Add single element to array in numpy

Question

append() creates a new array which can be the old array with the appended element.

I think it's more normal to use the proper method for adding an element:

a = numpy.append(a, a[0])

When appending only once or once every now and again, using np.append on your array should be fine. The drawback of this approach is that memory is allocated for a completely new array every time it is called. When growing an array for a significant amount of samples it would be better to either pre-allocate the array (if the total size is known) or to append to a list and convert to an array afterward.

Using np.append:

b = np.array([0])
for k in range(int(10e4)):
    b = np.append(b, k)
1.2 s ± 16.1 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

Using python list converting to array afterward:

d = [0]
for k in range(int(10e4)):
    d.append(k)
f = np.array(d)
13.5 ms ± 277 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

Pre-allocating numpy array:

e = np.zeros((n,))
for k in range(n):
    e[k] = k
9.92 ms ± 752 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

When the final size is unkown pre-allocating is difficult, I tried pre-allocating in chunks of 50 but it did not come close to using a list.

85.1 ms ± 561 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

a[0] isn't an array, it's the first element of a and therefore has no dimensions.

Try using a[0:1] instead, which will return the first element of a inside a single item array.

Try this:

np.concatenate((a, np.array([a[0]])))

http://docs.scipy.org/doc/numpy/reference/generated/numpy.concatenate.html

concatenate needs both elements to be numpy arrays; however, a[0] is not an array. That is why it does not work.

This command,

numpy.append(a, a[0])

does not alter a array. However, it returns a new modified array. So, if a modification is required, then the following must be used.

a = numpy.append(a, a[0])

t = np.array([2, 3])
t = np.append(t, [4])