python - If/Then apply different function depending on each value in array

Question

I have an allegedly easy to solve question, but i still cannot figure it out:

I have an array with 1000 numbers called "mu" like this:

array([2.25492522e-01, 2.21059993e-01, 2.16757006e-01,....)

Now i need to plug these values in two different functions: For numbers in the array, that are less than 0.009, i need to use equation1:

nu = 1 - 5.5 * mu**(0.66) + 3.77 * mu

For all other numbers in the array, i need to plug these into equation2:

nu = 0.819**(-11.5*mu)+0.0975**(-70.1*mu)

In the end, i need an array of the function values "nu".

I gave this code a try, but it didn't work

for item in mu:
    if item < 0.009:
       nu = 1 - 5.5 * mu**(0.66) + 3.77 * mu
    else:
       nu = 0.819**(-11.5*mu)+0.0975**(-70.1*mu)

print nu

How can I tell Python to put the right numbers in?

标签： pythonarrayspython-2.7numpy

Answer 1

一个问题是您没有item在for循环中使用。您也不是附加到列表或分配给新数组来存储结果。无论如何，NumPy 具有为此任务设计的特定功能。例如，使用numpy.where：

def func1(x):
    return 1 - 5.5 * x**(0.66) + 3.77 * x

def func2(x):
    return 0.819**(-11.5*x)+0.0975**(-70.1*x)

res = np.where(mu < 0.009, func1(mu), func2(mu))

虽然您可能会觉得处理的计算量是所需的两倍，但效率低下，但矢量化操作的好处远远超过了这一点。