Moving minimum (moving window)

[#1]

Does anyone know how to calculate a moving minimum (or for that matter how to iteratively select X number of lag rows and do anything to them such as calculate a min, max, range, etc. In this specific example i want to look back over say the last 100 rows and get the minimum value, then move to the next row down and do the same again (over and over). 

[#2]

The macro below isn't particularly elegant, but will do the job. 

For ( i= 100, i <= N Rows ( dt ), i++,

 dt << Select Rows (Index (i-99, i));
 
 dt << Subset(
	Selected Rows( 1 ),
	Selected Columns ( 0 ),
	Output Table ("out")
);

Current Data Table ( Data Table ("out"));

x = Col Minimum ( :YourDataColumn );

Close ( Data Table ( "out"), no save);

Column (  "YourMovingWindow" )[i] = x;

)

Sub-setting the selected data, the way I've done it, is slow. Perhaps another user has a better idea how to access the selected data and extract the desired product.

I also wonder how a column formula could look like to achieve the results without a macro.

Cheers,

Kofi

[#3 · Accepted solution]

Would something like this work?

Names default to here( 1 );
dt = open("$SAMPLE_DATA\Big Class.jmp");

dt << New Column("Min height 5 back", 
	Formula(
		Min( // since we want a minumum 
			:height[// the column we want the minimum of
				// a slice of the current row minus 5 (minimum 1 so it's still on the datatable)
				// and current row
				Min( Row() - 5 )::Row() 
			] 
		)
	)
);