qwaszxee - 1 year ago 153
Python Question

# python float vs numpy.float give different answers?

Hello i don't understand what is happening with my code. The way numpy.float64 acts seems different from float.

i think they should give the same answers but they don't. 2e-05 vs 0.2

main result

``````N = 500.0
1.0/N*(np.absolute((-1.2e-10 -1e+02j))) =  0.2
1.0/N*float(np.absolute(-1.2e-10 -1e+02j)) =  0.2
1.0/float(N*np.absolute(-1.2e-10 -1e+02j)) =  2e-05
N*np.absolute(-1.2e-10 -1e+02j) =  50000.0
``````

code

``````print 'N*np.absolute(-1.2e-10 -1e+02j) = ',N*np.abs(-1.2e-10 -1e+02j)
print 'type(N*np.abs(-1.2e-10 -1e+02j))',type(N*np.abs(-1.2e-10 -1e+02j))
print '1/5000 = ',1/5000
print ''
print '1.0/N*np.absolute(-1.2e-10 -1e+02j) = ',1.0/N*np.abs(-1.2e-10 -1e+02j)
print 'type(1.0/N*np.abs(-1.2e-10 -1e+02j))',type(1.0/N*np.abs(-1.2e-10 -1e+02j))
print ''
print '1.0/N(*np.absolute((-1.2e-10 -1e+02j))) = ',1.0/N*(np.abs(-1.2e-10 -1e+02j))
print 'type(1.0/N*np.abs(-1.2e-10 -1e+02j))',type(1.0/N*(np.abs(-1.2e-10 -1e+02j)))
print ''
print '1.0/N*float(np.absolute(-1.2e-10 -1e+02j)) = ',1.0/N*float(np.abs(-1.2e-10 -1e+02j))
print 'type(1.0/N*float(np.absolute(-1.2e-10 -1e+02j)))',type(1.0/N*float(np.absolute(-1.2e-10 -1e+02j)))
print ''
print '1.0/float(N*np.absolute(-1.2e-10 -1e+02j)) = ',1.0/float(N*np.abs(-1.2e-10 -1e+02j))
print 'type(1.0/float(N*np.abs(-1.2e-10 -1e+02j)))',type(1.0/float(N*np.abs(-1.2e-10 -1e+02j)))
``````

result

``````N*np.absolute(-1.2e-10 -1e+02j) =  50000.0
type(N*np.abs(-1.2e-10 -1e+02j)) <type 'numpy.float64'>
1.0/5000 =  0

1.0/N*np.absolute(-1.2e-10 -1e+02j) =  0.2
type(1.0/N*np.abs(-1.2e-10 -1e+02j)) <type 'numpy.float64'>

1.0/N(*np.absolute((-1.2e-10 -1e+02j))) =  0.2
type(1.0/N*np.abs(-1.2e-10 -1e+02j)) <type 'numpy.float64'>

1.0/N*float(np.absolute(-1.2e-10 -1e+02j)) =  0.2
type(1.0/N*float(np.absolute(-1.2e-10 -1e+02j))) <type 'numpy.float64'>

1.0/float(N*np.absolute(-1.2e-10 -1e+02j)) =  2e-05
type(1.0/float(N*np.abs(-1.2e-10 -1e+02j))) <type 'float'>
``````

thanks for any help.

This is an order of operations issue. Use a few extra parenthesis...

``````>>> 1.0/N*float(np.absolute(-1.2e-10 -1e+02j))
0.2
>>> 1.0/(N*float(np.absolute(-1.2e-10 -1e+02j)))
2e-05
>>> 1.0/N*(np.absolute((-1.2e-10 -1e+02j)))
0.20000000000000001
>>> 1.0/(N*(np.absolute((-1.2e-10 -1e+02j))))
2.0000000000000002e-05
``````

Basically, the difference is that you're comparing:

``````(1./N) * np.absolute((-1.2e-10 - 1e+02j))
``````

with:

``````1./(N * np.absolute((-1.2e-10 - 1e+02j)))
``````

And obviously those are going to have different values (irrespective of whether you're actually using `float` or `np.float64`).

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