[fdo-users] RE: RE: Conditional "IF" in Expressions

Wed Jul 18 11:11:09 EDT 2007

Hi Traian,

Actually, labeling via conditional expressions was just a lightweight
example of their use, but thanks for the tip and the source anyways. I was
thinking to using them to build provider-neutral function defintions and
gain better access to calculated property values, i.e.:

- Height of a Building is calculated using formula NumberOfFloors * 3.5
- but, if Building has RoofType of 1 (it's not flat), then overall Height is
increased by 2.5

if(Bulding.RoofType=1,Building.NumberOfFloors*3.5+2.5,Building.NumberOfFloors*3.5)

So, function IF has three parameters: A (which is always Boolean, and
evaluates to either True or False), and also B and C that actually get
returned as a result, depending on evaluated A. Using state machine it's
possible to create branching of any depth. Parameters B and C need to be of
the same type, since function returns exactly that type. 

As Orest proposed, one of possible conditional function definitions would
look like:

int32 = if(boolean, int32, int32)

or

string = if(boolean, string, string)

etc.

Possiblity of using conditional branching within defined expression is
something just very handy thing.

Regards,
Maksim Sestic

Traian Stanev wrote:
> 
> Hi,
> 
> I assume that since you are talking about labeling, you are using
> MapGuide and my advice below applies. Otherwise, ignore my email.
> 
> For the purposes of MapGuide theming expressions, you can simulate IF by
> creating several theming rules. So in your case you would have one
> theming Rule that is triggered with the condition (Lanes = 1) that draws
> in red for example and another Rule that triggers when (Lanes > 1) that
> draws in yellow for example.
> 
> That said, it is fairly simple to implement an IF as an expression in
> MapGuide, and it is not necessary that argument types match. One problem
> currently is that an FDO function cannot have a conditional as argument
> (the parser can't handle it). So for example something like 
> 
> IF(Lanes>2, 7, 8) 
> 
> would not parse.
> 
> However, if you implement the IF such that if the first argument is say
>>0, then it means true and otherwise false, you can do it. For example
> the following 
> 
> IF (Lanes-2, 'Hello', 12345)
> 
> would be equivalent and would also parse successfully. In order to
> evaluate such a thing in MapGuide, you can use the attached patch (if
> you are running from source of course :). MapGuide still does implicit
> type conversion, so the example I give above with the string and the
> number works fine for conditional labeling, that's what I used to test
> it.
> 
> 
> 
> Traian
> 
> 
> 
> -----Original Message-----
> From: fdo-users-bounces at lists.osgeo.org
> [mailto:fdo-users-bounces at lists.osgeo.org] On Behalf Of Maksim Sestic
> Sent: Wednesday, July 18, 2007 10:18 AM
> To: fdo-users at lists.osgeo.org
> Subject: [fdo-users] RE: Conditional "IF" in Expressions
> 
> 
> Hi Orest,
> 
> You're right, it returns different types depending on it's return
> parameters
> (true and false parts). As a rule, both parameters (therefor function's
> return value, too) must be of the same type, i.e.:
> 
> if(Road.Lanes>1,"Multiple lanes","Single lane")
> 
> Here, return value is String - so both params must conform to this rule.
> This sort of conditional branching is quite useful when building label
> expressions, here's an example:
> 
> - Parcel feature class has two data properties - Number and SubNumber
> (both
> are strings)
> - if there's subnumber set, then it should display "Number/SubNumber"
> (notice slash sign inbetween)
> - otherwise, it displays only "Number" part
> 
> if(Parcel.SubNumber="",Parcel.Number,concat(Parcel.Number,(concat("/",Pa
> rcel.SubNumber))))
> 
> This happens even more often when it comes to labeling infrastructure
> entities, since it's labels depend on multiple states of a network
> element,
> etc.
> 
> Certainly, labeling is not the only use of conditional branching of that
> sort :-)
> 
> Regards,
> Maksim Sestic
> 
> 
> 
> Orest Halustchak wrote:
>> 
>> Hi Maksim,
>> 
>> This seems doable. It's similar to a null value function that some
>> providers such as rdbms providers could support: nvl(a,b) which
> returns
>> a if a is not null, otherwise b. In your case it's if(a,b,c) where a
> is
>> boolean. This would need a number of signatures for different data
> types
>> that it would return, e.g. int16 = if(boolean, int16, int16), but
>> FdoFunctionDefinition can deal with it.
>> 
>> The main concern that I would have about it is implementation in an
>> efficient manner, although for rdbms providers, SQL includes a CASE
>> expression that could be used.
>> 
>> Thanks,
>> Orest.
>> 
>> -----Original Message-----
>> From: fdo-users-bounces at lists.osgeo.org
>> [mailto:fdo-users-bounces at lists.osgeo.org] On Behalf Of Maksim Sestic
>> Sent: Wednesday, July 18, 2007 4:52 AM
>> To: fdo-users at lists.osgeo.org
>> Subject: [fdo-users] Conditional "IF" in Expressions
>> 
>> 
>> I was wondering if future versions of FDO will support conditional
>> branching
>> in Expressions (and Filtering) mechanism, i.e.:
>> 
>> if(conditional_expression,true_part,false_part)
>> 
>> if(Road.Lanes>1,"Multiple lanes","Single lane")
>> 
>> also
>> 
>> if(Road.Lanes>0,if(Road.Lanes>1,"Multiple lanes","Single lane"),"No
>> lanes
>> set")
>> 
>> Useful link:
>> http://www.codeplex.com/dotMath Steve Heberts dotMath library 
>> 
>> Regards,
>> Maksim Sestic
>> 
>> 
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