问题
I have this array named a of 1242 numbers. I need to get the cosine value for all the numbers in Python.
When I use : cos_ra = math.cos(a) I get an error stating:
TypeError: only length-1 arrays can be converted to Python scalars
How can I solve this problem??
Thanks in advance
回答1:
Problem is you're using numpy.math.cos here, which expects you to pass a scalar. Use numpy.cos if you want to apply cos to an iterable.
In [30]: import numpy as np
In [31]: np.cos(np.array([1, 2, 3]))
Out[31]: array([ 0.54030231, -0.41614684, -0.9899925 ])
Error:
In [32]: np.math.cos(np.array([1, 2, 3]))
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-32-8ce0f3c0df04> in <module>()
----> 1 np.math.cos(np.array([1, 2, 3]))
TypeError: only length-1 arrays can be converted to Python scalars
回答2:
use numpy:
In [178]: from numpy import *
In [179]: a=range(1242)
In [180]: b=np.cos(a)
In [181]: b
Out[181]:
array([ 1. , 0.54030231, -0.41614684, ..., 0.35068442,
-0.59855667, -0.99748752])
besides, numpy array operations are very fast:
In [182]: %timeit b=np.cos(a) #numpy is the fastest
10000 loops, best of 3: 165 us per loop
In [183]: %timeit cos_ra = [math.cos(i) for i in a]
1000 loops, best of 3: 225 us per loop
In [184]: %timeit map(math.cos, a)
10000 loops, best of 3: 173 us per loop
回答3:
The problem is that math.cos expect to get a number as argument while you are trying to pass a list.
You need to call math.cos on each of the list elements.
Try using map :
map(math.cos, a)
回答4:
math.cos() can only be called on individual values, not lists.
Another alternative, using list comprehension:
cos_ra = [math.cos(i) for i in a]
回答5:
Easy Way Motivated by the answer of zhangxaochen.
np.cos(np.arange(start, end, step))
Hope this helps!
来源：https://stackoverflow.com/questions/21043644/calculating-cosine-values-for-an-array-in-python