Test Whether A Point Is In A Polygon Or Not

[Rick Rothstein]

However, i have a couple questions:

Poly = Polygon

why is this line needed? declaring an other variable seems to be unnecessary to me, can i use just polygon instead of poly?

The Polygon argument can be either a two-dimensional VBA array or a two-dimensional range. If it is a range, assigning that range to a Variant variable converts it to a two-dimensional VBA array so that the rest of the code ends up with the same kind of array as when a real two-dimensional array is inputted. The reason the assignment has to take place within the code is because the coercion to an array does not take place via the argument itself even if that argument is a Variant. So, that line of code makes the overall function flexible enough to handle a real array (assigning such an array to the Variant variable does not change its structure) or a range (because the range gets converted to a real array).

PtInPoly = CBool(NumSidesCrossed Mod 2)

what is the exact purpose of CBool? NumSidesCrossed Mod 2 gives 1 or 0 (remember, PtInPoly is a variant, not boolean) which can be used as the condition input for IF, no need to make the code longer with CBoolean.

I wanted the function to return either TRUE or FALSE directly in case the function was to be used directly in a MessageBox or to fill a TextBox or any other number of uses besides as the argument to an IF function... the idea being the user can implement the function without having to take special steps to format the output for the occasion. As for why PtInPoly is a Variant and not a Boolean... that was so it could accommodate the error messages when called as a UDF directly on a worksheet.

you mention that the first and last point of the polygon have to be the same. Since this is a _must_ condition, why not write the code in such a way, that the last point would be added as the first point. this way no error correction would be necessary, since the last point would be automatically copied from the first point.

It was a design decision to help a user not make a mistake. If the user accidentally grabbed the wrong number of rows from a table of coordinates and inputted that too short range of values, the function would alert the user to the error because the last point did not equal the first point... if I made the function close the polygon automatically, it would blindly calculate values and the user would not be aware some of the answers were incorrect... by forcing the user to make sure the polygon closed, he/she could never make a mistake of accidentally inputting too few points.