`Floor`

With math.floor we reduce a number so that the fractional part is removed. Floor() will make all numbers smaller. Negative values will become more negative.

With floor, we get integers from floating-point numbers. Floor has no effect on integers. We must "import math" at the top of our programs.

Initial example

Here we use math.floor on some numbers. Note that the first number 100 is returned unchanged. Floor() does not modify integers.

Detail Floor does not round upon the value 100.9. Instead it always reduces the number to be less.

import math

# Some numbers to take floors of.
value0 = 100
value1 = 100.1
value2 = 100.5
value3 = 100.9

# Take floor of number.
floor0 = math.floor(value0)
print(value0, ":", floor0)

# Take other floors.
print(value1, ":", math.floor(value1))
print(value2, ":", math.floor(value2))
print(value3, ":", math.floor(value3))100 : 100
100.1 : 100
100.5 : 100
100.9 : 100

`NameError`

Sometimes when writing Python programs I make this error. There is no "floor" in Python. We must use math.floor to use the floor function.

number = 78.6
# This will not work.
result = floor(number)Traceback (most recent call last):
  File "C:\programs\file.py", line 5, in <module>
    result = floor(number)
NameError: name 'floor' is not defined

Negative floors

With negative numbers, math.floor has the same logical result as with positive ones. Numbers are always reduced. Negative numbers will become more negative.

import math

# Use math.floor on a negative number.
result = math.floor(-1.1)
print(result)

result = math.floor(-1.9)
print(result)-2
-2

Benchmark, floor dictionary

I wanted to test whether a dictionary lookup could be faster than a math.floor call. I found math.floor is fast—much faster than calling get().

Version 1 This version of the code uses math.floor to get the floor of each number.

Version 2 Here we look up a value in a dictionary to get a cached floor value for a number.

Result Using a dictionary to memoize (cache) the result of math.floor is a big slow down. Just use math.floor directly.

import time, math

# Floor dictionary.
floor_dict = {100.5: 100}

print(time.time())

# Version 1: use math.floor.
for i in range(0, 100000000):
    y = 100.5
    z = math.floor(y)
    if z != 100:
        print(z)
        break

print(time.time())

# Version 2: use dictionary lookup, get method.
for i in range(0, 100000000):
    y = 100.5
    z = floor_dict.get(y)
    if z != 100:
        print(z)
        break

print(time.time())1454633830.142
1454633830.727    math.floor =     0.59 s
1454633839.844    floor_dict.get = 9.12 s
                  PyPy3 used

A review

In Python 3 we find many built-in functions like abs and round. With floor, though, we have to use the math module with an import statement.