17.6.1.0245 /STRING slash-string STRING
Adjust the character string at c-addr1 by n characters. The resulting character string, specified by c-addr2 u2, begins at c-addr1 plus n characters and is u1 minus n characters long.
See:
Rationale:
S" ABC" 2 /STRING 2DUP TYPE \ outputs "C"
-1 /STRING TYPE \ outputs "BC"
Testing:
T{ s1 10 /STRING -4 /STRING -> s1 6 /STRING }T
T{ s1 0 /STRING -> s1 }T
ContributeContributions
JimPeterson
[203] Possible Reference ImplementationSuggested reference implementation2021-05-21 17:58:29
Would this be a sufficient reference implementation?:
: /STRING DUP >R - SWAP R> CHARS + SWAP ;
Are there no checks for when u2 might go negative (or, really, wraps around, as it's considered unsigned)?
ruv
Are there no checks for when u2 might go negative?
Formally, u2 cannot be negative since it's an unsigned number (a number that is interpreted as an unsigned number). See data type symbols and their meaning.
In some use-cases it could be a problem if n > u1, but it's out of the scope of the standard, if a user tries to shoot himself in the foot. OTOH, it could be a correct and expected intermediate result.
Would this be a sufficient reference implementation?
Yes, I think. Other possible variants:
: /string ( c-addr1 u1 n -- c-addr2 u2 ) tuck - >r chars + r> ;
: /string ( c-addr1 u1 n -- c-addr2 u2 ) tuck - -rot chars + swap ;
AntonErtl
Gforth, iForth, lxf, SwiftForth 3.11, and VFX 5.11 do not check for n<=u; so if n>u, the result is broken (I explored whether you could do something useful with strings with negative length, but did not come up with anything). I dimly remember that there are systems where /STRING behaves like OVER MIN /STRING, so the resulting string is always valid, but I don't remember which systems behave that way.
JimPeterson
I prefer your first implementation over the other two options. It seems like it would be the most efficient on many systems.