Initializing numpy string array using for loop failing for some elements

Question

I am trying to create an array of binary strings each exactly 16 bits long.

I have declared an empty string array which holds up to 20 characters for each element:

bin_array = np.empty(len(dat_array), dtype="S20")

Then I assign each element using this loop:

for i in range(len(dat_array)):
binary = bin(dat_array[i])
if len(binary) < 18:
    number_of_zeros = 18 - len(binary)
    zeros = ""
    for i in range(number_of_zeros):
        zeros = zeros + "0"
    binary = "0b" + zeros + binary[2:]
bin_array[i] = binary
print bin_array[i]

at this stage, print bin_array[i] gives me every element entirely correctly, but once I am outside of the loop and check my array:

print bin_array

some elements have apparently changed to error messages and it returns something like this:

'0b1011000011010101' '0b1100000000110011' '0b1101000101101010' 
'0b1110000000101001' '0b1111000100011111' 'ject that raises an ' 
'ImportError if ctype' 's is not available.\n' '\n        Raises\n  
 ' '    ------\n        I' 'mportError\n         ' '   If ctypes is

not ' 'available.\n\n ' '0b1000000000110100' '0b1001000101110100' '0b1010000000011111'

How can it be that each element when assigned by the for loop is exactly correct, but inside the array, some elements (and only some!) produce errors?

Alternatively if I use an empty python list, and use the for loop to .append each element, everything works fine. But I would still like to know why this is happening in numpy.

edit: when initializing values in a predefined list of len(dat_array) using the same for loop, some elements are also failing to initialize, so it seems like this is not a numpy array issue...

Toby Speight · Accepted Answer · 2017-04-28 14:20:47Z

1

The problem is that you've used i as the index in both the inner and outer for loops. You can change the inner index to k, say or simply eliminate the inner loop and use zeros = number_of_zeros * "0"

edited Apr 28, 2017 at 14:20

Toby Speight

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answered Aug 5, 2015 at 16:10

saulspatz

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Comments

Toby Speight · Accepted Answer · 2017-04-28 14:21:07Z

0

You are using the same index i for your inner and outer loop, which is causing you your indexing into the numpy array to get corrupted. You should use a different index variable:

    for j in range(number_of_zeros):
        zeros = zeros + "0"

edited Apr 28, 2017 at 14:21

Toby Speight

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answered Aug 5, 2015 at 16:17

Conor

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