Python实现PLA算法，出现判断错误

import numpy as np

ITERATION = 100 # 迭代次数为100
W = 0

f = open("C:\Users\Administrator\PycharmProjects\PLA算法实现\PLA_train.txt")

f.readline() # skip the header

data = np.loadtxt(f)

X = data[:, :4] # select columns 1 through end

y = data[:, 4] # select column 0, the stock price

def sign(W, i): # 符号函数
sum = W * X[i]
if sum > 0: return 1
else:
return -1

def trainw(W,i): #更新W
multiply = sign(W, i)
if multiply == y[i]:
return W
else:
W = W + X[i] * y[i]
return W

for i in range(ITERATION):
W = trainw(W,i)

print(W) # 只写到求出 Wpla 为止

错误：
Traceback (most recent call last):
File "C:/Users/Administrator/PycharmProjects/PLA算法实现/test.py", line 33, in
W = trainw(W,i)
File "C:/Users/Administrator/PycharmProjects/PLA算法实现/test.py", line 25, in trainw
multiply = sign(W, i)
File "C:/Users/Administrator/PycharmProjects/PLA算法实现/test.py", line 19, in sign
if sum > 0: return 1
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

http://blog.csdn.net/xiangxianghehe/article/details/53262633