Faster float to int conversion in Python

Question

Here's a piece of code that takes most time in my program, according to timeit statistics. It's a dirty function to convert floats in [-1.0, 1.0] interval into unsigned integer [0, 2**32]. How can I accelerate floatToInt?

piece = []
rng = range(32)
for i in rng:
    piece.append(1.0/2**i)

def floatToInt(x):
    n = x + 1.0
    res = 0
    for i in rng:
        if n >= piece[i]:
            res += 2**(31-i)
            n -= piece[i]

    return res

Joachim Sauer · Accepted Answer · 2010-03-23 12:53:20Z

5

Did you try the obvious one?

def floatToInt(x):
    return int((x+1.0) * (2**31))

answered Mar 23, 2010 at 12:53

Joachim Sauer

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3 Comments

PaulMcG Over a year ago

231 evaluates to 2147483648 every time. Since you are focused on speed here, replacing 231 with this constant is one less calculation you'll need to do in your time-critical code segment.

pixelbeat Over a year ago

I checked with timeit and 2**31 is only slower when there is no "x" in the equation??

PaulMcG Over a year ago

Import the dis module to disassemble the code for floatToInt using dis.dis(floatToInt) to see that Python evaluates this constant internally, so this part of the calculation is not done at loop runtime.

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