How to make arrays from Lists

[gc_ottawa]

I want to construct a 2-dimensional array from a List but I cannot
find a simple way of changing any element. For example, construct a
3x3 array like this:-

x=[0,0,0]

x=[x]*3
this produces [[0,0,0],[0,0,0],[0,0,0]. So far so good.
How do I change the value of any element to produce (say)
[[99,0,0],[0,0,0],[0,0,0]] ?

gordc

 

[bearophileHUGS]

gc_ott:

How do I change the value of any element to produce (say)
[[99,0,0],[0,0,0],[0,0,0]] ?

gordc

To create a 2D list, that is a list of lists:
x = [[0] * ncols for i in nrows]
(Don't do what you were doing, because you end with many references to
the same list, and that will give you troubles.)

To set an item you do just:
x[0][0] = 99

Bye,
bearophile

 

[gc_ottawa]

gc_ott:

How do I change the value of any element to produce (say)
[[99,0,0],[0,0,0],[0,0,0]] ?

gordc

To create a 2D list, that is a list of lists:
x = [[0] * ncols for i in nrows]
(Don't do what you were doing, because you end with many references to
the same list, and that will give you troubles.)

To set an item you do just:
x[0][0] = 99

Bye,
bearophile

Many thanks, I don't think I would ever 'discovered' this.
gordc

 

[Robert Kern]

gc_ott:

How do I change the value of any element to produce (say)
[[99,0,0],[0,0,0],[0,0,0]] ?
gordc

To create a 2D list, that is a list of lists:
x = [[0] * ncols for i in nrows]
(Don't do what you were doing, because you end with many references to
the same list, and that will give you troubles.)

To set an item you do just:
x[0][0] = 99

Bye,
bearophile

Many thanks, I don't think I would ever 'discovered' this.

If you are trying to emulate numerical arrays, you may want to use numpy, instead.

http://numpy.scipy.org/

import numpy
x = numpy.zeros([3,3], dtype=int)
x[0,0] = 99

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

 

[Gabriel Genellina]

On Nov 11, 7:48 pm, (e-mail address removed) wrote:

To create a 2D list, that is a list of lists:
x = [[0] * ncols for i in nrows]

Many thanks, I don't think I would ever 'discovered' this.

Try reading the FAQ:
http://www.python.org/doc/faq/programming/#how-do-i-create-a-multidimensional-list