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Round number to specified number of significant digits (Python recipe) by Ben Hoyt
ActiveState Code (http://code.activestate.com/recipes/578114/)

This function rounds num to the specified number of significant digits (rather than decimal places). See doctests for more examples.

Python, 24 lines
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def round_sigfigs(num, sig_figs):
    """Round to specified number of sigfigs.

    >>> round_sigfigs(0, sig_figs=4)
    0
    >>> int(round_sigfigs(12345, sig_figs=2))
    12000
    >>> int(round_sigfigs(-12345, sig_figs=2))
    -12000
    >>> int(round_sigfigs(1, sig_figs=2))
    1
    >>> '{0:.3}'.format(round_sigfigs(3.1415, sig_figs=2))
    '3.1'
    >>> '{0:.3}'.format(round_sigfigs(-3.1415, sig_figs=2))
    '-3.1'
    >>> '{0:.5}'.format(round_sigfigs(0.00098765, sig_figs=2))
    '0.00099'
    >>> '{0:.6}'.format(round_sigfigs(0.00098765, sig_figs=3))
    '0.000988'
    """
    if num != 0:
        return round(num, -int(math.floor(math.log10(abs(num))) - (sig_figs - 1)))
    else:
        return 0  # Can't take the log of 0
Tags: decimalplaces, round, rounding, significantfigures