Brian Brian - 1 year ago 131
Python Question

numerical precision of cross product in numpy

I am experiencing what I think is weird behavior using cross product in

with somewhat large values.

E.g., the following seems correct:

r = 1e15
a = array([1, 2, 3]) * r;
b = array([-1, 2, 1]) * r;

c = cross(a / norm(a), b / norm(b));

print(dot(c, a)) # outputs 0.0

But if we raise the exponent by 1, we get:

r = 1e16
a = array([1, 2, 3]) * r;
b = array([-1, 2, 1]) * r;

c = cross(a / norm(a), b / norm(b));

print(dot(c, a)) # outputs 2.0

The numbers get even more bizarre for larger values of the exponent. Does anyone know what is going on here? Thanks!

Answer Source

You are seeing round-off error. By default, array() returns an object with dtype=float64. As you make r bigger and bigger, you're running out of mantissa spaces to represent the array products exactly. Here's a way to test this:

def testcross(r, dt):
    a = array([1, 2, 3], dtype=dt)*r
    b = array([-1, 2, 1], dtype=dt)*r
    c = cross(a/norm(a), b/norm(b))
    return dot(c, a)

for rr in logspace(4, 15, 10):
    print "%10.2f %10.2f %g" % (testcross(rr, float32), testcross(rr, float64)

With the result:

     -0.00       0.00 10000
      0.00      -0.00 166810
      0.00       0.00 2.78256e+06
     -4.00       0.00 4.64159e+07
    -64.00       0.00 7.74264e+08
   1024.00       0.00 1.29155e+10
      0.00       0.00 2.15443e+11
-524288.00       0.00 3.59381e+12
      0.00      -0.02 5.99484e+13
-134217728.00       0.00 1e+15

Notice things aren't "perfect" even for float64 with r=5.99484e13. This shows that the precision is starting to fall apart long before you get to r=1e15, even for float64. As expected, things are much worse with the less-precise float32.

Following up on OP's suggestion: the mantissa fields for 32 and 64 bit floating point representation are 24 and 53 bits, respectively (including the implied bit). Taking log10([2**24, 2**53]), we see that this corresponds with about 7 and 16 orders of magnitude respectively. This corresponds with the tabulated errors showing up around r=4.6e7 for float32 and r=1e16 as originally noted. The round-off happens when the dot-product causes the underlying matrix calculation to subtract large numbers, and the differences can't be represented one or the other large number's mantissa.

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