rule is overridden by another one: if the start is
more than the finish, then the result is the empty string. So...

	"abcdef"(5 TO 7)

...gives the error '3 Subscript wrong' because the string only
contains 6 characters and 7 is too many, but...

	"abcdef"(8 TO 7)

...and...

	"abcdef"(1 TO 0)

...are both equal to the empty string "" and are therefore permitted.

The start and finish must not be negative, or you get the error 'B
integer out of range'. This next program is a simple one illustrating
some of these rules...

	10 LET a$="abcdef"
	20 FOR n=1 TO 6
	30 PRINT a$(n TO 6)
	40 NEXT n

Type NEW when this program has been run and enter the next program.

	10 LET a$="1234567890"
	20 FOR n=1 TO 10
	30 PRINT a$(n TO 10),a$((11-n) TO 10)
	40 NEXT n

For string variables, we can not only extract substrings, but also
assign to them. For instance, type...

	LET a$="Velvet Donkey"

...and then...

	LET a$(8 TO 13)="Lips******"

...and...

	PRINT a$

Since the substring 'a$(8 TO 13)' is only 6 characters long, only its
first 6 characters ('Lips**') are used; the remaining 4 characters are
discarded. This is a characteristic of assigning to substrings: the
substring has to be exactly the same length afterwards as it was
before. To make sure this happens, the string that is being assigned
to it is cut off on the right if it is too long, or filled out with
spaces if it is too short - this is called 'Procrustean assignment'
after the inn-keeper Procrustes who used to make sure that his guests
fitted their beds by either stretching them out on a rack or cutting
their feet off!

Complicated string expressions will need brackets around them before
they can be sliced. For example...

	"abc"+"def"(1 TO 2)      is equal to "abcde"
	("abc"+def")(1 TO 2)     is equal to "ab"


Exercise...

1. Try writing a program to print the day of the week using string
slicing. (Hint - Let the string be 'SunMonTuesWednesThursFriSatur'.)