simplifying sums of numbers



Hi,
  I have a linear interpolation function f made from around 500 data
  points in the plane

Using linearinterpol gives a function with many summands (as a sum of
characteristic functions of affine maps).  I have two questions.

1. In actually evaluating the function f at some point x there seems
    to be a lot of wasted time since a lot of zeroes are being summed.
    Is there a better way, short of rewriting the routine?

2. Even using the routine, the answer shows up as a sum of many
    numbers times characteristic functions.  How do I see the actual
    sum as a floating point number?

TIA,
  -sen

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