# 中位数计算原理
1、先按升序排列 [2s, 10s, 100s, 1000s]
2、找到你需要用做统计的最后一个条目（向高取整）对应的数值，比如：TP50就是第 ceil(4*0.5)=2 个，即 10s ；TP90就是第 ceil(4*0.9)=4 个，即 1000s 。
# 方法一
def percentile(N, percent, key=lambda x:x):
"""
Find the percentile of a list of values.
@parameter N - is a list of values. Note N MUST BE already sorted.
@parameter percent - a float value from 0.0 to 1.0.
@parameter key - optional key function to compute value from each element of N.
@return - the percentile of the values
"""
if not N:
return None
k = (len(N)-1) * percent
f = math.floor(k)
c = math.ceil(k)
if f == c:
return key(N[int(k)])
d0 = key(N[int(f)]) * (c-k)
d1 = key(N[int(c)]) * (k-f)
return d0+d1
# 方法二
import numpy as np
a = np.array([1,2,3,4,5])
p = np.percentile(a, 50) # 中位数
print p
