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Using the _SUM function to improve the efficiency of complex yield arithmetic
Woodstock 3.25 includes a yield table summation function (_SUM) for adding together previously defined yield tables, implemented using a variant Dynamic Link Library (DLL) developed by Remsoft and included with the software. The _SUM function offers a much more efficient, and thus significantly faster, option for summing yields than standard complex yield arithmetic. The purpose of this tech tip is to describe complex yield arithmetic and how to use the _SUM function to add together yield tables.
Let's begin with a brief description of yield table arithmetic in Woodstock. Yield table arithmetic is any mathematical manipulation of a declared yield table to derive a new yield table. For example, you may create new yield tables by adding, subtracting, multiplying or dividing declared yield tables by another declared yield table or by a numeric constant. All of these types of yield table manipulations are collectively referred as yield arithmetic. Yield arithmetic may be performed either locally by creating new tables directly within each yield set, or globally by declaring a single complex yield set referencing previously declared yield tables.
With respect to resource use and processing speed, the first option-local yield arithmetic-is vastly more efficient than the second-complex yield arithmetic-because only one instance of the yield table is built and all matching development types assigned to it, rather than building a unique instance of the new table for every matching development type. Globally-masked complex yield tables are especially demanding, requiring a unique instance of the yield table built for every defined development type.
From a model coding perspective, however, global complex yield arithmetic is vastly more efficient than local because the yield arithmetic need only be declared once, as opposed to in every local yield set. Using the _SUM function, Woodstock checks to see if a complex yield table has already been built for a matching development type thus rendering complex yield arithmetic as efficient as local yield arithmetic.
The Yield section _SUM function is structured as follows:
mask - a valid sequence of landscape thematic attributes
acomp# - an age-dependent yield component code
# - a complex yield component code
*YC mask
ccomp1 _SUM(acomp1,acomp2)
Example: suppose that your model contains two age-dependent yield tables, one called PLPYLD representing standing pulpwood, and one called LOGYLD representing standing sawlogs, and that you wish to sum them to determine the total standing wood volume.
To create a total volume yield table for every development type in the model, declare the following in the Yields sections, AFTER the age-dependent yield declarations:
*YC ?
TOTYLD _SUM(PLPYLD,LOGYLD)
If the yield tables to sum are numerous, you may exceed the line length limit in Woodstock input files. You can workaround the limit by performing the summation across multiple complex yields and then summing them to arrive at the grand total. Structurally, this would look like:
*YC mask
ccomp1 _SUM(acomp1,acomp2)
ccomp2 _SUM(acomp3,acomp4)
ccomp3 _SUM(ccomp1,ccomp2)
The _SUM function presently works only with age-dependent yields. You must declare compound complex yields-those composed of age- and time-dependent yield tables-and time dependent complex yields using standard complex yield arithmetic.
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