# Re: Lazy expression evaluation

From: Dave Allured <dave.allured_at_nyahnyahspammersnyahnyah>
Date: Thu, 18 Jun 2009 12:14:32 -0600

Dave B,

I am okay with your strategy, making lazy evaluation the general
rule for array expressions as well as scalars.

So let's be more specific. Am I correct in thinking that this will
apply only to these two cases in the language, for both arrays and
scalars?

False .and. (any expression) always gives False
True .or. (any expression) always gives True

This will be the rule for *any* left side expression, not just when
the ismissing function is used?

There are no other NCL operators, logical or otherwise, where this
could possibly apply?

Also, in these cases, the right hand expression is never computed at
some array points, i.e. all errors and side effects from the right
side are suppressed at those positions? For example, suppressing
illegal math, such as divide by zero?

x = array
flags = (x .gt. 0) .and. (1/x .gt. 10)

Now, I see a problem when the right side contains a function that
returns an array result. This might be covered by the rule for
functions in array expressions -- but I can't find that rule in the
ref. manual! That function is called with which shape? Is the
function called once, returning an array result, e.g. func(x(0:99))?
Or is the function called a hundred times with the argument
iterated, e.g. func(x(0)), func(x(1)), etc? This is important to
know in general, not just for lazy evaluation.

Well thanks for talking this over with me. I hope others will chime
in if they see any related issues about lazy evaluation that we may
have missed.

