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up vote 2 down vote favorite

How do I add 0's to the beginning and end of each row of a multidimensional array? This is the function I am trying to apply to each row.

def zero(ltr):
  for x in range (1,int((N+1)/2)):
        ltr = append(([0]), ltr)
        ltr = append(ltr,([0]))
  return ltr

I have tried using both 

for row in a:
   zero(row)

and apply_along_axis(zero,1,a) Neither one of these commands does what I want.

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2 Answers 2

up vote 3 down vote accepted

It is not possible to add entries to single rows of a two-dimensional array. All rows must always have the same length. But you can add entries to all rows at once.

If a is a two-dimensional NumPy array, you can use numpy.hstack to add zeros to left and the right:

a = numpy.array([[  0.,   1.,   2.,   3.],
                 [  4.,   5.,   6.,   7.],
                 [  8.,   9.,  10.,  11.]])
numpy.hstack((numpy.zeros((a.shape[0], 2)), a, numpy.zeros((a.shape[0], 1))))
# array([[  0.,   0.,   0.,   1.,   2.,   3.,   0.],
#        [  0.,   0.,   4.,   5.,   6.,   7.,   0.],
#        [  0.,   0.,   8.,   9.,  10.,  11.,   0.]])

For the sake of example, I added 2 zeros to the left and 1 zero to the right.

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up vote 0 down vote

EDIT: I see you're already using numpy. I'll leave this for the sake of education, but you should use hstack as in Sven's answer.

>>> a = [[1, 1, 1], [1, 1, 1], [1, 1, 1]]
>>> for row in a:
...     row.insert(0, 0)
...     row.append(0)
...
>>> a
[[0, 1, 1, 1, 0], [0, 1, 1, 1, 0], [0, 1, 1, 1, 0]]

or if you prefer:

>>> import operator
>>> a = [[1, 1, 1], [1, 1, 1], [1, 1, 1]]
>>> map(operator.methodcaller("insert", 0, 0), a)
[None, None, None]
>>> map(operator.methodcaller("append", 0), a)
[None, None, None]
>>> a
[[0, 1, 1, 1, 0], [0, 1, 1, 1, 0], [0, 1, 1, 1, 0]]
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