[Tutor] inserting a vector into an array - numpy Q

[Danny Yoo]

> I know there must be a better way to do this with slices, but I can't 
> seem to figure it out - I keep getting errors about the need to have the 
> same dimensions:


Look at the error message more closely.

     ValueError: arrays must have same number of dimensions


What does this mean?


The 1-d array that's being passed in has a different dimension than the 2d 
array.  If we trust the error message, we should make the two arrays the 
same dimension before we pass them to functions that glue these arrays 
together.


We want both of them to be two-dimensional.  If we look in:

     http://www.scipy.org/Numpy_Functions_by_Category

we should see several ways to do this.  One way is with reshape:

     http://www.scipy.org/Numpy_Example_List_With_Doc#reshape

For example:

########################################
>>> numpy.array([1, 2, 3]).reshape(1, 3)
array([[1, 2, 3]])
>>>
>>>
>>> numpy.append(numpy.array([1, 2, 3]),
...              numpy.array([[4, 5, 6],
...                           [7, 8, 9]]),
...              axis=0)
Traceback (most recent call last):
   File "<stdin>", line 4, in <module>
   File "/usr/lib/python2.5/site-packages/numpy/lib/function_base.py", line 
1492, in append
     return concatenate((arr, values), axis=axis)
ValueError: arrays must have same number of dimensions
#####################################################


As we expect, this fails.  But let's try to reshape the first array into 
two dimensions and try again.


#####################################################
>>> numpy.append(numpy.array([1, 2, 3]).reshape(1, 3),
...              numpy.array([[4, 5, 6],
...                           [7, 8, 9]]),
...              axis=0)
array([[1, 2, 3],
        [4, 5, 6],
        [7, 8, 9]])
########################################


Another way is to use numpy.atleast_2d(), and for your particular example, 
it's probably more appropriate than using reshape() on the 1-d array.

####################################################
>>> numpy.atleast_2d(numpy.array([3,1,4,1,5]))
array([[3, 1, 4, 1, 5]])
>>> numpy.atleast_2d(numpy.array([3,1,4,1,5])).shape
(1, 5)
####################################################

See:

     http://www.scipy.org/Numpy_Example_List_With_Doc#atleast_2d