[Numpy-discussion] genfromtxt behaviour
Hi all
first, please forgive me for my ignorance - I am taking my first
stumbling steps with numpy and scipy.
I am having some difficulty with the behaviour of genfromtxt.
s = SIO.StringIO(1, 2, 3
4, 5, 6
7, 8, 9)
g= genfromtxt(s, delimiter=', ', dtype=None)
print g[:,0]
This produces the output I expected, a slice for column 0...
array([1, 4, 7])
BUT
s = SIO.StringIO(a, 2, 3
b, 5, 6
c, 8, 9)
g= genfromtxt(s, delimiter=', ', dtype=None)
g[:,0]
Produces the following error:
IndexError: invalid index
In the first case, genfromtxt returns me a 2d array (list of lists),
in the second it returns a list of tuples with an associated dtype
list of tuples.
How can I do my nice 2d slicing on the latter?
array([('a', 2, 3), ('b', 5, 6), ('c', 8, 9)],
dtype=[('f0', '|S1'), ('f1', 'i4'), ('f2', 'i4')])
thanks in advance for any assistance
Matt
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Re: [Numpy-discussion] genfromtxt behaviour
On Oct 29, 2010, at 2:35 PM, Matt Studley wrote:
Hi all
first, please forgive me for my ignorance - I am taking my first
stumbling steps with numpy and scipy.
No problem, it;s educational
I am having some difficulty with the behaviour of genfromtxt.
s = SIO.StringIO(1, 2, 3
4, 5, 6
7, 8, 9)
g= genfromtxt(s, delimiter=', ', dtype=None)
print g[:,0]
This produces the output I expected, a slice for column 0...
array([1, 4, 7])
BUT
s = SIO.StringIO(a, 2, 3
b, 5, 6
c, 8, 9)
g= genfromtxt(s, delimiter=', ', dtype=None)
g[:,0]
Produces the following error:
IndexError: invalid index
In the first case, genfromtxt returns me a 2d array (list of lists),
Well, not exactly.
When you use dtype=None, genfromtxt tries to guess the type of the columns.
Because in this case all your variables can be safely casted to integers,
genfromtxt considers that the dtype is uniform and it outputs a 2D array.
in the second it returns a list of tuples with an associated dtype
list of tuples.
Well, in this case, the first column is detected to be of type string, while
the other columns are of type integers (as the dtype shows you).
Therefore, genfromtxt considers that the dtype is structured and the output is
a 1D array (each line is a tuple of 3 elements, the first one '|S1', the 2nd
and 3rd ints
How can I do my nice 2d slicing on the latter?
array([('a', 2, 3), ('b', 5, 6), ('c', 8, 9)],
dtype=[('f0', '|S1'), ('f1', 'i4'), ('f2', 'i4')])
Select a column by its name:
yourarray['f0']
More info:
http://docs.scipy.org/doc/numpy/user/basics.rec.html
http://docs.scipy.org/doc/numpy/user/basics.io.genfromtxt.html
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[Numpy-discussion] genfromtxt behaviour
snip
How can I do my nice 2d slicing on the latter?
array([('a', 2, 3), ('b', 5, 6), ('c', 8, 9)],
dtype=[('f0', '|S1'), ('f1', 'i4'), ('f2', 'i4')])
Select a column by its name:
yourarray['f0']
Super!
So I would need to get the dtype object...
myData[ myData.dtype.names[0] ]
in order to index by column.
Matt
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Re: [Numpy-discussion] genfromtxt behaviour
On Oct 29, 2010, at 2:59 PM, Matt Studley wrote:
snip
How can I do my nice 2d slicing on the latter?
array([('a', 2, 3), ('b', 5, 6), ('c', 8, 9)],
dtype=[('f0', '|S1'), ('f1', 'i4'), ('f2', 'i4')])
Select a column by its name:
yourarray['f0']
Super!
So I would need to get the dtype object...
myData[ myData.dtype.names[0] ]
in order to index by column.
For a structured array, yes, you get it.
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