how to declare a 2D array in python

[krypto_wizard]

I am trying to execute the following code


a =[][]
for i in range(3):
for j in range(3):
a[i][j] = i+j

print a


I get an error "invalid syntax". How to fix this bug.

Thanks for the help

[thumper]

You need to append values into the lists

>>> a = []
>>> for i in xrange(3):
... a.append([])
... for j in xrange(3):
... a[i].append(i+j)
...
>>> a
[[0, 1, 2], [1, 2, 3], [2, 3, 4]]
>>>

[krypto_wizard]

Thanks, it worked like a charm.

You need to append values into the lists

>>> a = []
>>> for i in xrange(3):
... a.append([])
... for j in xrange(3):
... a[i].append(i+j)
...
>>> a
[[0, 1, 2], [1, 2, 3], [2, 3, 4]]
>>>

[zobayer1]

:popcorn: Just to put, there is an easier way, have a look:

To create an n by n 2D array:
a = [[]*n for x in xrange(n)]
In case you want to initialize each element with some value v
a = [[v]*n for x in xrange(n)]
Test it

print a

[Tony Flury]

if it is going to be a sparsley populated array, that you need to index with two integers, you might find that a dictionary with a tuple (x,y) as the key might work just as well.

[ssam]

and if you are dealing with arrays of numbers then have a look at numpy.

[Pipeliner]

You should also be able to type this in:
l_variable = [[[2]*3]*4]*5This will create a three dimensional array with the following properties


all values will be populated with '2'
the first dimension will be length 3
the second dimension will be length 4
the third dimension will be length 5

Because Python does not require type declaration when creating a new variable, I choose to put a prefix in front of my variables. In this instance I use the "l_" in front of my list variables.

This has been tested with Python 2.7.1

[Senesence]

You should also be able to type this in:
l_variable = [[[2]*3]*4]*5This will create a three dimensional array

Yes, it will, but a three dimensional array of what?

Independent lists, or references to a single list?

This would be a good test to run:

l_variable[0][0][0] = 5
print l_variable

Be careful.

[Pipeliner]

This will be the result of:
l_variable = [[[2]*3]*4]*5[[[2,2,2], [2,2,2], [2,2,2], [2,2,2]], [[2,2,2], [2,2,2], [2,2,2], [2,2,2]],[[2,2,2], [2,2,2], [2,2,2], [2,2,2]],[[2,2,2], [2,2,2], [2,2,2], [2,2,2]],[[2,2,2], [2,2,2], [2,2,2], [2,2,2]]]

[Senesence]

Run this:


l_variable = [[[2]*3]*4]*5
print l_variable

l_variable[0][0][0] = 5
print l_variable


See the problem now?

[Pipeliner]

Good Point,

Next time, instead of the hints, just post the error in the thread so everyone can follow.

[Senesence]

Good Point,

Next time, instead of the hints, just post the error in the thread so everyone can follow.

It's not an error. The syntax is correct, and results in correct behavior, but it might be behavior that you didn't really expect, because you misunderstood the nature of the statement.

Anyway, I think hints are actually preferable to just dumped answers, because they teach people how to run their own tests, and think critically.

Also, it's not like these were "cryptic" hints; I essentially gave you the test to run, which would (with basic analysis) answer the questions posed.