Python math module | 易学教程

问题

Whenever I try to use any of the built-in functions of Python's exponentiation and logarithms module, I get an error like this:

NameError: name 'sqrt' is not defined

I have tried using math.sqrt(4),sqrt(4) and sqrt(4.0), but none of them work. The exception is pow, which works as it's supposed to. This is really strange and I'm not sure what's wrong.

回答1:

pow is built into the language(not part of the math library). The problem is that you haven't imported math.

Try this:

import math
math.sqrt(4)

回答2:

You can also import as

from math import *

Then you can use any mathematical function without prefixing math. e.g.

sqrt(4)

回答3:

add:

import math

at beginning. and then use:

math.sqrt(num)  # or any other function you seem neccessary

回答4:

You need to say math.sqrt when you use it. Or, do from math import sqrt.

Hmm, I just read your question more thoroughly.... How are you importing math? I just tried import math and then math.sqrt which worked perfectly. Are you doing something like import math as m? If so, then you have to prefix the function with m (or whatever name you used after as).

pow is working because there are two versions: an always available version in __builtin__, and another version in math.

回答5:

import math #imports math module

import math as m
print(m.sqrt(25))

from math import sqrt #imports a method from math module
print(sqrt(25))

from math import sqrt as s
print(s(25))

from math import *
print(sqrt(25))