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Beware abs function in Core Data

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Beware abs function in Core Data

Subject: Beware abs function in Core Data
From: Scott Ellsworth <email@hidden>
Date: Wed, 18 May 2005 13:00:35 -0700

Hi, all.

I just posted bugs

4123401 Use of abs funtion in sqllite store produces 'unsupported expression type (4)'

and

4123382 Core Data XML store throws selectorNotRecognized when key path uses abs

The first is a killer, the second merely annoying.

My World object has a 1-1 with a Location object. Inside the Location are normalizedRow and normalizedColumn int16 attributes.

I want those worlds that are close. The formula is the one I mentioned yesterday:

    int deltaNormCol = abs(originNormCol-destinationNormCol);
    int deltaNormRow = abs(originNormRow-destinationNormRow);
    if (deltaNormCol>deltaNormRow){
        return deltaNormCol/2;
    } else {
        return (deltaNormCol + deltaNormRow)/4;
    }

(Those who saw my post of yesterday where I was talking about this - I worked out the data costs of storing a pair of int16s in each World, and decided that it was just not that big a deal.)

To implement this, I created the predicate against the Location entity:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"((abs(%@ - normalizedColumn) > abs(%@ - normalizedRow) AND (abs(%@ - normalizedColumn) / 2 < %@)) OR ((abs(%@ - normalizedColumn) <= abs(% @ - normalizedRow)) AND (abs(%@ - normalizedColumn) + abs(%@ - normalizedRow) <= %@)))",[world valueForKeyPath:@"location.normalizedColumn"], [world valueForKeyPath:@"location.normalizedRow"], [world valueForKeyPath:@"location.normalizedColumn"], [NSNumber numberWithInt:3],[world valueForKeyPath:@"location.normalizedColumn"], [world valueForKeyPath:@"location.normalizedRow"], [world valueForKeyPath:@"location.normalizedColumn"], [world valueForKeyPath:@"location.normalizedRow"], [NSNumber numberWithInt:3]];

When you run this in sqlite, you get

2005-05-18 12:21:35.354 Astrogator[16849] unsupported expression type (4): abs:(-88 - normalizedColumn)

but you get correct answers with an XML store.

One tedious problem - xml stores seems to not handle predicates with keypaths inside functions, so abs(%@ - location.normalizedColumn) fails with

2005-05-18 12:28:18.672 Astrogator[17213] *** -[NSXMLDocumentMapNode objCType]: selector not recognized [self = 0x1d18ab50]

Scott
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