Error message: unsupported operand type(s) for ___

Question

This program is used to calculate prime numbers and save them to a file. The saving function hasn't been properly programmed yet so please ignore. The program works by comparing an odd number to previous prime numbers. If it is not a multiple of these numbers then it is prime. In theory it should work however when the I try to divide the number by a prime number from the list it returns the error message:

Traceback (most recent call last): File "C:\Users\Archie\Desktop\maths python\prime\prime v1.3.py", line 51, in primeCheck(num) File "C:\Users\Archie\Desktop\maths python\prime\prime v1.3.py", line 8, in primeCheck check = int(num) / listImport TypeError: unsupported operand type(s) for /: 'int' and 'list'

Can you please either suggest how I can fix this problem or suggest a different approach to the problem.

def primeCheck(num):
    divider = 2
    refresh = 0
    firstPoint = 0
    secondPoint = 1
    while refresh == 0:
        listImport = primeList[firstPoint:secondPoint]
        check = int(num) / listImport
        if (check.is_integer()):
            refresh = 1
        else:
            firstPoint = firstPoint + 1
            secondPoint = secondPoint + 1
        if secondPoint > len(primeList):
            file.write(str(num) + "\n")
            print(str(num))
            global x
            x = x + 1
            refresh = 1
            primeList.append


\\        if (int(num)/divider).is_integer():
\\            if divider == num:
\\                file.write(str(num) + "\n")
\\                print(str(num))
\\                global x
\\                x = x + 1
\\                refresh = 1
\\            else:
\\                refresh = 1
\\        else:
\\            divider = divider + 1

global file
repeat = input("How many numbers do you want to add to the existing file?\n")
file = open("Prime results v1.3.txt", "r")
global x
x = 1
num = file.readline()
file.close()
global file
file = open("Prime results v1.3.txt", "a")
num = int(num)

global primeList
primeList = [2]

while x <= int(repeat):
    primeCheck(num)
    num = num + 2

file.close()

The area double slashed is a previous approach I tried, which worked, however this way is more efficient.

Answer 1

There is a lot of ways how to improve your code. But the reason for the error is that you get a list when you do this primeList[firstPoint:secondPoint].

More on this topic is nicely explained here in SO question: Explain Python's slice notation

When you want to index only a single item in a list, you can do it by using my_list[idx] (note: Python indexes from 0) which returns the item at the position idx of list my_list.

It seems to me that the difference between firstPoint and secondPoint is always equal to 1 (if I understand your code well). So you dont have to use secondPoint at all. Just write primeList[firstPoint] and you get the same result as when you use primeList[firstPoint:secondPoint].

There is also an error on the line

primeList.append

Which is a function and not a function call. You probably want to do:

primeList.append(num)

And another tricky part could be, that if you use Python2.x and not Python 3.0 division of two integers is again an integer (e.g. 4/3 = 0). So I suggest slight modification:

check = float(num) / listImport

Than 4/3 = 1.3333333333333333 and moreover is_integer() function raises an error when called on int (and if you use Python 2.x then int/int returns int and therefore raises an error.

Example (Python 2.7):

>>> 1/4
0
>>> float(1)/4
1.3333333333333333