How to understand the error in my Python program

Question

code for printing factors of a number

I'm trying to code a program that prints the factors of a number, just as shown in the attached image, but the outcome is totally not what I'm expecting. (I just want the code to return factors of the specified number.) What am I doing wrong and what is the correct way of doing it?

Code:

number=42
factor_list=[]
for num in range(1,number+1):
    num=number/num
    if (number%num)==0:
        factor_list.append(num)
print(factor_list)

Output:

[42.0, 21.0, 14.0, 10.5, 7.0, 6.0, 5.25, 3.5, 3.0, 2.625, 2.0, 1.75, 1.5, 1.3125, 1.0]

Post the code in the question directly and not as pictures - stackoverflow.com/help/how-to-ask — matszwecja
– matszwecja, Commented Feb 24, 2022 at 9:54
Please add your code to the question. Hyperlinks to images are not acceptable — jackal
– jackal, Commented Feb 24, 2022 at 9:54
tutorialspoint.com/How-to-Find-Factors-of-Number-using-Python — Devang Sanghani
– Devang Sanghani, Commented Feb 24, 2022 at 9:56
Thanks for adding the code as text. Would you add the console output as text too, in a separate block? — halfer
– halfer, Commented Feb 24, 2022 at 18:44

Roopa M · Accepted Answer · 2022-02-24 10:03:54Z

1

Just remove the 4th line i.e num=number/num which produces quotients. you are trying to find modulus of number from those quotients that is the reason you are getting wrong answer. Here is the modified code which works.

number = 42
factor_list=[]
for num in range(1,number+1):
   if(number%num==0):
       factor_list.append(num)
print(factor_list)

answered Feb 24, 2022 at 10:03

Roopa M

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Comments

matszwecja · Accepted Answer · 2022-02-24 10:09:04Z

0

As to why your program does not work, let's see what is going on step by step:

    number=42
    factor_list=[]
    for num in range(1,number+1):      #for num in [1, 2, 3, 4, 5... 42] (Let's take 28 for example)
        num=number/num                 # num = 42 / 28; num = 1.5 now
        if (number%num)==0:            # 42 mod 1.5 == 0.0
            factor_list.append(num)    # so we add 1.5 to the list.
    print(factor_list)

See the problem in logic now?

answered Feb 24, 2022 at 10:09

matszwecja

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Comments

jackal · Accepted Answer · 2022-02-24 10:10:44Z

0

In Python, the division operator / will result in a float data type even if the values either side of it are int. For integer division you should use //

Having said that, what you need here is the modulo operator.

Here's a concise (but not necessarily most efficient) way of achieving your objective:

def factors(n):
    return [1] + [x for x in range(2, n + 1) if n % x == 0]

print(factors(4203))

Output:

[1, 3, 9, 467, 1401, 4203]

Fun fact:

If the length of the returned list from this function is 2 then n must be a prime number. Don't go using this function to test for primes though as it would be woefully inefficient

edited Feb 24, 2022 at 10:10

answered Feb 24, 2022 at 10:04

jackal

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2 Comments

Crimson Ninja Over a year ago

Ah certainly, that makes sense. In the 2nd line of your code though, I see that you have defined a list- 1. without using a variable or labeling the list 2. also used '+' sign to further add the 'for' loop (which I think is similar to list comprehension) but you have also used an 'if' condition within it, all of which helps define the list So out of curiosity my question is- can I add multiple such conditions using the '+' sign similar to what you have done to define the list above? Also, if it is possible to label the list to permanently store list elements for further usage?

jackal Over a year ago

@CrimsonNinja Instead of printing the output from factors(n) just assign it to some variable. Yes, it's a list comprehension. Remember that all positive integers have a factor of 1. So that's just a constant that we can add to the output of the comprehension

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