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Step by step evaluation of IR_FILTER

This article shows the step by step evaluation of the operation performed by the IR_FILTER function. To start, let us evaluate run this function with IDL setting all of the parameters to [1,1,1]:

IDL> x = [1,1,1]

IDL> c = [1,1,1]

IDL> d = [1,1,1]

IDL> result = ir_filter(c,d,x)

IDL> print, result

      1.00000      3.00000      7.00000

To understand how IDL comes produces this result, let us evaluate the equation provided in the documentation step by step (in this case d0=1 so it has been dropped it out of the equation):

We can evaluate the first element of y with x[0] and c[0]:

We can now use the first element of y to evaluate the second element:

Now we can use the results of the second and first element to evaluate the third element:

Thus, the result [y1,y2,y3] is  [1,3,7] which is the value that is returned by IDL.

Written by DS and Reviewed by MM (3/16/2018)

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