python: dividing a number by a numpy array

Question

I have tried the top two solutions here and the bottom solution since it dealt with numpy but nothing worked.

I wanted to take 80.0 divided each element of my array name that a new array dx.

import numpy

L = 80.0
N = []


for n in range(-4, 10):
    N.append(str(2 ** N))


N = np.array([N])

This is the set up. So what I have tried is:

dx = L / N

dx = map(lambda x: L / x, N)
dx = np.array([dx])

Lastly, keeping N as a list and keeping N as a numpy array and doing
```
dx = [x / N for x in N]
dx = np.array([dx])
```

Unfortunately, I haven't been able to find a post that helped or anything in the documentation. What can I do to achieve the desired result?

@georgesl I thought it had to be a string to go into a list. — dustin
– dustin, Commented Sep 25, 2013 at 12:25
Nope, list can support any type, also different ones in the same list. — Aleksander Lidtke
– Aleksander Lidtke, Commented Sep 25, 2013 at 12:27
Further, this would give you a numpy array of strings, which cannot be divided with ints. — SethMMorton
– SethMMorton, Commented Sep 25, 2013 at 13:41

tamasgal · Accepted Answer · 2013-09-25 13:14:21Z

4

Your code contains several bugs and you have a lot of unnecessary casts, but however: why don't you try this with numpy directly?

Something like this:

import numpy as np

L = 80.0
N = 2 ** np.arange(-4, 10, dtype=np.float64)
dx = L / N

gives you the expected result

array([  1.28000000e+03,   6.40000000e+02,   3.20000000e+02,
         1.60000000e+02,   8.00000000e+01,   4.00000000e+01,
         2.00000000e+01,   1.00000000e+01,   5.00000000e+00,
         2.50000000e+00,   1.25000000e+00,   6.25000000e-01,
         3.12500000e-01,   1.56250000e-01])

Btw. you can also implicitly force the dtype to be float when using dots:

N = 2 ** np.arange(-4., 10.)

edited Sep 25, 2013 at 13:14

answered Sep 25, 2013 at 12:26

tamasgal

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1 Comment

dustin Over a year ago

Thanks. You have an extra r in arange.

Nipun Batra · Accepted Answer · 2013-09-25 12:51:10Z

2

You could do it in a single line using List comprehensions.

In [8]: N=[80.0/(2**n) for n in range(-4,10)]

In [10]: print N
[1280.0, 640.0, 320.0, 160.0, 80.0, 40.0, 20.0, 10.0, 5.0, 2.5, 1.25, 0.625, 0.3125, 0.15625]

You can avoid using Numpy for such tasks.

The for loop equivalent of this would be (without preallocating N):

In [11]: N=[]

In [12]: for n in range(-4,10):
   ....:     N.append(80.0/(2**n))
   ....:     

In [13]: print N
[1280.0, 640.0, 320.0, 160.0, 80.0, 40.0, 20.0, 10.0, 5.0, 2.5, 1.25, 0.625, 0.3125, 0.15625]

edited Sep 25, 2013 at 12:51

answered Sep 25, 2013 at 12:45

Nipun Batra

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