[Numpy-discussion] numpy.floor() is supposed to return an int, but returns a float

Tim Hochberg tim.hochberg at cox.net
Sun Apr 9 16:07:02 CDT 2006


Webb Sprague wrote:

>I think the docstring implies that numpy.floor() returns an integer
>value.  
>
You've been programming to much!

Everywhere but the computer programming world, 1.0 is integer.  Even 
their, many (most?) computer languages avoid the term integer using int, 
Int or something similar. The distinction made between ints and integral 
floating point values is mostly an artificial one resulting from 
implementation issues. Making this distinction is also a handy, if 
imperfect,  proxy for exact / versus inexact numbers.

>One can cast the float value to a usable integer value, but
>either the docstring should read something different or the function
>should be changed (my preference).
>
>"y = floor(x) elementwise largest integer <= x" is the docstring.
>
>As far as "integral valued float" versus "integer", this distinction
>seems a little obscure...
>
An integral floating point value *is* an integer, just ask any 12 year 
old. What's obscure is the way concepts of integers and reals get mapped 
to ints and floats. Don't get me wrong, these are reasonable comprises 
given the sad reality that computers are not so hot at representing 
inifinte quantities.  However, we get sucked into thinking that integers 
and ints are really the same things at our peril. Similarly for floats 
and reals.

>  I am sure the difference is very important
>in some contexts, but I for one think that floor should return a
>straight up integer,
>
It's a ufunc. Ufuncs in general return the same type that they operate 
on. So, not only would this be difficult, it would make the signature of 
ufuncs harder to remember.

Also, as Robert Kern just pointed out, not all intergral FP values can 
be represents as ints.

> if just for code style (see example below). Plus
>it will be upcast to a float whenever necessary, so floor(4.5) + .75
>== 4.75 whether floor() returns an int or a float.
>  
>
    Not every two-line Python function has to come pre-written -- Tim 
Peters on C.L.P

def webbsfloor(x):
    return numpy.floor(x).astype(int)

>fooMatrix[numpy.floor(age/ageWidth)]
>
>is better (easier to type, read, and debug) than
>
>fooMatrix[numpy.floor(age/ageWidth).astype(int)]
>
>If there is a explanation as to why an integral valued float is a
>better return value, I would be interested in a link.
>  
>
I think there's at least four reasons:

1. It would be a pain.
2. It would make the ufuncs inconsistent.
3. It's a thin wrapper over C's floor, so people coming from that 
language be confused.
4. It wouldn't work for numbers with very large magnitudes.

Pick any three


Regards,

-tim





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