Python Floating Point Formatting

Question

I've seen a few questions about this already, but none that I read helped me actually understand why what I am trying to do is failing.

So I have a bunch of floating point values, and they have different precisions. Some are 0.1 others are 1.759374, etc. And I want to format them so they are ALL in the form of "+0.0000000E+00" I tried doing

number = '%1.7f' % oldnumber

but that didn't work. I thought what I was telling it to do was "one digit perfor the decimal point, and 7 after, float" but it doesn't work. I'm not really getting the examples in the docs, which don't seem to even bother with "before and after decimal point" issues, and I didn't find a question that was about before and after decimal point fixing.

Now, I know that some of my numbers are 0.0437 or similar, and I want them to appear as 4.3700000E-02 or something. I was sort of hoping it would do the E bit on it's own, but if it doesn't how do I do it?

Here is the exact line I have:

RealValConv =   '%1.7g' % struct.unpack('!f',    RealVal.decode('hex'))[0]

RealVal is a hex number that represents the value I want.

Also, this is in Python 2.7

You want to use g rather than f if you want scientific notation. For instance, "%1.7g" % .0000012323 becomes '1.2323e-06'. Is this the behavior you want? — David Robinson
– David Robinson, Commented Sep 10, 2013 at 0:30
Hmm, I'm not sure why, but that isn't working. I'm using python 2.7, does that matter? That is the behavior I wanted though. — CamelopardalisRex
– CamelopardalisRex, Commented Sep 10, 2013 at 0:39
@DavidRobinson: %g uses %f in some cases, e.g., formatting math.pi with %1.7g results in 3.141593. %e always uses "e" notation. — torek
– torek, Commented Sep 10, 2013 at 0:39

planestepper · Accepted Answer · 2013-09-10 00:29:49Z

11

>>> '{:.7e}'.format(0.00000000000000365913456789)
'3.6591346e-15'

answered Sep 10, 2013 at 0:29

planestepper

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2 Comments

CamelopardalisRex Over a year ago

My problems with length were an excel problem, not a program problem. (I open the file as .csv and excel defaulted it to being shorter)

SethMMorton Over a year ago

Always, always, prefer .format over % formatting. Here is yet another reason it is preferred.

Manoj Pandey · Accepted Answer · 2013-09-10 00:50:12Z

3

You can use the scientific notation format: Something like this:

number = '%e' % oldnumber

>>> x = 1.759374
>>> print '%e' % x
1.759374e+00
>>>
>>> x = 1.79
>>> print '%e' % x
1.790000e+00
>>>
>>> x = 1.798775655
>>> print '%e' % x
1.798776e+00
>>>

Or, if you want to control precision, you can use the format method as sugged by @leon approach (+1).

>>> x = 1.759374
>>>
>>> print('{:.2e}'.format(x))
1.76e+00
>>>
>>> print('{:.10e}'.format(x))
1.7593740000e+00
>>>
>>> print('{:.4e}'.format(x))
1.7594e+00
>>>

edited Sep 10, 2013 at 0:50

answered Sep 10, 2013 at 0:32

Manoj Pandey

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3 Comments

Manoj Pandey Over a year ago

I actually get a default precision of 7 decimal values. If you want to control, then you can use format() method as suggested by @leon. I have updated my answer, accordingly.

CamelopardalisRex Over a year ago

My problems with length were an excel problem, not a program problem. (I open the file as .csv and excel defaulted it to being shorter) Sorry for assuming it was you, when it was me. Dizzy days.

Manoj Pandey Over a year ago

Thnx for telling me that. Appreciate that!

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