CODEWITHSUNDEEP
python
李林泽
李林泽

Reputation: 31

Question about python math.ceil() and math.floor()

This is my code:

num = math.log(536870912) / math.log(2)  # num = 29.0
ceil = math.ceil(num)
floor = math.floor(num)
print(ceil, floor)

The output is (30.0, 29.0)

My question is when I just print math.ceil(29.0) the result is 29.0, why the above code gives me the 30.0 as ceiling?

Upvotes: 2

Views: 376

Answers (3)

Edison
Edison

Reputation: 1020

I could say because

math.log(536870912) / math.log(2) = 29.000000000000004

And

29.000000000000004 != 29.0

As usr2564301 points out, floating point math is inaccurate.

Upvotes: 1

Red
Red

Reputation: 27567

Because num does not equal to 29.0 exactly. If you print it, you will get something like 29.000000000000004:

import math
num = math.log(536870912) / math.log(2)  # num = 29.0
print(num)
ceil = math.ceil(num)
floor = math.floor(num)
print(ceil, floor)

Output:

29.000000000000004
30 29

You can check out this article for more details.

Upvotes: 1

leopardxpreload
leopardxpreload

Reputation: 768

This is due to the inaccuracies of floating point math in python.

Print the number too:

import math
num = math.log(536870912) / math.log(2)  # num = 29.0 (not actually)
ceil = math.ceil(num)
floor = math.floor(num)
print(num, ceil, floor)

29.000000000000004 30 29

ceil() will return the the next highest integer.

I suggest using round()

Upvotes: 3

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