Sinclair ZX81 - Chapter 21 - Substrings

Chapter 21 - Substrings
Given a string, a substring of it consists of some consecutive characters from it, taken in sequence. Thus
"STRING" is a substring of "BIGGER STRING", but "B STRING" & "BIG REG" are not.
There is a notation called slicing
for describing substrings, & 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) = "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) = "ABCDEF" (1
TO
5) = "ABCDE"
"ABCDEF" (2
TO
) = "ABCDEF" (2
TO
6) = "BCDEF"
&
"ABCDEF" (
TO
) = "ABCDEF" (1
TO
6) = "ABCDEF"
(you can also write this last one as "ABCDEF" (), for what it's worth.)
A slightly different for misses out the
TO
& just has one number:
"ABCDEF" (3) = "ABCDEF" (3
TO
3) = "C"
Although normally both start & finish must refer to existing parts of the string, this rule is overridden by
one other: if the start is more than the finish, then the result is the empty string. So
"ABCDEF" (5
TO
7)
gives error 3 (subscript error) because, the string only contains 6 characters, & 7 is too many, but
"ABCDEF" (8
TO
7) = ""
&
"ABCDEF" (1
TO
0) = ""
The start & finish must not be negative, or you get error B.
This next program makes B$ equal to A$, but imitting any trailing spaces.
10
INPUT
A$
20
FOR
N=
LEN
A$
TO
1
STEP
-1
30
IF
A$(N)<>""
THEN GOTO
50
40
NEXT
N
50
LET
B$=A$(
TO
N)
60
PRINT
"
""
";A$;"
""
","
""
";B$;"
""
"
70
GOTO
10
Note how if A$ is entirely spaces, then in line 50 we have N = 0 & A$(
TO
N) = A$(1
TO
0) = "".
For string variables, we can not only extract substrings, but also assign to them. For instance type
LET
A$="LOR LOVE A DUCK"
& then
LET
A$(5 TO 8)="******"
&
PRINT
A$
Notice how since the substring A$(5
TO
8) is only 4 characters long, only the first four stars have been
used. This is a characteristic of assigning to substrings: the substring has to be exactly the same length