如何在Python中执行向量化运算？

Question

I'm having an issue with my simple code that is suppose to be a mortgage calculator where all the rates from 0.03 to 0.18 are listed in a table. Here is my code and error.

l = 350000 #Loan amount
n = 30 #number of years for the loan
r = [0.03,0.04,0.05,0.06,0.07,0.08,0.09,0.10,0.11,0.12,0.13,0.14,0.15,0.16,0.17,0.18] #interest rate in decimal

n = n * 12
a = l
int1 = 12
u = [x / int1 for x in r]

D = (((u+1)**n)-1) /(u*(u+1)**n)

z = (a / D)
print(z)

---

File "test.py", line 23, in <module>
    D = (((u+1)**n)-1) /(u*(u+1)**n)
TypeError: can only concatenate list (not "int") to list

Thanks

Answer 1

The problem is that u is a list which cannot be used for vectorized operation which you are doing while computing D . You can convert your list to a NumPy array to make your code work.

u = np.array([x / int1 for x in r])

Alternatively, you can use a for loop or list comprehension to store D for each element of u as

D = [(((i+1)**n)-1) /(i*(i+1)**n) for i in u]

but this will again complain during z = (a / D) because D is still a list. Therefore, converting to array seems to be a convenient approach.

The another alternative answer is to compute z using list comprehension directly without involving extra variable D

z = [a / (((i+1)**n)-1) /(i*(i+1)**n) for i in u]

Answer 2

The current error you're facing is because u is a list (made via a list comprehension ), and D tries to perform math operations between u (a list) and numbers. That won't work.

Try this:

import numpy as np
u = np.array([x / int1 for x in r])

u will be a NumPy array , which allows you to do vector math with it. If you've never used the numpy module, it's an easy install using the pip package manager. If it's not installed then

import numpy as np

will throw an error, and you will not be able to use a NumPy array. If you find yourself doing similar work often, it's likely worth the installation.