r_metal - 10 months ago 34

Python Question

This program suppose to find 1000 prime numbers and pack them into a list

Here's the code:

`num = raw_input('enter a starting point')`

primes = [2]

num = int(num)

def prime_count(num):

if num % 2 == 0: #supposed to check if the number is divided by 2 evenly

num = num +1 #if it is, then add 1 to that number and check again

return num

elif num % num == 0:

primes.append(num) #supposed to add that prime to a list

num = num + 1 #add 1 and check again

return num

while len(primes) <= 999:

prime_count(num)

So what actually happens when I run it:

it asks me raw_input and then goes to various things depending on what I choose as input:

- If I choose a prime, let's say 3, it runs and adds 999 of 3s to the list instead of adding it just one time and going on to try 4
- If I choose a non-prime, let's say 4, it just breaks, after that I can't even print out a list

What am i doing wrong?

Answer

You're never using the return value from prime_count. Try this:

```
while len(primes) <= 999:
num = prime_count(num)
```

You've set your self up for confusion by using the name `num`

as both a parameter (also a local variable) inside of `prime_count`

, and also as a global variable. Even though they have the same name, they are different variables, due to Python's rules for the scope of variables.

Also, `prime_count`

is (probably unintentionally) leveraging the fact that `primes`

is a global variable. Since you're not *assigning* to it, but rather just calling a method (append), the code will work without using the `global`

keyword.

However, your algorithm isn't even correct. `if num % num == 0`

says *"if a number divided by itself has a remainder of zero"* which will *always* be true. This program will find a lot of "primes" that aren't primes.

Real Python programs do very little in the global scope; your current code is just asking for confusion. I suggest you start with this template, and also do some reading of existing Python code.

```
def add_three(a_param):
a_local_var = 3 # This is *different* than the one in main!
# Changes to this variable will *not* affect
# identically-named variables in other functions
return a_local_var + a_param
def main():
a_local_var = 2
result = add_three(a_local_var)
print result # prints 5
if __name__ == '__main__':
main()
```

Source (Stackoverflow)