How can I iterate through two lists and compare the values of each in Python?

Question

Suppose I have the following lists:

x = [0.5, 0.9, 0.1, 0.3, 0.6]

y = [0.4, 0.3, 0.6, 0.1, 0.9]

I want to write a function that iterates through the list and compares each value in order.

 def compare_numbers(prob,thresh):
 output = []
 for x, y in zip(prob, thresh):
   if x >= y:
    output.append(1)
   else:
    output.append(0)
  return output

  compare_numbers(x,y)

This gives me Type Error: 'float' object is not iterable'

Why is this?

I would expect the output to look like this:
[1, 1, 0, 1, 0]

Is there anyway I could write this function such that if the "threshold" value was only one digit it would still work?

My current code to make the latter function is:

def compare_numbers(prob, thresh):
 output = []
 for n in prob:
  if n >= thresh:
   output.append(1)
  else:
    output.append(0)
return output

compare_numbers(x,y)

Yet this returns the Type Error: TypeError: '>=' not supported between instances of 'int' and 'list' when thresh is a list.

Asked By: brandooo23

||

Source

Answer 1

The problem is that you are comparing the value n from prob with the entire list thresh. What you need to do is to also iterate over thresh.
Here is a small fix that will allow your code to work:

   def compare_numbers(prob, thresh):
     output = []
     for n in range(len(prob)):
       if prob[n] >= thresh[n]:
         output.append(1)
       else:
         output.append(0)
     return output

Answered By: Franfrancisco9

Answer 2

Psychic debugging:

Your first function is passed a float for either prob or thresh, but the code assumes both inputs are list (really, any iterable) of numbers.
Your second function has the opposite problem, it assumes thresh is a single scalar value (e.g. float), but you passed it as a list.

Seems like confusion on your part; you passed the arguments for the first function to the second and vice-versa. I can’t say exactly how it went wrong, because you didn’t show enough information about how you called the functions, but the tracebacks make the ultimate problem obvious.

If you want a function that accepts either of:

A single float
A list with some number of floats (that can be extended to match prob if it’s too short)

for thresh, you could do:

from itertools import cycle

def compare_numbers(prob,thresh):
    try:
        iter(thresh)  # Check if it's iterable, so we can wrap if it's not
    except TypeError:
        thresh = (thresh,)  # Convert float to one-tuple of float for compatibility later
    output = []
    for x, y in zip(prob, cycle(thresh)):
        if x >= y:
            output.append(1)
        else:
            output.append(0)
    return output

The loop itself could also be one-lined to just:

return [int(x >= y) for x, y in zip(prob, cycle(thresh)]

or:

return [1 if x >= y else 0 for x, y in zip(prob, cycle(thresh)]

for simplicity and a small performance bump.

Answered By: ShadowRanger

Answer 3

This should work whether thresh is a list, tuple or a single value:

def compare_numbers(prob,thresh):
    if isinstance(thresh, (list, tuple)):
        return [int(x >= y) for x,y in zip(prob,thresh)]
    else:
        return list(map(lambda x: int(x >= thresh), prob))

print(compare_numbers(x,y))

Or:

def compare_numbers(prob,thresh):
    return [int(x >= y) for x,y in zip(prob, thresh)] if isinstance(thresh, (list, tuple)) else [int(x >= thresh) for x in prob]

Output:

# list
x = [0.5, 0.9, 0.1, 0.3, 0.6]
y = [0.4, 0.3, 0.6, 0.1, 0.9]

[1, 1, 0, 1, 0]

# int or float
x = [0.5, 0.9, 0.1, 0.3, 0.6]
y = 0.4

[1, 1, 0, 0, 1]

Answered By: Arifa Chan

How can I iterate through two lists and compare the values of each in Python?

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