第21章 PageRank算法 - 第21章 PageRank算法 - 实现pageRank函数

#https://gist.github.com/diogojc/1338222/84d767a68da711a154778fb1d00e772d65322187

import numpy as np
from scipy.sparse import csc_matrix


def pageRank(G, s=.85, maxerr=.0001):
    """
    Computes the pagerank for each of the n states
    Parameters
    ----------
    G: matrix representing state transitions
       Gij is a binary value representing a transition from state i to j.
    s: probability of following a transition. 1-s probability of teleporting
       to another state.
    maxerr: if the sum of pageranks between iterations is bellow this we will
            have converged.
    """
    n = G.shape[0]

    # transform G into markov matrix A
    A = csc_matrix(G, dtype=np.float)
    rsums = np.array(A.sum(1))[:, 0]
    ri, ci = A.nonzero()
    A.data /= rsums[ri]

    # bool array of sink states
    sink = rsums == 0

    # Compute pagerank r until we converge
    ro, r = np.zeros(n), np.ones(n)
    while np.sum(np.abs(r - ro)) > maxerr:
        ro = r.copy()
        # calculate each pagerank at a time
        for i in range(0, n):
            # inlinks of state i
            Ai = np.array(A[:, i].todense())[:, 0]
            # account for sink states
            Di = sink / float(n)
            # account for teleportation to state i
            Ei = np.ones(n) / float(n)

            r[i] = ro.dot(Ai * s + Di * s + Ei * (1 - s))

    # return normalized pagerank
    return r / float(sum(r))