python, numpy; How to insert element at the start of an array

Question

I have an numpy array of complex numbers. So I want to insert zero at start of the array, and shift the rest of the array one place forward.

example:

a = [1 + 2j, 5 + 7j,..]

I want to make:

a = [0 + 0j, 1 + 2j, 5 + 7j,..]

What's the simplest way to do this?

Answer 1

Simplest way:

a = np.array([1 + 2j, 5 + 7j])
a = np.insert(a, 0, 0)

Then:

>>> a
array([ 0.+0.j,  1.+2.j,  5.+7.j])

Note that this creates a new array, it does not actually insert the 0 into the original array.

There are several alternatives to np.insert, all of which also create a new array:

In [377]: a
Out[377]: array([ 1.+2.j,  5.+7.j])

In [378]: np.r_[0, a]
Out[378]: array([ 0.+0.j,  1.+2.j,  5.+7.j])

In [379]: np.append(0, a)
Out[379]: array([ 0.+0.j,  1.+2.j,  5.+7.j])

In [380]: np.concatenate([[0], a])
Out[380]: array([ 0.+0.j,  1.+2.j,  5.+7.j])

In [381]: np.hstack([0, a])
Out[381]: array([ 0.+0.j,  1.+2.j,  5.+7.j])

In [382]: np.insert(a, 0, 0)
Out[382]: array([ 0.+0.j,  1.+2.j,  5.+7.j])

Answer 2

An alternative is "horizontal stack" (also creates a new array):

np.hstack((0,a))