User guide

NumPy User Guide, Release 1.9.0
>>> x.dtype.names
(’col1’, ’col2’)
>>> x.dtype.names = (’x’, ’y’)
>>> x
array([(0, 0.0), (0, 0.0), (0, 0.0)],
dtype=[((’title 1’, ’x’), ’|i1’), ((’title 2’, ’y’), ’>f4’)])
>>> x.dtype.names = (’x’, ’y’, ’z’) # wrong number of names
<type ’exceptions.ValueError’>: must replace all names at once with a sequence of length 2
Accessing field titles
The field titles provide a standard place to put associated info for fields. They do not have to be strings.
>>> x.dtype.fields[’x’][2]
’title 1’
Accessing multiple fields at once
You can access multiple fields at once using a list of field names:
>>> x = np.array([(1.5,2.5,(1.0,2.0)),(3.,4.,(4.,5.)),(1.,3.,(2.,6.))],
dtype=[(’x’,’f4’),(’y’,np.float32),(’value’,’f4’,(2,2))])
Notice that x is created with a list of tuples.
>>> x[[’x’,’y’]]
array([(1.5, 2.5), (3.0, 4.0), (1.0, 3.0)],
dtype=[(’x’, ’<f4’), (’y’, ’<f4’)])
>>> x[[’x’,’value’]]
array([(1.5, [[1.0, 2.0], [1.0, 2.0]]), (3.0, [[4.0, 5.0], [4.0, 5.0]]),
(1.0, [[2.0, 6.0], [2.0, 6.0]])],
dtype=[(’x’, ’<f4’), (’value’, ’<f4’, (2, 2))])
The fields are returned in the order they are asked for.:
>>> x[[’y’,’x’]]
array([(2.5, 1.5), (4.0, 3.0), (3.0, 1.0)],
dtype=[(’y’, ’<f4’), (’x’, ’<f4’)])
Filling structured arrays
Structured arrays can be filled by field or row by row.
>>> arr = np.zeros((5,), dtype=[(’var1’,’f8’),(’var2’,’f8’)])
>>> arr[’var1’] = np.arange(5)
If you fill it in row by row, it takes a take a tuple (but not a list or array!):
>>> arr[0] = (10,20)
>>> arr
array([(10.0, 20.0), (1.0, 0.0), (2.0, 0.0), (3.0, 0.0), (4.0, 0.0)],
dtype=[(’var1’, ’<f8’), (’var2’, ’<f8’)])
34 Chapter 2. Numpy basics