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I have seen some posts on this, but only for ArcMap. I am looking to take a range of values from two fields and concatenate them into one field to make a unique value. For example:

XMAX     YMAX       GRIDID
332500   5366000    33255366
333000   5366000    33305366

In ArcMap you can do something like (Mid([XMAX]),1,4)+(Mid([YMAX]),1,4).

I know this can be done in a PostGIS database, but for the moment it needs to be done on a shapefile and I am looking to use QGIS.

Thanks

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Are "XMAX" and "YMAX" actually string fields or are they really numeric? – whuber♦ Jun 22 '12 at 15:47
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They are numbers. But I converted the number values to string in new columns and the calculation Jef supplied still didn't work. – Ryan Garnett Jun 22 '12 at 15:50
3  
Why not just combine them numerically then? The simplest example assumes all values of "XMAX" are six-digit multiples of 100 and all values of "YMAX" are seven-digit multiples of 1000: "XMAX"*100+"YMAX"/1000 does the trick. More generally you may need to do some rounding of the values before carrying out this calculation. – whuber♦ Jun 22 '12 at 15:54
Thank you Whuber, that totally did it. Thanks! – Ryan Garnett Jun 22 '12 at 17:06

1 Answer

up vote 2 down vote accepted

In QGIS it's almost the same: substr("XMAX",1,4)||substr("YMAX",1,4) in the field calculator.

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Thank you Jef. I have tried this, but it does not return anything. The field remains NULL. – Ryan Garnett Jun 22 '12 at 15:21
Hm, works for me in 1.8. If you are using an earlier version try: substr(tostring("XMAX"),1,4)||substr(tostring("YMAX"),1,4) – jef Jun 22 '12 at 15:51
I was am using 1.7.4. Is your output new field a String? My input fields are Whole Numbers. – Ryan Garnett Jun 22 '12 at 17:04
no, xmax, ymax and gridid are whole numbers - but the conversions should happen transparently. But I think the expression engine was completely redone in 1.8, so ymmv with 1.7. toint(...) should still help there. – jef Jun 22 '12 at 17:35
Yeah the 1.8 and 1.7 expression engines are different – Nathan W Jun 23 '12 at 5:07

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