CODEWITHSUNDEEP
pythonpython-3.x
RMRiver
RMRiver

Reputation: 645

Format float to 3 significant figures after decimal point

How would you format a float to three signicant figures after a decimal place? If the float has no decimal places then the number should remain the same.

This is correct:

'{0:.3}'.format(0.000325658)
'0.000326'

This is incorrect:

'{0:.3}'.format(325256.023)
'3.25e+05'

and should be

325256.023

The value 325256.0 should be output as 325256.0.

I'd like this to be done with a single ''.format() if possible.

Test cases:

0.000325658 - > 0.000326
325256.023 - > 325256.023
325256.0 - > 325256.0 or 325256.000

Edit add new test case:

'{0:.3f}'.format(325256.000023)
'325256.000'

should be

325256.000023

Upvotes: 0

Views: 570

Answers (2)

Robson
Robson

Reputation: 2032

If you're happy to include numpy then you could use the numpy.format_float_positional function, with an extra step:

import numpy as np

def to_significant_decimals(n, amount):
    integer = int(n)
    n -= integer
    return integer + float(np.format_float_positional(n, precision=amount, unique=False, fractional=False))

tests = [ 0.000325658, 325256.023, 325256.0, 325256.000023, 0, 7, 1000000, 1000.3, 1000.0001, 1234.4321 ]
for test in tests:
    print(' ' + str(test) + ' =  ' + str(to_significant_decimals(test, 3)))
    print(str(-test) + ' = ' + str(to_significant_decimals(-test, 3)))

The numpy function allows you to choose the amount of significant figures, but this includes the integer part. I've solved that by taking the integer part away, doing the format, then adding the integer back on. There's probably a more elegant solution, but this gives the results that you want.

Outputs:

 0.000325658 =  0.000326
-0.000325658 = -0.000326
 325256.023 =  325256.023
-325256.023 = -325256.023
 325256.0 =  325256.0
-325256.0 = -325256.0
 325256.000023 =  325256.000023
-325256.000023 = -325256.000023
 0 =  0.0
 0 =  0.0
 7 =  7.0
-7 = -7.0
 1000000 =  1000000.0
-1000000 = -1000000.0
 1000.3 =  1000.3
-1000.3 = -1000.3
 1000.0001 =  1000.0001
-1000.0001 = -1000.0001
 1234.4321 =  1234.432
-1234.4321 = -1234.432

Upvotes: 1

Bartosz Karwacki
Bartosz Karwacki

Reputation: 331

Just like that

'{0:.3f}'.format(325256.023)

Upvotes: 0

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