Python:"遍历numpy.ndarray时使用a.any()或a.all()"

Question

在numpy.ndarray这样的一个:

myarray=
array([[ 0.47174344,  0.45314669,  0.46395022,  0.47440382,  0.50709627,
         0.53350065,  0.5233444 ,  0.49974663,  0.48721607,  0.46239652,
         0.4693633 ,  0.47263569,  0.47591957,  0.436558  ,  0.43335574,
         0.44053621,  0.42814804,  0.43201894,  0.43973886,  0.44125302,
         0.41176999],
       [ 0.46509004,  0.46221505,  0.48824086,  0.50088744,  0.53040384,
         0.53592231,  0.49710228,  0.49821022,  0.47720381,  0.49096272,
         0.50438366,  0.47173162,  0.48813669,  0.45032002,  0.44776794,
         0.43910269,  0.43326132,  0.42064458,  0.43472954,  0.45577299,
         0.43604956]])

我想计算有多少单元格超过给定值,比方说0.5,并设置那些不超过给定值的单元格0.0.这就是我做的:

count=0 
value=0.5
for i in range(myarray.shape[0]):
   for j in range(myarray.shape[1]):
       if myarray[i][j]<value:
          myarray[i][j]=0 
       elif myarray[i][j]>=value:     
          count=count+1 
percentage=round(100*count/(myarray.shape[0]*myarray.shape[1]),2)

但是,我收到这个错误:ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all(),指着我检查的行if myarray[i][j]<value.

为什么会发生这种情况以及如何解决？什么是真值？

Answer 1

通常,您可以比较两个数字以获得真值.例如:

elem = 5
if elem < 6:
    # do something

相当于:

if true:
    # do something

但是,您无法将数组与值进行比较.例如:

elem = [5,7]
if elem < 6:
    # this doesn't make sense

相反,您可以获得任何或所有元素是否满足条件的真值.例如:

elem = np.array([5,7])
if np.any(elem<6):
    # this is true, because 5 < 6
if np.all(elem<6):
    # this isn't true, because 7 > 6

我运行了上面的示例代码并发现没有错误,所以我不确定问题是什么.但这是你应该注意的.考虑打印您正在比较的元素,看它是否是一个数组.

此外,这是做你想做的事情的一种较短的方式:

myarray = np.array( putarrayhere )
count = sum(myarray >= value)