Snippet: sgn function

From time to time, I find that I need a function to return +1, 0, or -1 representing the sign of a value: +1 or -1 for positive or negative values, or 0 for a zero value. I remember learning this as the sgn function in high school. Python’s standard lib leaves this out, but does supply cmp as a built-in, so the standard approach would probably be to define sgn using:

sgn = lambda x : cmp(x,0)

I found a nice website of Bit-Twiddling Hacks the other day, and it had a nice alternative for sgn, which in Python would be:

sgn = lambda x : (x>0) - (x<0)

The elegance of this appeals to me and I did a quick timing pass:

C:\Users\Paul>python -m timeit "[cmp(x,0) for x in (-100,0,14)]"
1000000 loops, best of 3: 0.645 usec per loop

C:\Users\Paul>python -m timeit "[lambda x:x>0 - x<0 for x in (-100,0,14)]"
1000000 loops, best of 3: 0.496 usec per loop

So by cutting out the repeated function calls to cmp, this also has the benefit of being just a tad faster.

  1. #1 by Anton Chikin on May 13, 2011 - 3:45 am

    Parsing namespaces takes the most time. you can just say:
    >>>local_cmp = cmp
    >>>[local_cmp(x,0) for x in (-100,0,14)]

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