## Chapter 7 – Strings ### Introduction As we discussed previously strings are series of characters stored as numbers in memory. Which number represents which character is usually governed by a standard. For *NextBASIC* this standard is a modified version of the *ASCII* standard that apart from characters, also defines *tokens* and *UDGs*. See *Chapters 13* and *23* as well as *Appendix A* in order to better understand how characters, tokens and UDGs are stored. This chapter deals with ways of manipulating strings and reiterates some functions touched upon by previous chapters. ### String slicing, using TO 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 sting” and “big reg” are not. 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)="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** and just has one number: **"abcdef"(3)="abcdef"(3 TO 3)="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 error **3 Subscript wrong** because the string only contains 6 characters and 7 is too many, but: **"abcdef"(8 TO 7)="" (an empty string)** and: **"abcdef"(1 TO 0)="" (again, an empty string)** The start and finish must not be negative, or you get error **B integer out of range**. This next program is a simple one illustrating some of these rules. ``` 10 a$="abcdef" 20 FOR n=1 TO 6 30 PRINT a$(n TO 6) 40 NEXT n ```