[Tutor] Re: If elif not working in comparison

[gerardo arnaez]

Hi all. I would like to post the  very small py files I have written
while doing this.
Would anyone object.
I think at most there be 20 lines of code all the files put together.
I woul dlike to hear some crituqe on them
Thanks

On Apr 4, 2005 11:08 AM, Christopher Smith <smiles at saysomething.com> wrote:
> 
> On Monday, Apr 4, 2005, at 05:01 America/Chicago,
> tutor-request at python.org wrote:
> 
> > Would I used an if else: construction to determine where the INR value
> > lay and decide what precentage to increase it by?
> >
> 
> Yes, that seems to be the right way to do that.  The caveat is that
> when you are using boolean tests like "if INR < 1.15:" you are using
> floating point numbers that *by definition* have limited ability to
> represent the numbers you type.
> 
> "1.15" is actually "1.1499999999999999", so calculation that "on paper"
> would have led to a value of 1.15 will appear to be larger than the
> floating point value of 1.15. When you are checking to see if your
> program is running right and you find that a test like "if INR < 1.15"
> doesn't work like you think it should this can lead to a lot of hair
> pulling...until you remember the floating point issue.  e.g. on paper,
> a and b as shown below should be the same, but they aren't
> 
> ###
> >>> a=.23*5 # which equals 1.15 on paper
> >>> b=1.15
> >>> a==b
> False
> >>> a
> 1.1500000000000001
> >>> b
> 1.1499999999999999
> ###
> 
> If one cares about what side of the boundary values get evaluated to
> then either exact numbers should be used (e.g. the new decimal type of
> python 2.4) or else the tests should use the round function to get both
> numbers to a compatible precision for comparison (e.g. use "if
> round(INR,2) < round(1.15,2):" instead of "if INR < 1.15".
> 
> It is instructive to see what the round function does to the a and b
> used in the example above:
> 
> ###
> >>> a,b=round(.23*5,2),round(1.15,2)
> >>> a==b
> True
> >>> a
> 1.1499999999999999
> >>> b
> 1.1499999999999999
> ###
> 
> Both numbers are still floats, but they are floats that both best
> represent, to 2 decimal places, their respective values.
> 
> /c
> 
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