While loop not exiting Python

Question

Given a list of inputs. The last x of the list, are a number that I want to use for a test. So, the beginning n-x elements are the ones I want to test using the elements in x.

For example:

test_case = [0.18677649597722776, 0.21992417009282958, 0.21001370207789635, 0.2576939078119566, -0.26790678064174844, 0.23723906040549575, 0.23796810219833633, 0.12311570730540798, 0.291222989748139, -0.46589179980005796, -0.5679935337540711, -0.541967302717414, 0.2797199715268191, -0.0462338707795437, 0.3352382038488532, -0.6395453091791992, -0.7116194799285872, -0.6827853559995019, 0.4131897184013285, 0.07125041194386302, 0.47179441094288416, -0.5670171363969451, -0.6493889334859158, -0.6214861349381114, 0.6332084272531783, 0.2946607775328391, 0.7252115985158697, -0.48494480580385074, -0.5584250339723696, -0.5329318548632481, 0, 1, 0, 5, 5, 5]

The last 6 numbers I want to use as part of my test on the first 30 numbers. I want to cycle through the 30 numbers such that when test_case[31] < 3, give me test_case[0], else give me -999. This iterates until test_case[36] < 3, give me test_case[5]. Then I want test_case[31] to go back and be used on test_case[6] and loop again.

After I'm at test_case[30], I want it to stop.

Here's what I have:

def test_inputs(x, comp_size):
 counts = x[-comp_size:]
 inputs = x[:(len(x)-comp_size+1)]
 counts_pos = 0
 inputs_pos = 0
 while inputs_pos < (len(x)-comp_size+1):
 if counts_pos == 6:
 counts_pos = 0
 if counts[counts_pos] < 3:
 x.append(inputs[inputs_pos])
 print inputs_pos
 print counts_pos
 inputs_pos += 1
 counts_pos += 1
 else:
 x.append(-999)
 print inputs_pos
 print counts_pos
 inputs_pos += 1
 counts_pos += 1

I'm trying to make a generalized function. In this case, should be able to run:

test_inputs(test_case, 6)

However, this doesn't stop at inputs_pos == 31. I put in print statements, and it looks like it just keeps going.

Is there a simpler way using a filter?

• jarmod: Your loop condition depends on len(x) but you're appending one item to x every iteration through the loop, so len(x) keeps increasing.
• WhitneyChia: That's it, thank you!!

Answer 1

Am I understanding correctly that you want:

from itertools import cycle
def test_inputs(x, comp_size):
 return [(input if count<3 else -999) for (input,count)
 in zip(x[:-comp_size], cycle(x[-comp_size:]))]

You can restore the in-place modification behaviour by using x.extend instead of return. As the slices are performed before the extend call, it will produce the exact same items. Still, I don't think it's generally a good idea to mix data types in one list like this, when it would be as easy to pass (inputs,counts) tuples.

Answer 2

The loop doesn't stop because len(x) is evaluated at each iteration. In your cycle you increment count_pos and append a new element to x.