I'm trying to plot a histogram with matplotlib. I need to convert my one-line 2D Array
[[1,2,3,4]] # shape is (1,4)
into a 1D Array
[1,2,3,4] # shape is (4,)
How can I do this?
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5 Answers
Adding ravel as another alternative for future searchers. From the docs,
It is equivalent to reshape(-1, order=order).
Since the array is 1xN, all of the following are equivalent:
arr1d = np.ravel(arr2d)arr1d = arr2d.ravel()arr1d = arr2d.flatten()arr1d = np.reshape(arr2d, -1)arr1d = arr2d.reshape(-1)arr1d = arr2d[0, :]
Comments
You can directly index the column:
>>> import numpy as np
>>> x2 = np.array([[1,2,3,4]])
>>> x2.shape
(1, 4)
>>> x1 = x2[0,:]
>>> x1
array([1, 2, 3, 4])
>>> x1.shape
(4,)
Or you can use squeeze:
>>> xs = np.squeeze(x2)
>>> xs
array([1, 2, 3, 4])
>>> xs.shape
(4,)
Comments
reshape will do the trick.
There's also a more specific function, flatten, that appears to do exactly what you want.
1 Comment
Eric O. Lebigot
More specifically,
arr.reshape(-1) converts an array to 1D. But the equivalent ravel() is better, as it is meant specifically to indicate a conversion to 1D.the answer provided by mtrw does the trick for an array that actually only has one line like this one, however if you have a 2d array, with values in two dimension you can convert it as follows
a = np.array([[1,2,3],[4,5,6]])
From here you can find the shape of the array with np.shape and find the product of that with np.product this now results in the number of elements. If you now use np.reshape() to reshape the array to one length of the total number of element you will have a solution that always works.
np.reshape(a, np.product(a.shape))
>>> array([1, 2, 3, 4, 5, 6])
Comments
Use numpy.flat
import numpy as np
import matplotlib.pyplot as plt
a = np.array([[1,0,0,1],
[2,0,1,0]])
plt.hist(a.flat, [0,1,2,3])
The flat property returns a 1D iterator over your 2D array. This method generalizes to any number of rows (or dimensions). For large arrays it can be much more efficient than making a flattened copy.
