CODEWITHSUNDEEP
python
casso
casso

Reputation: 329

Calculation error with pow operator

What should print (-2 ** 2) return? According to my calculations it should be 4, but interpreter returns -4.
Is this Python's thing or my math is that terrible?

Upvotes: 18

Views: 2087

Answers (4)

Jaco
Jaco

Reputation: 27

Python has a problem and does not see the -2 as a number. This seems to be by design as it is mentioned in the docs.

-2 is interpreted as -(2) {unary minus to positive number 2}

That usually doesn't give a problem but in -a ** 2 the ** has higher priority as - and so with - interpreted as a unary operatoe instead of part of the number -2 ** 2 evaluates to -2 instead of 2.

Upvotes: -2

vaultah
vaultah

Reputation: 46533

According to docs, ** has higher precedence than -, thus your code is equivalent to -(2 ** 2). To get the desired result you could put -2 into parentheses

>>> (-2) ** 2
4

or use built-in pow function

>>> pow(-2, 2)
4

or math.pow function (returning float value)

>>> import math
>>> math.pow(-2, 2)
4.0

Upvotes: 29

angel
angel

Reputation: 332

you can also use math library...

math.pow(-2,2) --> 4
-math.pow(2,2) --> -4
math.pow(4,0.5) --> 2

Upvotes: 1

Maxime Lorant
Maxime Lorant

Reputation: 36161

The ** operation is done before the minus. To get the results expected, you should do 

print ((-2) ** 2)

From the documentation:

Thus, in an unparenthesized sequence of power and unary operators, the operators are evaluated from right to left (this does not constrain the evaluation order for the operands): -1**2 results in -1.

A full detail of operators precedence is also available in the documentation. You can see the last line is (expr) which force the expr to be evaluated before being used, hence the result of (-2) ** 2 = 4

Upvotes: 6

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