擅长:python、mysql、java
<p>Python在<code>itertools</code>中的<a href="https://docs.python.org/2/library/itertools.html#itertools.product" rel="nofollow">^{<cd1>}</a>函数也有帮助,它可以与<code>enumerate</code>结合,返回{<cd4>}所需的索引,如下所示:</p>
<pre><code>from itertools import product
X = [1, 2]
Y = [1, 2, 3]
P = [[0.25,0.25,0.0], [0.0, 0.25, 0.25]]
mean_x = float(sum(X) / len(X))
mean_y = float(sum(Y) / len(Y))
print sum((x[1] - mean_x) * (y[1] - mean_y) * P[x[0]][y[0]] for x, y in product(enumerate(X), enumerate(Y)))
</code></pre>
<p>给出结果:</p>
^{pr2}$