在for循环内和在for循环外声明变量的区别

Question

I try to make a program that print the factors of a number and print its abundant factors. But when im working with it, i found some problem. I don't know why the output is different if i declare a variable named "sum_abundant_factor" inside the for loop and outside the for loop.

"sum_abundant_factor" is a variable that i use to check whether the factor is abundant or not. (abundant number is a number that is smaller than the sum of its proper divisors).

This is my code and output when i declare "sum_abundant_factor" inside the for loop:

input_number = int(input('Input number : '))
factor = ''
sum_factor = 0
abundant_factor = ''

for i in range(1, input_number+1):
    if input_number % i == 0:
        sum_abundant_factor = 0
        factor += str(i) + ' '
        if i < input_number :
           sum_factor += i
        for j in range(1, i):
           if i % j == 0:
              sum_abundant_factor += j
        if sum_abundant_factor > i:
           abundant_factor += str(i) + ' '

print('Factors of {} :'.format(input_number), factor)
print('Abundant Factors :', abundant_factor)

Output :
Input number : 54
Factors of 54 : 1 2 3 6 9 18 27 54
Abundant Factors : 18 54

And this is my code and output when i declare "sum_abundant_factor" before (outside) the for loop:

input_number = int(input('Input number : '))
factor = ''
sum_factor = 0
abundant_factor = ''
sum_abundant_factor = 0

for i in range(1, input_number+1):
    if input_number % i == 0:
        factor += str(i) + ' '
        if i < input_number:
           sum_factor += i
        for j in range(1, i):
           if i % j == 0:
              sum_abundant_factor += j
       if sum_abundant_factor > i:
           abundant_factor += str(i) + ' '

print('Factors of {} :'.format(input_number), factor)
print('Abundant Factors :', abundant_factor)

Output :
Input number : 54
Factors of 54 : 1 2 3 6 9 18 27 54
Abundant Factors : 6 9 18 27 54

I have no idea why the abundant factors output is different when i declaring the variable inside and outside for loop. Can anyone help me explain it to me?

Answer 1

If the sum_abundant_factor = 0 is declared inside the loop, every time it resets the sum_abundant_factor to 0 when it is the next element of the range .

Here is an easier example:

a = [1, 2, 3]
for i in a:
    b = 0
    b += i
    print(b)

Every time it is the next element in the a list, it will reset b to 0 , so then 0 added to i would change nothing to the iterator i , so the above code would output:

1
2
3

Whereas if you define the b variable outside the loop:

a = [1, 2, 3]
b = 0
for i in a:
    b += i
    print(b)

It will output:

1
3
6

Because it won't reset, it will keep adding, so 0 + 1 is 1 (the first printed value), and 1 + 2 is 3 (the second printed value) and 3 + 3 is 6 (the third printed value).

Answer 2

If you put the sum_abundunt_factor inside the for loop, every time when you loop through it, it will be set to 0 if the if input_number % i == 0: happens, even though you have code such as sum_abundant_factor += j . It outputs that number, but then resets to 0 the next time you loop through it when the if statement happens. If you leave it outside, the number will only be added every time sum_abundant_factor += j occurs. The number won't be reset to 0 as it is outside the for loop