How do you print significant figures in python?

For a scientific application I need to output very precise numbers, so I have to print 15 significant figures. There are already questions on this topic here, but they all concern with truncating the digits, not printing more.

I realized that the print function converts the input float to a 10 character string. Also, I became aware of the decimal module, but that does not suit my needs.

So the question is, how can I easily print a variable amount of signifcant figures of my floats, where I need to display more than 10?

asked Feb 23, 2012 at 15:26

3

Let:

>>> num = 0.0012345

For 3 significant figures:

>>> f'{num:.3}'
'0.00123'

For 3 decimal places:

>>> f'{num:.3f}'
'0.001'

See the "presentation types for floating point and decimal" table at the bottom of this section for any additional requirements provided by e, E, f, F, g, G, n, %, None.

answered Oct 19, 2018 at 20:26

Mateen UlhaqMateen Ulhaq

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4

You could use the string formatting operator %:

In [3]: val = 1./3

In [4]: print['%.15f' % val]
0.333333333333333

or str.format[]:

In [8]: print[str.format['{0:.15f}', val]]
Out[8]: '0.333333333333333'

In new code, the latter is the preferred style, although the former is still widely used.

For more info, see the documentation.

answered Feb 23, 2012 at 15:27

NPENPE

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3

Thought the original question wanted to format n significant figures, not n decimal points. So a custom function might be required until some more native built-in types are on offer? So you'll want something like:

def float_nsf[q,n]:
    """
    Truncate a float to n significant figures.  May produce overflow in 
    very last decimal place when q < 1.  This can be removed by an extra 
    formatted print. 
    Arguments:
      q : a float
      n : desired number of significant figures
    Returns:
    Float with only n s.f. and trailing zeros, but with a possible small overflow.
    """
    sgn=np.sign[q]
    q=abs[q]
    n=int[np.log10[q/10.]] # Here you overwrite input n!
    if q>> x = 1.23
>>> print["%.2g" % x]
1.2
>>> x = 12.3
>>> print["%.2g" % x]
12

See format spec for details on precision:

The precision is a decimal number indicating how many digits should be displayed after the decimal point for a floating point value formatted with 'f' and 'F', or before and after the decimal point for a floating point value formatted with 'g' or 'G'. For non-number types the field indicates the maximum field size - in other words, how many characters will be used from the field content. The precision is not allowed for integer values.

answered Jun 3, 2019 at 0:10

quantoidquantoid

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1

Use these two common print idioms for formatting. Its a matter of personal taste on which is better.

value = 10/3            #gives a float of 3.33333.....

print '%.15f' % value
print str.format['{0:.15f}', value]

Personally I think the first is more compact and the 2nd is more explicit. Format has more features when working with multiple vals.

answered Feb 23, 2012 at 16:05

Matt AlcockMatt Alcock

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1

You could use this function I wrote, it seems to be working fine and it's quite simple!:

def nsf[num, n=1]:
    """n-Significant Figures"""
    numstr = ["{0:.%ie}" % [n-1]].format[num]
    return float[numstr]

1. It first converts the number to a string using exponent notation
2. Then returns it as float.

Some tests:

>>> a = 2./3
>>> b = 1./3
>>> c = 3141592
>>> print[nsf[a]]
0.7
>>> print[nsf[a, 3]]
0.667
>>> print[nsf[-a, 3]]
-0.667
>>> print[nsf[b, 4]]
0.3333
>>> print[nsf[-b, 2]]
-0.33
>>> print[nsf[c, 5]]
3141600.0
>>> print[nsf[-c, 6]]
-3141590.0

I hope this helps you ;]

answered Aug 2, 2014 at 17:24

1

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