[Qgis-developer] QgsExpression: replacement for QgsSearchString

[Tim Sutton]

Hi

I'm getting  a lot of win build errors relating to qgsexpression:


http://pastie.org/2444030


It could just be some stale binaries left behind that msvc is hanging
on to - still investigating...

Regards

Tim

On Sun, Aug 21, 2011 at 1:56 PM, Nathan Woodrow <madmanwoo at gmail.com> wrote:
> Looking good Martin.  No objections from me.  The old QgsSearchString was a
> bit string in the way it handled a few things so I'm glad it's been
> simplified in your chances.
> I'm just merging with my expression labeling branch, which used
> QgsSearchString at the moment. I will report if I find any bugs.
> - Nathan
>
> On Sat, Aug 20, 2011 at 6:51 AM, Martin Dobias <wonder.sk at gmail.com> wrote:
>>
>> Hi everyone
>>
>> recently I have been trying to fix some flaws in search string
>> implementation and it turned out that rather than fixing them
>> one-by-one it will be easier to rewrite that code completely. Search
>> string and associated searching in attribute table was one of my first
>> contributions to QGIS and my first attempt to write a parser and an
>> evaluator. QgsExpression is my second attempt :-)
>>
>> The main reason for the redesign was to bring the evaluation closer to
>> SQL semantics and to clean the code which started to be hard to
>> maintain.
>>
>> QgsExpression is meant to be a replacement for QgsSearchString class.
>> Most of the changes are under the hood:
>> - fixed and simplifed the parser grammar and tree creation
>> - simplified lexer
>> - only one evaluation routine (instead of separate getValue/checkAgainst)
>> - correct handling of NULL values and three-value logic from SQL
>> (true, false, unknown)
>> - fixed error reporting
>> - easily extensible list of functions, saner evaluation of expressions
>> - using QVariant for return values instead of a special type
>> - slightly faster evaluation
>> - a more suitable class name
>> - tests included (!!!) :-)
>>
>> Only few changes are visible for users:
>> - spaces were removed from "to int", "to real" and "to string"
>> functions because function names must not contain spaces
>> - null values are handled correctly by the functions and operators
>> - errors are reported when conversions between types fail
>>
>> Btw. type conversions (currently supporting null, int, double, string)
>> are implemented in many different ways in databases. I have done some
>> tests on sqlite3, postgresql and mysql and many times there was no
>> consensus what the outcome of an evaluation should be:
>> - 7 > '6x' :: mysql=1, sqlite=0, postgresql=error
>> - 7 > '6' :: mysql=1, sqlite=0 (!), postgresql=1
>> - '6x' :: mysql=6 (+warning), sqlite=6, postgresql=error
>> - '3'+'5' :: mysql=8, sqlite=8, postgresql=error
>> - 3/5 :: mysql=0.6, sqlite=0, postgresql=0
>> - length(123) :: mysql=3, sqlite=3, postgresql=error
>> - 0 or 1 :: mysql=1, sqlite=1, postgresql=error
>> - 0 or 'x' :: mysql=0 (+warning), sqlite=0, postgresql=error
>>
>> PostgreSQL is very strict about type conversions, Sqlite is not strict
>> at all and MySQL is somewhere in the middle. I have ended up with
>> these conversion rules:
>> - implicit conversions to desired types
>> - string->number conversions with invalid input return error
>> - arithmetic operators convert to numeric
>> - comparison with numeric type => numeric comparison
>>
>>
>> The code is here:
>> https://github.com/wonder-sk/Quantum-GIS/tree/expr
>>
>> If there are no objections I will apply the changes to master in few
>> days. Given that I have been simultaneously writing unit tests I am
>> quite confident that there are not too many bugs (unit tests really
>> help!). In that branch I have replaced all the occurrences of search
>> string with expressions. So playing with search in attribute table or
>> doing calculation in field calculator will use the new engine.
>>
>>
>> Martin
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>
>
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>



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Tim Sutton - QGIS Project Steering Committee Member (Release  Manager)
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