how to extract value from a numpy.ndarray

Question

I'm using Scipy.Optimize.fmin to find the maximum of a function. The output is in the form of a numpy.ndarray with additional information on the process. I need just the x value returned as a float.

def f(x):
    """returns the value of f(x) with the input value x"""
    import math
    f = math.exp(-x ** 2.0) / (1.0 + x ** 2.0) + \
        2.0 * (math.cos(x) ** 2.0) / (1.0 + (x - 4.0) ** 2.0)
    return f

def find_max_f():
    """returns the x for which f(x) takes the maximum value"""
    import scipy.optimize as o
    m = o.fmin(lambda x: -f(x), 0)
    return m

This is what it returns:

>>> find_max_f()
Optimization terminated successfully.
     Current function value: -1.118012
     Iterations: 12
     Function evaluations: 24
array([ 0.0131875])

I just need the final number inside the brackets labeled array

What is the return type of fmin()? If it's an iterable you might be able to grab it using the index. — kylieCatt
– kylieCatt, Commented Jan 9, 2014 at 20:42

Glorfindel · Accepted Answer · 2023-01-09 21:03:51Z

3

Simply bind the result to something and then you can index the first element as if it were a list or tuple:

>>> xopt = find_max_f()
Optimization terminated successfully.
         Current function value: -1.118012
         Iterations: 12
         Function evaluations: 24
>>> xopt
array([ 0.0131875])
>>> xopt[0]
0.013187500000000005
>>> type(xopt[0])
<type 'numpy.float64'>

I recommend reading the NumPy Tutorial, in particular the section on "Indexing, Slicing and Iterating".

edited Jan 9, 2023 at 21:03

Glorfindel

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answered Jan 9, 2014 at 20:44

DSM

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2 Comments

Joe Kington Over a year ago

You might also want to mention unpacking. E.g. xopt, = find_max_f() (or m, = o.fmin(...) inside find_max_f.) It winds up confusing a lot of newcomers, but it's a common idiom, so it might be good for the OP to see it as well.

DSM Over a year ago

I think you just did. :^) Can't quite explain why I don't use that pattern, but I never use it in my own code.

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