[英]Numpy : Add a 1d array (row) to an empty 2d array in Numpy Python
I don't know the number of rows and columns of a 2d array ( a
) I need in advance: 我不知道我事先需要的2d数组(
a
)的行数和列数:
a = np.empty( (0,0), dtype=np.uint8 )
a.ndim
2
I managed to convert each line I read from a file to an 1d array of bytes called line
我设法将从文件中读取的每一行都转换为称为
line
一维字节数组
I want to add/append each line to a
, so I tried : 我想添加/追加每行
a
,所以我尝试:
np.vstack( (a,line) )
but I get : ValueError: all the input arrays must have same number of dimensions
但我得到:
ValueError: all the input arrays must have same number of dimensions
And if I call a=np.append(a,line)
, the number of dimensions becomes 1
如果我调用
a=np.append(a,line)
,则维数变为1
Can you help ? 你能帮我吗 ?
Numpy arrays cannot be appended to. 不能将Numpy数组附加到后面。 The easiest way for you to do what you are trying to achieve would be to use a list and call
np.asarray()
on it. 您要做的最简单的方法是使用列表并在其上调用
np.asarray()
。
a = []
# Build list from file
a = np.asarray(a)
You're on the right track with np.vstack
. 您在使用
np.vstack
的方向正确。 The only requirement is that the arrays being stacked must have the same number of columns. 唯一的要求是,要堆叠的阵列必须具有相同数量的列。 Take a look at this:
看看这个:
array = None # initialise array to None first
for ...: # now inside the loop
line = ... # read line from somewhere and get something like [201, 81, 237, 230]
if array is None:
array = line
else:
array = np.vstack((array, line))
In terms of performance, this is actually a little more wasteful than just creating an array at the end out of a list. 就性能而言,这实际上比仅在列表末尾创建数组要浪费更多。 This is the problem with numpy arrays - they're immutable in terms of dimensions.
这是numpy数组的问题-在尺寸方面它们是不可变的。
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