Problem to read from array

[Peter Otten]

vostrushka at gmail.com wrote:

> Hi,
> I have a file with one parameter per line:
> a1
> b1
> c1
> a2
> b2
> c2
> a3
> b3
> c3
> ...
> The parameters are lines of characters (not numbers)
> 
> I need to load it to 2D array for further manipulations.
> So far I managed to upload this file into 1D array:
> 
> ParametersRaw = []

No need for this assignment as you throw the value away.
> with open(file1) as fh:
>     ParametersRaw = fh.readlines()
> fh.close()

The with open() ... statement already ensures that the file is closed.

> NumberOfColumns = 7
> NumberOfRows = len(ParametersRaw)/NumberOfColumns
> 
> Parameters = [[],[]]
> i=0
> j=0
> k=0
> while (i < NumberOfRows):
>     while (j < NumberOfColumns):
>         k = (i*NumberOfColumns)+j
>         Parameters[i][j] = ParametersRaw[k]

The inner lists Parameters[i] are empty so every assignment

Parameters[i][k] = ...

will fail. While the minimal fix is to use append()

          Parameters[i].append(ParametersRaw[k])

you will run into the same problem with the rows if there are more than the 
two you prepared for in the line

> Parameters = [[],[]]

>         j = j + 1
>     i = i + 1
>     j = 0
> 
> it fails at the line "Parameters[i][j] = ParametersRaw[k]" with error:
> 
> IndexError: index 0 is out of bounds for axis 0 with size 0
> 
> In case of populating 1D array I would use append() method.
> But in case of 2D I am lost of how append() can be applied.
> Could some one help newbie.

As a general remark in idiomatic Python the loop

i = 0
while i < THRESHOLD:
   ...
   i = i + 1

is written as a for loop with range():

for i in range(THRESHOLD):
   ...

For your specific problem list slicing is a good approach:

COLUMNS = 7
with open(file1) as f:
    parameters_raw = f.readlines()

parameters = []
for row_start in range(0, len(parameters_raw), COLUMNS):
    parameters.append(parameters_raw[row_start:row_start+COLUMNS])