How to achieve this? I have a numpy array containing:
[1, 2, 3]
I want to create an array containing:
[8, 1, 2, 3]
That is, I want to add an element on as the first element of the array.
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3 Answers
The most basic operation is concatenate:
x=np.array([1,2,3])
np.concatenate([[8],x])
# array([8, 1, 2, 3])
np.r_ and np.insert make use of this. Even if they are more convenient to remember, or use in more complex cases, you should be familiar with concatenate.
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Use numpy.insert(). The docs are here: http://docs.scipy.org/doc/numpy/reference/generated/numpy.insert.html#numpy.insert
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You can also use numpy's np.r_, a short-cut for concatenation along the first axis:
>>> import numpy as np
>>> a = np.array([1, 2, 3])
>>> b = np.r_[8, a]
>>> b
array([8, 1, 2, 3])
