given this code fragment:

            if    (i < (line.length - 3) )
            {    writeln ("in c4: i = ", i, ", line.length = ", line.length);
                  add2 (c4, line [i..i+4]);
I get this result:

in c4: i = 0, line.length = 2
core.exception.RangeError@source/freqs.d(32): Range violation
----------------
??:? _d_arrayboundsp [0x56041325a70d]
??:? _Dmain [0x560413233beb]

Why did this get executed?  The if test was supposed to prevent this.

DMD64 D Compiler v2.097.2

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On Tuesday, 19 October 2021 at 16:20:39 UTC, Charles Hixson wrote:
> given this code fragment:
>
>             if    (i < (line.length - 3) )
>
> in c4: i = 0, line.length = 2

line.length is an unsigned value. Arithmetic on an unsigned thing is still unsigned.

So UNSIGNED 2 - 3 is not -1, but instead it is size_t.max since it rolls over.

Then the comparison also becomes unsigned. So 0 < size_t.max is true, meaning it goes in there.


you should be able to fix it if you do

 if(i < (cast(int) line.length) - 3)

to force it to become signed. But this isn't great either. You probably want to  change the code to avoid going negative in the first place. Maybe test `i + 3 < line.length` instead. Or maybe `if(!(i > ... wahtever that is)`. You get the idea im brain farting.


I personally hate that array.length is unsigned. And I hate that the signed/unsigned mixing prefers unsigned instead of just about anything else. What a pain in the butt. But that's how it is.

On Tuesday, 19 October 2021 at 16:38:50 UTC, Adam D Ruppe wrote:
> test `i + 3 < line.length` instead

BTW this is my personal preference, I have gotten into the habit of using this style tests with lengths all the time now.

On Tuesday, 19 October 2021 at 16:20:39 UTC, Charles Hixson wrote:
> core.exception.RangeError@source/freqs.d(32): Range violation
> ----------------
> ??:? _d_arrayboundsp [0x56041325a70d]
> ??:? _Dmain [0x560413233beb]

> DMD64 D Compiler v2.097.2

By the way, if you upgrade to 2.098.0, you get a better error message for out of bounds array access.

Thank you.  That seems to have solved the problem (bar additional testing).  And also thanks for your recommendation to add to the index rather than casting the length.  It wasn't as "nice" to my eyes at first, but it's a cleaner answer.

On 10/19/21 9:38 AM, Adam D Ruppe via Digitalmars-d-learn wrote:
> On Tuesday, 19 October 2021 at 16:20:39 UTC, Charles Hixson wrote:
>> given this code fragment:
>>
>>             if    (i < (line.length - 3) )
>>
>> in c4: i = 0, line.length = 2
>
> line.length is an unsigned value. Arithmetic on an unsigned thing is still unsigned.
>
> So UNSIGNED 2 - 3 is not -1, but instead it is size_t.max since it rolls over.
>
> Then the comparison also becomes unsigned. So 0 < size_t.max is true, meaning it goes in there.
>
>
> you should be able to fix it if you do
>
>  if(i < (cast(int) line.length) - 3)
>
> to force it to become signed. But this isn't great either. You probably want to  change the code to avoid going negative in the first place. Maybe test `i + 3 < line.length` instead. Or maybe `if(!(i > ... wahtever that is)`. You get the idea im brain farting.
>
>
> I personally hate that array.length is unsigned. And I hate that the signed/unsigned mixing prefers unsigned instead of just about anything else. What a pain in the butt. But that's how it is.

-- 
Javascript is what you use to allow third part programs you don't know anything about and doing you know not what to run on your computer.