topic How to use column name as a variable. in Discussions

How to use column name as a variable.

qt — Wed, 17 Oct 2012 23:36:47 GMT

I want to select some rows from a column ( I have a column named as 'RESOLUTION' in the current data table)

This two lines will work:

======================================

dt = Current Data Table();

dt<<Select Where(RESOLUTION > 0.);

======================================

If I define 'RESOLUTION' as another variable, it doesn't work:

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dt = Current Data Table();

colA="RESOLUTION";

dt<<Select Where(column(dt, colA) > 0.);

============================================

This will not select any row.

Could someone kindly tell me how could I do this?

Thanks.

Re: How to use column name as a variable.

pmroz — Thu, 18 Oct 2012 20:24:33 GMT

qt,

This will do the trick.

dt = Current Data Table();

colA = "Resolution";

col_expr = "dt << Select Where(:name(\!"" || colA || "\!") > 0.)";

eval(parse(col_expr));

Basically you're creating some dynamic code, and then executing it.

Re: How to use column name as a variable.

qt — Thu, 18 Oct 2012 20:58:56 GMT

It works!!

Many thanks!

Re: How to use column name as a variable.

David_Burnham — Thu, 18 Oct 2012 21:07:18 GMT

Alternatively use the AsColumn function:

colA="RESOLUTION";

dt<<Select Where(AsColumn(colA) > 0.)

Re: How to use column name as a variable.

tsl — Fri, 18 Jun 2021 20:00:00 GMT

One thing to watch with using As Column() is that it seems to be much slower than using the eval( parse(..)) syntax for a large data table.

In my case I had a data table with 390k rows and 17 columns.

Using Eval ( Parse ( .... )) took 67 msec to select the rows I was looking for while using the As Column() syntax took 3,600 msec ! ( using HPTime() to measure )

So while the As Column() syntax is a whole lot more readable in the code, if speed matters, it may not be the best option

Re: How to use column name as a variable.

jthi — Fri, 18 Jun 2021 20:35:19 GMT

It might make AsColumn() faster if you combine it with Column().

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