Ken, when I first read your code I said "That wont work and k(0) will be wrong."
however it seems to work. I cannot explain why your code works and my original code did not. I never modify the string content. ------------------------------------- your line: 20 FOR X=0TO3:F=VARPTR(A$(X)):K(X)=256*PEEK(F+2)+PEEK(F+1):NEXT compared with 20 F=VARPTR(A$(0)):*K(0)*=256*PEEK(F+2)+PEEK(F+1) yield the same answers for K(0) ----------------------------------- your line: 20 FOR X=0TO3:F=VARPTR(A$(X)):K(X)=256*PEEK(F+2)+PEEK(F+1):NEXT compared with 20 F=VARPTR(A$(0)):*K*=256*PEEK(F+2)+PEEK(F+1) yield different answers for K, K(0) the only difference is using K vs K(0). I still don't know a good rule for how to safely use VARPTR!! On Tue, May 29, 2018 at 4:52 PM, Ken Pettit <petti...@gmail.com> wrote: > Steve, > > Are you are trying to pre-calculate the addresses of the strings to save > time or something? > > One thing to note is that the address of the A$() or D$() array variable > will move around in RAM. However, as long as you don't modify the strings > *in* the array after they are first assigned, then the > "256*PEEK(K+2)+PEEK(K+1)" address values pointing to the actual text will > NOT change. Those addresses will be pointing back to the actual string > text in the BASIC program (which doesn't move around during program > execution). > > This is an optimization BASIC makes. When strings are first assigned, the > variable is created with an address pointing to the actual string text. > The initial value simply points to the BASIC program. If the contents of > that string variable are changed after that point, then BASIC moves the > text into high RAM to make the modification. Otherwise the text simply > lives in the BASIC program. > > So you could create an array of pre-calculated addresses as follows: > > 10 DIM A$(3):DIMK(3):A$(0)="blah0":A$(1)="blah1":A$(2)="blah2":A$( > 3)="blah3" > 20 FOR X=0TO3:F=VARPTR(A$(X)):K(X)=256*PEEK(F+2)+PEEK(F+1):NEXT > 30 REM > 40 REM Now any variable can be created and the values in K() will still be > correct. > > Ken > > > On 5/29/18 9:25 AM, Stephen Adolph wrote: > > John, I wasn't able to declare variables in any meaningful way to solve > this problem. Only assignment seemed to work. Do you know a trick for that? > steve > > On Tue, May 29, 2018 at 12:21 PM, John R. Hogerhuis <jho...@pobox.com> > wrote: > >> >> On Tue, May 29, 2018 at 5:13 AM Jeffrey Birt <bir...@soigeneris.com> >> wrote: >> >>> >>> Anytime a new scalar (i.e. non-array) variable is created, the >>> addresses of the array variables must all change to make room for the new >>> scalar variable. >>> >>> >>> >>> So BASIC copies all the arrays to a new memory address? That does not >>> seem very efficient. >>> >>> >>> >>> Jeff >>> >>> >>> >> >> I think it’s a trade off. Extra levels of indirection are also >> inefficient. >> >> It’s either take the hit on a “move” when a variable is created or take >> a guaranteed hit every variable access. >> >> And you can avoid the cost by declaring your variables up front. >> >> — John. >> >> >> > >
