Re: ensure numeric comparison

[Peng Yu]

On 5/8/22, Neil R. Ormos <ormos-gnulists17@ormos.org> wrote:
> Peng Yu wrote:
>> david kerns wrote:
>>> Peng Yu wrote:
>>>> david kenrs wrote:
>>>>> Peng Yu wrote:
>
>>>>>> I want to make sure whether the following
>>>>>> code is guaranteed to compare $1 and $2
>>>>>> numerically as long as the input of the 1st
>>>>>> and 2nd fields are legitimate numbers or
>>>>>> empty strings (which are considered as 0).
>
>>>>>> awk -e '{ print ($1 < $2) }' < input
>
>>>>>> Or there is any corner case when even the
>>>>>> 1st and 2nd fields are legitimate numbers or
>>>>>> empty strings, they may be compared as
>>>>>> strings.
>
>>>>> pretty sure the only way to guarantee numeric
>>>>> comparison is to add 0
>
>>>>> awk -e '{ print ($1 +0 < $2 + 0) }' < input
>
>>>>> (not sure if it's required on both sides, but
>>>>> it can't hurt)
>
>>>> Is there a test case for which `awk -e '{
>>>> print ($1 < $2) }' < input` can not compare $1
>>>> and $2 numerically given the input columes 1
>>>> and 2 are numbers or empty strings?
>
>>> see 6.3.2.1 String Type versus Numeric Type in
>>> https://www.gnu.org/software/gawk/manual/gawk.html
>
>> But my question involves empty string which is
>> "unassigned". It is not explained in the 3x3
>> table in that section.
>
> The table in the manual does appear to answer your question because the
> cited section of the manual also states, "Uninitialized variables also have
> the strnum attribute."
>
> See also Sec. 4.2 Examining Fields:
>
> | If you try to reference a field beyond the last
> | one (such as $8 when the record has only seven
> | fields), you get the empty string. (If used in a
> | numeric operation, you get zero.)
>
> I don't know if there are any test cases that satisfy the "[...] can not
> compare $1 and $2 numerically [...]" constraint.
>
> However, if you do not trust that missing elements of $N are "uninitialized
> variables" that have the strnum attribute, you can resolve the question at a
> practical level by following David Kerns' suggestion of explicitly coercing
> the comparands to numeric type.
>
> As a simplification to David Kern's suggestion, I would use the unary "+"
> operator:
>
>   print ( +$1 < +$2 )

But this does not answer my question. If I have to use "+" or "+0" to
ensure numeric comparison, what would cause the vanilla comparison "<"
to fail under the condition "the input of the 1st and 2nd fields are
legitimate numbers or empty strings (which are considered as 0)."? I
can not see a test case for this to fail, so I don't see "+" or "+0"
as necessary under this condition.

-- 
Regards,
Peng