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From: bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: Interval Comparisons
Date: Tue, 4 Jun 2024 16:58:43 +0100
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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.