17

I can't seem to find the answer to this on this site, though it seems like it would be common enough. I am trying to output a double for the ratio of number of lines in two files.

#Number of lines in each file
inputLines = sum(1 for line in open(current_file))
outputLines = sum(1 for line in open(output_file))

Then get the ratio:

ratio = inputLines/outputLines

But the ratio always seems to be an int and rounds even if I initialize it like:

ratio = 1.0

Thanks.

The Nightman
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4 Answers4

32

In python 2, the result of a division of two integers is always a integer. To force a float division, you need to use at least one float in the division:

ratio = float(inputLines)/outputLines

Be careful not to do ratio = float(inputLines/outputLines): although that results in a float, it's one obtained after doing the integer division, so the result will be "wrong" (as in "not what you expect")

In python 3, the integer division behavior was changed, and a division of two integers results in a float. You can also use this functionality in python 2.7, by putting from __future__ import division in the begging of your files.

The reason ratio = 1.0 does not work is that types (in python) are a property of values, not variables - in other words, variables don't have types.

a= 1.0 # "a" is a float
a= 1   # "a" is now a integer
loopbackbee
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4

Are you using Python 2.x?

Try using

>>> from __future__ import division
>>> ratio = 2/5
0.4

to get a float from a division.

Initializing with ratio = 1.0 doesn't work because with ratio = inputLines/outputLines you are re-assigning the variable to int, no matter what it was before.

adrianus
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3

Indeed, Python 2 returns an integer when you divide an integer. You can override this behaviour to work like in Python 3, where int / int = float by placing this at the top of your file:

from __future__ import division

Or better yet, just stop using Python 2 :)

Underyx
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2

You need to cast one of the terms to a floating point value. Either explicitly by using float (that is ratio = float(a)/b or ratio=a/float(b)) or implicitly by adding 0.0: ratio = (a+0.0)/b.

If using Python 2 you can from __future__ import division to make division not be the integral one but the float one.

Mihai Maruseac
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