Adding comma every 3 digit problem, in python, what's different using list or using format

Question

I am practicing an algorithm on a website.

I want to add data(number) comma(,) every 3 digit.

But 'a', which variable I made, can't be the collect answer.

But 'b', which variable I searched, is the collect answer.

Can you tell me why 'a' is not the same as 'b'

length = 8
data = "12421421"

inv_result = []
for index in range(length):
    if index % 3 == 0:
        inv_result.append(',')
        inv_result.append(str(data[index]))
    else:
        inv_result.append(str(data[index]))

result = inv_result[::-1]

#first comma delete
result.pop()

a = ''.join(result)
b = format(int(datas),",")

print(a)
print(b)
print(a == b)

result is

12,412,421
12,421,421
False

Answer 1

Your problem is that you didn't reverse the data in the beginning. The following (slightly cleaned up) code works:

length = 8
data = "12421421"
inv_data = data[::-1]
inv_result = []
for index in range(length):
    if index % 3 == 0:
        inv_result.append(',')
    inv_result.append(str(inv_data[index]))

result = inv_result[::-1]

#first comma delete
result.pop()

a = ''.join(result)
b = format(int(data),",")

print(a)
print(b)
print(a == b)