Module « numpy.random »

Fonction lognormal - module numpy.random

Signature de la fonction lognormal

Description

lognormal.__doc__

        lognormal(mean=0.0, sigma=1.0, size=None)

        Draw samples from a log-normal distribution.

        Draw samples from a log-normal distribution with specified mean,
        standard deviation, and array shape.  Note that the mean and standard
        deviation are not the values for the distribution itself, but of the
        underlying normal distribution it is derived from.

        .. note::
            New code should use the ``lognormal`` method of a ``default_rng()``
            instance instead; please see the :ref:`random-quick-start`.

        Parameters
        ----------
        mean : float or array_like of floats, optional
            Mean value of the underlying normal distribution. Default is 0.
        sigma : float or array_like of floats, optional
            Standard deviation of the underlying normal distribution. Must be
            non-negative. Default is 1.
        size : int or tuple of ints, optional
            Output shape.  If the given shape is, e.g., ``(m, n, k)``, then
            ``m * n * k`` samples are drawn.  If size is ``None`` (default),
            a single value is returned if ``mean`` and ``sigma`` are both scalars.
            Otherwise, ``np.broadcast(mean, sigma).size`` samples are drawn.

        Returns
        -------
        out : ndarray or scalar
            Drawn samples from the parameterized log-normal distribution.

        See Also
        --------
        scipy.stats.lognorm : probability density function, distribution,
            cumulative density function, etc.
        Generator.lognormal: which should be used for new code.

        Notes
        -----
        A variable `x` has a log-normal distribution if `log(x)` is normally
        distributed.  The probability density function for the log-normal
        distribution is:

        .. math:: p(x) = \frac{1}{\sigma x \sqrt{2\pi}}
                         e^{(-\frac{(ln(x)-\mu)^2}{2\sigma^2})}

        where :math:`\mu` is the mean and :math:`\sigma` is the standard
        deviation of the normally distributed logarithm of the variable.
        A log-normal distribution results if a random variable is the *product*
        of a large number of independent, identically-distributed variables in
        the same way that a normal distribution results if the variable is the
        *sum* of a large number of independent, identically-distributed
        variables.

        References
        ----------
        .. [1] Limpert, E., Stahel, W. A., and Abbt, M., "Log-normal
               Distributions across the Sciences: Keys and Clues,"
               BioScience, Vol. 51, No. 5, May, 2001.
               https://stat.ethz.ch/~stahel/lognormal/bioscience.pdf
        .. [2] Reiss, R.D. and Thomas, M., "Statistical Analysis of Extreme
               Values," Basel: Birkhauser Verlag, 2001, pp. 31-32.

        Examples
        --------
        Draw samples from the distribution:

        >>> mu, sigma = 3., 1. # mean and standard deviation
        >>> s = np.random.lognormal(mu, sigma, 1000)

        Display the histogram of the samples, along with
        the probability density function:

        >>> import matplotlib.pyplot as plt
        >>> count, bins, ignored = plt.hist(s, 100, density=True, align='mid')

        >>> x = np.linspace(min(bins), max(bins), 10000)
        >>> pdf = (np.exp(-(np.log(x) - mu)**2 / (2 * sigma**2))
        ...        / (x * sigma * np.sqrt(2 * np.pi)))

        >>> plt.plot(x, pdf, linewidth=2, color='r')
        >>> plt.axis('tight')
        >>> plt.show()

        Demonstrate that taking the products of random samples from a uniform
        distribution can be fit well by a log-normal probability density
        function.

        >>> # Generate a thousand samples: each is the product of 100 random
        >>> # values, drawn from a normal distribution.
        >>> b = []
        >>> for i in range(1000):
        ...    a = 10. + np.random.standard_normal(100)
        ...    b.append(np.product(a))

        >>> b = np.array(b) / np.min(b) # scale values to be positive
        >>> count, bins, ignored = plt.hist(b, 100, density=True, align='mid')
        >>> sigma = np.std(np.log(b))
        >>> mu = np.mean(np.log(b))

        >>> x = np.linspace(min(bins), max(bins), 10000)
        >>> pdf = (np.exp(-(np.log(x) - mu)**2 / (2 * sigma**2))
        ...        / (x * sigma * np.sqrt(2 * np.pi)))

        >>> plt.plot(x, pdf, color='r', linewidth=2)
        >>> plt.show()