Re: Missing values and divide by zero

The original thread

    z = x/y

To prevent divsion by 0

    x = x/where(y.eq.0, y@_FillValue, y)

On 4/17/12 2:33 PM, David Brown wrote:
> Hi Dave,
>
> I see the behavior you describe for arrays. However, contrary to (1) below, I see the same behavior for scalars:
>
> ncl 9> z1 = -999.0
> ncl 10> z1@_FillValue = -999.0
> ncl 11> print(z1 / 0)
> fatal:divide: Division by 0, Can't continue
> fatal:Div: operator failed, can't continue
> fatal:["Execute.c":7514]:Execute: Error occurred at or near line 11
>
> ncl 12> z1 = (z1 / 0)
> fatal:divide: Division by 0, Can't continue
> fatal:Div: operator failed, can't continue
> fatal:["Execute.c":7514]:Execute: Error occurred at or near line 12
>
> ncl 13> print(z1)
>
> Variable: z1
> Type: float
> Total Size: 4 bytes
> 1 values
> Number of Dimensions: 1
> Dimensions and sizes: [1]
> Coordinates:
> Number Of Attributes: 1
> _FillValue : -999
> (0) -999
>
> Of course, since the "operator failed" the value remains the same as before. That is true whether the value is scalar or an array. In the case of an array, it seems that all values remain the same as before the operation, even those that could be operated on without error.
>
> If you have a counter-example of scalar division of a missing value by 0 working as in (1), please let us know.
>
> This is not to say that your proposed behavior does not have a lot of merit. We will consider making this change, considering that the docs say _FillValues should always propagate.
> -dave
>
>
> On Apr 15, 2012, at 6:14 PM, Dave Allured wrote:
>
>> NCL support team,
>>
>> There seems to be inconsistent behavior when the numerator is a
>> missing value in simple division. In the scalar case:
>>
>> (1) missing value / 0 gives missing value
>>
>> In the array case:
>>
>> (2) missing value / 0 gives a fatal error
>>
>> My interpretation of the current NCL manual is that (1) is the correct
>> behavior, and (2) is wrong. Also, (1) is best behavior for clean user
>> programming. I request that NCL be changed to correctly handle the
>> array case. Either way, would you please explicitly document the
>> expected behavior for (missing divided by zero) in the NCL manual?
>>
>> There is a general workaround for user programs, by setting zeros in
>> the denominator array to missing values. This is less desirable in
>> user cases when missing value information is already contained in the
>> numerator array.
>>
>> I found this behavior consistently through several versions of NCL
>> 5.2.0 through NCL 6.0.0 on Linux and Mac.
>>
>> Following is a short test case which ends in a fatal error. Thank you
>> for your consideration.
>>
>> begin
>> a = new (3, float)
>> a(0) = 3.3
>> a(2) = 5.5
>> print ("A missing = " + ismissing (a))
>> print (a)
>>
>> ; Note, a(1) should be a missing value.
>>
>> b = (/ 4, 0, 4 /)
>> print (b)
>>
>> c = a / b
>> print (c)
>> end
>>
>> --Dave
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Received on Tue Apr 17 14:44:59 2012