I have a list of lists I've read from a file. Each of the inner lists is six elements in length, and has 3 strings and 5 floats. How do I convert this list of lists into a numpy array? Thanks!
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How does a length-6 list contain 3 strings and 5 floats? Isn't that 8 objects?Curt F.– Curt F.2015-08-31 06:54:20 +00:00Commented Aug 31, 2015 at 6:54
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oops, 8 objects is correct.Hugo Kitano– Hugo Kitano2015-08-31 17:18:14 +00:00Commented Aug 31, 2015 at 17:18
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2 Answers
You want a structured array, one that has a compound dtype:
A sample list of lists:
In [4]: ll = [['one','two',1,1.23],['four','five',4,34.3],['six','seven',4,34.3]]
trying to make a regular array, produces an array of strings:
In [5]: np.array(ll)
Out[5]:
array([['one', 'two', '1', '1.23'],
['four', 'five', '4', '34.3'],
['six', 'seven', '4', '34.3']],
dtype='|S5')
But if I specify a dtype that contains 2 strings, and int and a float, I get a 1d structured array:
In [8]: np.array([tuple(x) for x in ll],dtype='S5,S5,i,f')
Out[8]:
array([('one', 'two', 1, 1.2300000190734863),
('four', 'five', 4, 34.29999923706055),
('six', 'seven', 4, 34.29999923706055)],
dtype=[('f0', 'S5'), ('f1', 'S5'), ('f2', '<i4'), ('f3', '<f4')])
Note that I had to convert the inner lists to tuples. That's how a structured array takes its input, and also how it displays it. It helps distinguish the structured 'row' from the uniform 'row' of a regular (2d) array.
This the same sort of structured array that genfromtxt or loadtxt produces when reading from a csv file.
There are other ways of specifying the dtype, and a couple of other ways of loading the data into such an array. But this is a start.
Further testing, https://stackoverflow.com/a/47774915/901925, shows that this tuple conversion is not that time consuming. Simply creating the array takes more time.
I had the same problem, but tuples are no solution. So I found (python 3.7.1):
ll = [['one','two',1,1.23],['four','five',4,34.3],['six','seven',4,34.3]]
np.array(ll, dtype = 'object')
result:
array([['one', 'two', 1, 1.23],
['four', 'five', 4, 34.3],
['six', 'seven', 4, 34.3]], dtype=object)
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