--Dave

David Brown wrote:
> Dave A.,
>
> My view is that the 'ismissing' function is an explicit exception to
> the rule you quote (otherwise it would return a missing value itself
> instead of True) whose main purpose is to allow you to work around
> the rule when necessary. I think the 'check1' statement below behaves
> properly because by using 'ismissing' and lazy evaluation the right
> hand side of the '.and.' expression never gets evaluated and
> therefore does not figure into the result of the expression as a whole.
>
> I agree with your original premise that 'check2' should work the same
> way, and I now think it is a bug that it does not.
> By the way I found another reference to lazy evaluation in the NCL
> glossary. It is discussed without reference to 'if' statements
> although it does not talk about array logical expressions either:
>
> lazy evaluation
> NCL: The process whereby relational expressions are assigned a value
> as soon as it is possible to do so, without necessarily evaluating
> all of the components in the expression. For example, the expression
> (1 .lt. 3) .or. (2 .lt. 1) can be assigned the value True immediately
> after evaluating (1 .lt. 3) without having to evaluate (2 .lt. 1).
>
> -dave
>
>
> On Jun 16, 2009, at 7:04 PM, Dave Allured wrote:
>
>> Dave B,
>>
>> On the other hand, the missing value rule for general expressions,
>> logical and others, is elegantly stated on the Expressions page:
>>
>> "When any NCL expression is being evaluated, NCL ignores
>> elements that are equal to the value of the "_FillValue"
>> attribute for each variable. When a missing value is
>> ignored, the result of the expression will contain a
>> missing value at the corresponding array index."
>>
>> There are other contexts in which I would really not want to have
>> complications added to this rule. With full knowledge I would
>> probably vote for keeping logical expressions in conformance with
>> this simple rule.
>>
>> This means that (a) a logical expression within an "if" statement
>> must explicitly be an exception to this rule, with some
>> dimensionality and nesting considerations; and (b) the assignment
>> statement for check1 below is not in compliance, and might be a bug
>> (which you already said).
>>
>> This is reminding me of a saying, "Be careful what you ask for!"
>>
>> --Dave
>>
>> Dave Allured wrote:
>>> Dave B,
>>>
>>> Thanks for looking at this. I think that lazy expression evaluation
>>> for array expressions would be beneficial.
>>>
>>> It seems to me that that lazy evaluation was never implemented in
>>> NCL just for "if" statements. Lazy evaluation also works in general
>>> scalar expressions, just not in array expressions. In this example
>>> for current NCL versions, check1 is a scalar expression assignment
>>> which could not have the indicated result without lazy evaluation:
>>>
>>> a = (/ 1,2,3 /)
>>> a@_FillValue = 2
>>> check1 = (.not.ismissing(a(1)) .and. (a(1) .gt. 0))
>>> check2 = (.not.ismissing(a) .and. (a .gt. 0))
>>>
>>> print (check1)
>>> print (check2(1))
>>>
>>> (0) False
>>> (0) Missing
>>>
>>> Also I see in the same documentation under "if statements", there is
>>> an almost explicit reference to using the ismissing function in
>>> array mode with lazy expression evaluation. "The function ismissing
>>> returns an array ..." "Combined with lazy conditional expression
>>> evaluation..." There are other suggestive statements in the same
>>> section. This seems like simply an incomplete implementation of
>>> lazy evaluation.
>>>
>>> Meanwhile, here is a workaround that I have started to use. This
>>> needs only one extra line, and it does not depend on lazy
>>> evaluation. x is an array; the output vmask is also an array of the
>>> same dimensionality:
>>>
>>> vmask = (x .gt. -130. .and. x .lt. 130.)
>>>
>>> This is surely not as efficient as true lazy evaluation in a single
>>> line, but it will do for now.
>>>
>>> --Dave
>>>
>>> David Brown wrote:
>>>> Hi Dave,
>>>>
>>>> This is a very interesting observation. In the NCL reference
>>>> manual lazy
>>>> expression evaluation is only documented in the context of 'if'
>>>> statements, which require a scalar logical expression. The
>>>> documentation
>>>> for '.and.' and '.or.' says only that the operands must be
>>>> logical, but
>>>> does not mention lazy evaluation. Apparently lazy evaluation was
>>>> implemented specifically for 'if' statement evaluation but was never
>>>> generalized to work for array logical expressions.
>>>>
>>>> This leads to the inconsistency that you have pointed out here.
>>>> The code
>>>> could easily be updated to use lazy evaluation for .and. and .or.
>>>> in the
>>>> context of array logical expressions. I am not totally confident
>>>> that
>>>> there might not be some backwards-compatibility issue, but it
>>>> does seem
>>>> like a bug of sorts, so my inclination is to go ahead and make the
>>>> change, noting that the behavior should be clearly documented. The
>>>> development team will discuss.
>>>> -dave
>>>>
>>>>
>>>> On Jun 12, 2009, at 7:22 PM, Dave Allured wrote:
>>>>
>>>>> NCL team,
>>>>>
>>>>> Is lazy expression evaluation supposed to work for array
>>>>> expressions?
>>>>>
>>>>> See attached script. mask1 is scalar and shows the expected
>>>>> result.
>>>>> This is basically the example in the NCL manual under "If
>>>>> statements",
>>>>> with assignment rather than if statement.
>>>>>
>>>>> http://www.ncl.ucar.edu/Document/Manuals/Ref_Manual/
>>>>> NclStatements.shtml
>>>>>
>>>>> For mask2 I expect True, False, True, but NCL returns True,
>>>>> Missing,
>>>>> True. This creates problems for subsequent usage of the mask.
>>>>>
>>>>> I checked this with NCL versions 5.0.1 (pre-release, ca. May
>>>>> 2008) and
>>>>> 5.1.1 (pre-release). The problem was the same in both.
>>>>>
>>>>> uname -a
>>>>> Darwin mac56.psd.esrl.noaa.gov 9.7.0 Darwin Kernel Version
>>>>> 9.7.0: Tue
>>>>> Mar 31 22:54:29 PDT 2009; root:xnu-1228.12.14~1/RELEASE_PPC Power
>>>>> Macintosh powerpc PowerMac7,3 Darwin
>>>>>
>>>>> Please advise. Thank you for taking a look.
>>>>>
>>>>> Dave Allured
>>>>> CU/CIRES Climate Diagnostics Center (CDC)
>>>>> NOAA/ESRL/PSD, Climate Analysis Branch (CAB)
>>>>> http://www.cdc.noaa.gov/psd1/
>>>>> ; Test program for lazy expression evaluation.
>>>>> ; 2009-jun-13 By Dave Allured, NOAA/PSD/CU/CIRES/CDC.
>>>>>
>>>>> begin
>>>>> a = (/ 1,2,3 /)
>>>>> a@_FillValue = 2
>>>>>
>>>>> i=1
>>>>> mask1 = (.not.ismissing(a(i)) .and. (a(i) .gt. 0))
>>>>>
>>>>> mask2 = (.not.ismissing(a) .and. (a .gt. 0))
>>>>> end
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Received on Thu Jun 18 2009 - 12:14:32 MDT

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