Creating a 2d Grid in Python

Question

I am trying to create a 2d array or list or something in Python. I am very new to the language, so I do not know all the ins and outs and different types or libraries.

Basically, I have a square list of lists, g, and I want to transpose it (turn rows into columns and columns into rows). Later I will be traversing this list of lists and the transposed list of lists. In the transposed list, the order of the columns does not matter. Is this the "Python way"? Is there a much faster way to do this?

I like using lists because I am comfortable with the syntax that is so similar to arrays in the languages I know, but if there is a better way in Python, I would like to learn it.

def makeLRGrid(g):
    n = []
    for i in range(len(g)):
        temp = []
        for j in range(len(g)):
            temp.append(g[j][i])
        n.append(temp)
    return n

Thank you for any help you can offer!

Edit: Sorry for the confusion, apparently I mean transpose, not invert!

Sven Marnach · Accepted Answer · 2012-06-15 15:07:33Z

6

This implementation does the same as yours for "square" lists of lists:

def makeLRGrid(g):
    return [row[:] for row in g]

A list can be copied by slicing the whole list with [:], and you can use a list comprehension to do this for every row.

Edit: You seem to be actually aiming at transposing the list of lists. This can be done with zip():

def transpose(g):
    return zip(*g)

or, if you really need a list of lists

def transpose(g):
    return map(list, zip(*g))

3 Comments

William Over a year ago

Sorry, I meant to describe inverting the list of lists by turning rows into columns and vice-versa. I believe this code just copies the list of lists? Is there slick syntax like this to turn rows into columns and columns into rows?

William Over a year ago

This is really nice, thank you! One question I have still is what does the * operator do to g? Does it simply put each of the lists in g as a separate argument to zip? The documentation does not quite clear this question up for me.

Sven Marnach Over a year ago

@user1458948: It does exactly what you said. See the Python tutorial, some section on functions and parameters, for more information. It's called "argument unpacking", by the way.

Glorfindel · Accepted Answer · 2023-03-24 19:00:27Z

4

For numerical programming I would strongly recommend NumPy (and the related SciPy). NumPy implements very fast multi-dimensional arrays. Have a look at here for the available array manipulation routines.

edited Mar 24, 2023 at 19:00

Glorfindel

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answered Jun 15, 2012 at 14:51

Chris

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3 Comments

William Over a year ago

Thank you for the suggestion, but I am just doing one small project with this, so I do not think that NumPy is what I need. However, I will take a look at it and might use it if it is not too complicated to learn.

Simon Bergot Over a year ago

as much as I like numpy, you should try to give an answer to the question without relying on a link. @user1458948 even if your project is small, if your grid is big, numpy will be useful.

Chris Over a year ago

@Simon I generally do try to - I agree that answers that are just links aren't particularly helpful. In this case I think I supplement the other answers by suggesting NumPy. Also, I was waiting to find out what invert the lists meant before giving a solid example.

Jdog · Accepted Answer · 2012-06-15 15:20:53Z

2

To add to Chris's comment, I really cannot recommend numpy enough. You say it is for one project, but you will probably make use of it many times over for the sake of learning some (simple) syntax just once.

For example if you have a list of lists g:

g = [[1,2,3],[4,5,6],[7,8,9]]

You can make this into an array simply by:

>>> import numpy as np
>>> g_array = np.array(g)
>>> g_array
array([[1, 2, 3],
      [4, 5, 6],
      [7, 8, 9]])

access any item by simply:

>>> g_array[0,1]
2
>>> g_array[1,2]
6

and perform your 'invert' (actually transpose- i.e. row one becomes column one up to for each row) by:

>>> g_transpose = g_array.transpose() # or  = g_array.T from Simon's comment
>>> g_transpose
array([[1, 4, 7],
      [2, 5, 8],
      [3, 6, 9]])

edited Jun 15, 2012 at 15:20

answered Jun 15, 2012 at 15:15

Jdog

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1 Comment

Simon Bergot Over a year ago

or g_array.T. +1 for numpy answer

shihongzhi · Accepted Answer · 2012-06-16 02:53:33Z

1

I guess invert the list of lists is like this:

g = [[1,2,3], [4,5,6], [7,8,9]]
result = [p[::-1] for p in g]
print result

Output：

[[3,2,1], [6,5,4], [9,8,7]]

Maybe I'm worry. Can you give some example?

EDIT: for the comment example

result = [list(p) for p in zip(*g)]
print result

Output:

[[1,4,7], [2,5,8], [3,6,9]]

edited Jun 16, 2012 at 2:53

answered Jun 15, 2012 at 15:01

shihongzhi

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1 Comment

William Over a year ago

g = [[1,2,3], [4,5,6], [7,8,9]] result = [[1,4,7], [2,5,8], [3,6,9]]

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