Question

When appending longer statements to a list, I feel append becomes awkward to read. I would like a method that would work for dynamic list creation (i.e. don’t need to initialize with zeros first, etc.), but I cannot seem to come up with another way of doing what I want.

Example:

import math
mylist = list()
phi = [1,2,3,4] # lets pretend this is of unknown/varying lengths
i, num, radius = 0, 4, 6

while i < num:
    mylist.append(2*math.pi*radius*math.cos(phi[i]))
    i = i + 1

Though append works just fine, I feel it is less clear than:

mylist[i] = 2*math.pi*radius*math.cos(phi[i])

But this does not work, as that element does not exist in the list yet, yielding:

IndexError: list assignment index out of range

I could just assign the resulting value to temporary variable, and append that, but that seems ugly and inefficient.

Answer 1

In this particular case you could use a list comprehension:

mylist = [2*math.pi*radius*math.cos(phi[i]) for i in range(num)]

Or, if you’re doing this sort of computations a lot, you could move away from using lists and use NumPy instead:

In [78]: import numpy as np

In [79]: phi = np.array([1, 2, 3, 4])

In [80]: radius = 6

In [81]: 2 * np.pi * radius * np.cos(phi)
Out[81]: array([ 20.36891706, -15.68836613, -37.32183785, -24.64178397])

I find this last version to be the most aesthetically pleasing of all. For longer phi it will also be more performant than using lists.