1D to 2D array - python

Question

I would like to change the data stored in 1D into 2D: I mean:

from

x|y|a
1|1|a(1,1)
2|1|a(2,1)
3|1|a(3,1)
1|2|a(1,2)
...

into:

x\y|1     |2     |3
1  |a(1,1)|a(1,2)|a(1,3
2  |a(2,1)|a(2,2)|a(2,3)...
3  |a(3,1)|a(3,2)|a(3,3)...
    ...

I did it by 2 loops:

(rows - array of x,y,a)
for n in range(len(rows)):
            for k in range(x_len):
                    for l in range(y_len):
                            if ((a[2, n] == x[0, k]) and (a[3, n] == y[0, l])):
                                    c[k, l] = a[0, n]

but it takes ages, so my question is if there is a smart and quick solution for that in Python.

So to clarify what I want to do:

I know the return() function, the point is that it's randomly in array a.

So:

a = np.empty([4, len(rows)]

I read the data into array a from the database which has 4 columns (1,2,x,y) and 'len(rows)' rows.

I am interested in '1' column - this one I want to put to the new modified array.

x = np.zeros([1, x_len], float)

y = np.zeros([1, y_len], float)

x is a vector of sorted column(x) from the array a, but without duplicitas with a length x_len

(I read it by the sql query: select distinct ... )

y is a vector of sorted column(y) from the array a (without duplicitas) with a length y_len

Then I am making the array:

c = np.zeros([x_len, y_len], float)

and put by 3 loops (sorry for the mistake before) the data from array a: >

 for n in range(len(rows)):
     for k in range(x_len):
        for l in range(y_len):
          if ((a[2, n] == x[0, k]) and (a[3, n] == y[0, l])):
              c[k, l] = a[0, n]

Example:

Array a

   array([[1, 3, 6, 5, 6], 
          [1, 2, 5, 5, 6], 
          [1, 4, 7, 1, 2],   ## x
          [2, 5, 3, 3, 4]])  ## y

Vectors: x and y

 [[1,2,4,7]]   ## x with x_len=4

 [[2,3,4,5]]   ## y with y_len=4

Array c

   array([[1, 5, 0, 0], 
          [0, 0, 0, 0],  
          [0, 0, 0, 3],  
          [0, 6, 0, 0]])

the last array c looks like this (the first a[0] is written into):

 x\y 2|3|4|5
 -----------
 1   1|5|0|0
 2   0|0|0|0
 4   0|0|0|3
 7   0|6|0|0

I hope I didn't make mistake how it's written into the array c.

Thanks a lot for any help.

I know you probably spent a lot of time formatting your question, but all the <br /> make it hard to help with the formatting. — Levon
– Levon, Commented Jun 18, 2012 at 20:02
got rid of the <br /> and made it look easier. Next time use the code tags so its easier to see your code :) — hammythepig
– hammythepig, Commented Jun 18, 2012 at 20:09
There are three loops here, not two. And what's the value of len(rows)? You seem to be looping through x and y values, testing each pair against a single a row. There's almost certainly a better way of doing this, but I can't tell what x, y, and a are; your indexing makes it look as though they are more complex data structures. Give us a simple example -- say, a 9 row example, written out in correct Python syntax, so that we can simply copy and paste the code. Then give the 3x3 grid that should result. — senderle
– senderle, Commented Jun 18, 2012 at 20:59
I edited my question, as I couldn't post the new answer, thanks — Purchawka
– Purchawka, Commented Jun 18, 2012 at 22:28

fraxel · Accepted Answer · 2012-06-18 20:04:24Z

7

You could use numpy:

>>> import numpy as np
>>> a = np.arange(9)
>>> a
array([0, 1, 2, 3, 4, 5, 6, 7, 8])
>>> a.reshape(3,3)
array([[0, 1, 2],
       [3, 4, 5],
       [6, 7, 8]])
#or:
>>> a.reshape(3,3).transpose()
array([[0, 3, 6],
       [1, 4, 7],
       [2, 5, 8]])

answered Jun 18, 2012 at 20:04

fraxel

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1 Comment

Purchawka Over a year ago

Does the reshape() function mind the order of data in the array? Cause it's random in my array a. And I can't crearly see that reshape() solves my problem...

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