Re: Command Languages Versus Programming Languages

scott@slp53.sl.home (Scott Lurndal) writes:

mas@a4.home writes:

On 2024-11-20, Rainer Weikusat <rweikusat@talktalk.net> wrote:

Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
>
Assuming that p is a pointer to the current position in a string, e is a
pointer to the end of it (ie, point just past the last byte) and -
that's important - both are pointers to unsigned quantities, the 'bulky'
C equivalent of [0-9]+ is
>
while (p < e && *p - '0' < 10) ++p;
>
That's not too bad. And it's really a hell lot faster than a
general-purpose automaton programmed to recognize the same pattern
(which might not matter most of the time, but sometimes, it does).

>
int
main(int argc, char **argv) {
unsigned char *p, *e;
unsigned char mystr[] = "12#45XY ";
>
   p = mystr;
   e = mystr + sizeof(mystr);
   while (p < e && *p - '0' < 10) ++p;
>
   size_t xlen = p-mystr;
   printf("digits: '%.*s'\n", (int) xlen, mystr);
   printf("mystr %p p %p e %p\n", mystr, p, e);
   printf("xlen %zd\n", xlen);
>
   return 0;
}
>
./a.out
digits: '12#45'
mystr 0x7ffc92ac55ff p 0x7ffc92ac5604 e 0x7ffc92ac5608
xlen 5
>

>
Indeed.    Rainer's while loop should be using isdigit(*p).

I'm even using

    c &= ~0x20;
    if (c - 'A' < 6) return c - 'A' + 10;

for detecting hex digits (type of c is unsigned). JSON demands UTF8
which implies demanding ASCII (which means that the code point of a
lowercase letter is that of the corresponding uppercase letter but with
the sixth bit additionally set).

Constructs like this are great insurance policy against people inventing
spurious character encodings.