Imagine you have a structured numpy array, generated from a csv with the first row as field names. The array has the form:
dtype([('A', '<f8'), ('B', '<f8'), ('C', '<f8'), ..., ('n','<f8'])
Now, lets say you want to remove from this array the 'ith' column. Is there a convenient way to do that?
I'd like a it to work like delete:
new_array = np.delete(old_array, 'i')
Any ideas?
It's not quite a single function call, but the following shows one way to drop the i-th field:
In [67]: a
Out[67]:
array([(1.0, 2.0, 3.0), (4.0, 5.0, 6.0)],
dtype=[('A', '<f8'), ('B', '<f8'), ('C', '<f8')])
In [68]: i = 1 # Drop the 'B' field
In [69]: names = list(a.dtype.names)
In [70]: names
Out[70]: ['A', 'B', 'C']
In [71]: new_names = names[:i] + names[i+1:]
In [72]: new_names
Out[72]: ['A', 'C']
In [73]: b = a[new_names]
In [74]: b
Out[74]:
array([(1.0, 3.0), (4.0, 6.0)],
dtype=[('A', '<f8'), ('C', '<f8')])
Wrapped up as a function:
def remove_field_num(a, i):
names = list(a.dtype.names)
new_names = names[:i] + names[i+1:]
b = a[new_names]
return b
It might be more natural to remove a given field name :
def remove_field_name(a, name):
names = list(a.dtype.names)
if name in names:
names.remove(name)
b = a[names]
return b
Also, check out the drop_rec_fields
function that is part of the mlab
module of matplotlib.
Update : See my answer at How to remove a column from a structured numpy array *without copying it*? for a method to create a view of subsets of the fields of a structured array without making a copy of the array.
Having googled my way here and learned what I needed to know from Warren's answer, I couldn't resist posting a more succinct version, with the added option to remove multiple fields efficiently in one go:
def rmfield( a, *fieldnames_to_remove ):
return a[ [ name for name in a.dtype.names if name not in fieldnames_to_remove ] ]
Examples:
a = rmfield(a, 'foo')
a = rmfield(a, 'foo', 'bar') # remove multiple fields at once
Or if we're really going to golf it, the following is equivalent:
rmfield=lambda a,*f:a[[n for n in a.dtype.names if n not in f]]
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