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Yeah this is one of the big stumbling blocks for novices. What you're seeing is a side-effect of C's int division. 7/4 is 1.75, but 1.75 is a float, not an int. ints can only be whole numbers.

Fortunately, the developers of python recognized this problem, but they couldn't simply change it right away because it might break existing python programs. So, to enable "real-world" division where 7/4 = 1.75, add this line to the top of your python script and everything will magically work:
{{{
from __future__ import division
}}}

See also this for more info about this and other stumbling blocks:
http://developers.coedit.net/BeginnerErrorsWithPythonProgramming

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I am not troubled by the integer division which it seems is what you thought I was asking about. Let me put this in context: The students had an improper fraction, say 25/12. To obtain the whole number part for this fraction you should just be able to do integer division and obtain 2 (12 divides into 25 2 times). This is okay as long as the 25 is positive. However if it is -25/12 Python returns -3 which is not the correct answer for the whole number part of the fraction. It is possible to work around this but I was curious why the Python developers chose to handle negatives in this way. Thanks for any insight anyone might have. Terry S
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Please see the Python Programming FAQ, [http://www.python.org/doc/faq/programming.html#why-does-22-10-return-3 question 1.3.2], for an explanation.

I discovered this when my students were creating a fraction class. 7/4 = 1 however -7/4 = -2 It seems to me the second result should be -1 It appears it is using the floor function but it isn't what my students wanted nor can I think of when anyone would want this result. Anyone know why? Thanks for any help Terry S - tscott@fisher.unco.edu


That is what floor division does, it reduces the value to the next lowest whole number. If you want to separate the whole number and fractional part, use math.modf:

from __future__ import division
import math
print math.modf(25/12)
--> (0.08333, 2.0)
print math.modf(-25/12)
--> (-0.08333, -2.0)

#to just print the whole number as an int:
print int(math.modf(25/12)[1])


-3 is the correct answer to the expression -25/12

Here is why.

Consider the statement print 25/12 and print 25%12 {{{>>> print 25/12 2 >>> print 25%12 1}}}

This is because 25 = 2 * 12 + 1

So print -25/12 and print -25%12 gives you {{{>>> print -25/12 -3 >>> print -25%12 11}}}

Because -25 = -3 * 12 + 11

Your suggested answer -25/12 = -2 implies that -25%12 = -1

-25 = -2 * 12 + -1

But the result of A%B must be positive so -25%12 = 11 instead of -1 thus -25/12 must be -3 instead of -2


Please see the Python Programming FAQ, [http://www.python.org/doc/faq/programming.html#why-does-22-10-return-3 question 1.3.2], for an explanation.

Asking for Help/SubtractionQuestion (last edited 2011-03-26 23:11:56 by PaulBoddie)

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