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| Chapter 21.1 - 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 form 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$, but omitting any trailing spaces. 10 INPUT A$ 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$(5 TO 8) is only 4 characters long, only the first four stars have been used. This is a characteristic of assigning to substrings: the substring has to be exactly the same length afterwards as it was before. To make sure this happens, the string that is being assigned to it is cut off on the right if it is too long, or filled out with spaces if it is too short - this is called Procrustean assignment after the inn-keeper Procrustes who used to make sure that his guests fitted the bed by either stretching them out on a rack or cutting their feet off. If you now try LET A$()="COR BLIMEY" & PRINT A$;"." you will see that the same thing has happened again (this time with spaces put in) because A$() counts as a substring. LET A$="COR BLIMEY" will do it properly. Slicing may be considered as having priority 12, so, for instance LEN "ABCDEF"(2 TO 5) = LEN("ABCDEF"(2 TO 5)) = 4 Complicated string expressions will need brackets round them before they can be sliced. For example, "ABC"+"DEF"(1 TO 2) = "ABCDE" ("ABC"+"DEF")(1 TO 2) = "AB" Summary Slicing, using TO. Note that this notation is non-standard. |
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