Re: Interval Comparisons

On 04/06/2024 16:27, Scott Lurndal wrote:

David Brown <david.brown@hesbynett.no> writes:

On 04/06/2024 13:23, bart wrote:

 

It is incredibly useful:
>
     if c in [' ', '\t', '\n'] then ... # whitespace

 if (strpbrk(c, " \t\n") != NULL) it_is_whitespace.

That doesn't do the same thing. In my example, c is a character, not a string.
To achieve the same thing using strpbrk requires code like this:
     char c[2];
     c[0]=rand()&255;            // Create a string
     c[1]=0;
     if (strpbrk(c, " \t\n") != NULL) puts("whitespace");
If I compile this with gcc -O3, then the checking part is this:
     lea rcx, 46[rsp]
     mov BYTE PTR 47[rsp], 0
     lea rdx, .LC0[rip]
     mov BYTE PTR 46[rsp], al
     call    strpbrk             // CALL TO LIBRARY FUNCTION
     test    rax, rax
     je  .L2
     lea rcx, .LC1[rip]
     call    puts
I don't know what it gets up to inside strprbk. If I write this in my language:
     if c in [9,10,32] then
         puts("whitespace")
     fi
The generated code is this (using alternate register names, D0 = rax):
     mov   D0, D3      # (could have tested D3 (= c) directly.)
     cmp   D0, 9
     jz    L4
     cmp   D0, 10
     jz    L4
     cmp   D0, 32
     jnz   L3
L4:
     lea   D10, [L5]
     call  puts*
L3:
Anyway, the construct is not limited to character codes that can be contained within a string. It works for 64-bit values which can include 0. And it could be extended to other scalar types like floats and pointers.