The thing is, "%f" rounds differently than "%d".
>>> "%.1f" % 1.56 ### I expect 1.6 '1.6' >>> "%d" % 1.6 ### By analogy, I expect 2 '1'The correct solution is either "%.0f" (yes - it exists) or explicitly round it yourself how you like, e.g. "%d" % int(x+0.5) or using round().
This was causing off-by-one results in my code which I fortunately identified prior to submitting a manuscript. But I'm disappointed, Python; and don't blame C.
Note: As a further issue, if you need to handle negative values also, doublecheck the rounding behaviour of your code...it may not always be what you expect.
6 comments:
With a jump to python3 I've started to use "".format().
In this case, it gives me:
>>> a=1.56
>>> "{0:.0f} is rounded {0:.2f}".format(a)
'2 is rounded 1.56'
Interesting in Java there's no compiler warning but it will panic with a runtime exception.
IllegalFormatConversionException: d != java.lang.Double
In C there is a compiler warning but it ploughs on regardless giving 1435806904 for 1.56.
@Nikita: Is that the preferred syntax now, rather than using %?
@John: What's the equivalent in Java, just for interest?
So some C convention isn't to blame. Hmmm...I don't get it. I never heard of %.0f before, but it seems that you need to know it even though it's counter-intuitive.
Yes, I think now it's preferred syntax (and it works in 2.7)
Here you have some syntax and examples.
http://docs.python.org/3.2/library/string.html#format-specification-mini-language
String.format("%.0f", 1.56);
or
System.out.printf("%.0f", 1.56);
String.format("%d", 1.56);
Will fail with with the exception.
This behavior of Python is perfectly consistent with the intention of how data is represented in most programming languages, including C and Java. The only thing "broken" in Python is that it allows a float value to be directly formatted into an integer, whereas the other languages actually expect any data formatted as %d to actually be integer data.
%d is intended to format integer data, and in all these computer languages, casting a float to an int is handled via truncation, not rounding. If you want to round a float to the nearest integer value, then %.0f is more appropriate to your intention.
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