[Tutor] Extracting bits from an array

[Peter Otten]

Colin Ross wrote:

> Hello,
> 
> I have an array that takes on the following form:
> 
> x = [1000,1001,1011,1111]

But these are actually integers in decimal representation. You could treat 
them as binary, but I recommend that you use integers in binary 
representation to avoid confusion:

>>> x = [0b1000, 0b1001, 0b1011, 0b1111]
>>> x
[8, 9, 11, 15]

> The array elements are meant to be binary representation of integers.
> 
> Goal: Access array elements and extract the first two bits.

Is the number of bits fixed to four? If so you can shift the bits to the 
right:
 
>>> y = [v>>2 for v in x]
>>> y
[2, 2, 2, 3]
>>> y
[2, 2, 2, 3]

All that is left to do now is to convert the result to binary for display 
purposes:

>>> for v in y: print "{:02b}".format(v)
... 
10
10
10
11

> e.g. Final result would look something like this:
> 
> x_new = [10,10,10,11]
> 
> What I have tried:
> 
> data_indices = range(4)      #  Set up array of values to loop over
> 
> for idx in data_indices:
>      f = x[idx]                          # Index into array of x values
>      f_idx = f[:2]                      # Extract first two elements

This works for strings and sequences, but not for numbers.

>      print f_idx
> 
> I then receive the following error:
> 
> IndexError: invalid index to scalar variable.