python之为什么 Python 不操作这个数组

我有以下代码片段:

d = [[0,558,437,142,45,290], [558,0,232,500,600,523], [437,232,0,408,492,572],  
 [142,500,408,0,180,197], [45,600,492,180,0,254], [290,523,572,197,254,0]] 
 
for row in d: 
    for elem in row: 
        elem = elem * 2 
print d[0][1] 
print '\n' 

为什么不打印新值，还是打印558