Python Cookbook

Spanning a Range Defined by Floats

Credit: Dinu C. Gherman, Paul M. Winkler

Problem

You need an arithmetic progression, just like the built-in function range, but with float values (range works only on integers).

Solution

Although this functionality is not available as a built-in, it’s not hard to code it with a loop:

def frange(start, end=None, inc=1.0):
    "A range-like function that does accept float increments..."

    if end == None:
        end = start + 0.0     # Ensure a float value for 'end'
        start = 0.0
    assert inc                # sanity check

    L = []
    while 1:
        next = start + len(L) * inc
        if inc > 0 and next >= end:
            break
        elif inc < 0 and next <= end:
            break
        L.append(next)

    return L

Discussion

Sadly missing in the Python standard library, the function in this recipe lets you use ranges, just as with the built-in function range, but with float arguments.

Many theoretical restrictions apply, but this function is more useful in practice than in theory. People who work with floating-point numbers all the time have many war stories about billion-dollar projects that failed because someone did not take into consideration the strange things that modern hardware does when comparing floating-point numbers. But for pedestrian cases, simple approaches like this recipe generally work.

You can get a substantial speed boost by preallocating the list instead of calling append repeatedly. This also allows you to get rid of the conditionals in the inner loop. For one element, this version is barely faster, but with more than 10 elements ...