Josh - 4 months ago 13

Python Question

I know this has been asked before, but I'm trying to learn Python and I don't want the answer handed to me in the form of completed code. I'm hoping someone can just point me in the right direction.

I know how to do loops and functions and stuff, but I don't know how I should set up a check to determine if a number is prime or not. This might actually be a simple math question really.

What math process do I use to find out if a number is a prime?

Answer

Here's how it could be broken down:

- First, you'll need to ask the user to input a number and store that number in a variable, say
`num_to_test`

- Check that it is valid, i.e. it is an integer larger than 2
- Then, for each number
`d`

between`2`

and`num_to_test-1`

, we will try and figure out if`d`

divides`num_to_test`

- If we find one that does, we can break the loop ! There exists an integer that is not 1 and not our number, and that divides our number. Thus
`num_to_test`

is not a prime. - If we reach the end of the loop and have found nothing, then our number is a prime.
- Finally, tell the user about the result and you're done.

Now, how to do this *pythonically*?

**User input**

With Python 3, all you have to do to request an integer input is a line like this:

```
num_to_test = int(input('What\'s your number?'))
```

Of course, you want to be sure that the user enters a number, that's why you'll surround this by a `try`

/`except`

block and a `while True`

loop, asking again and again until the answer is valid.

```
while True:
try:
# request input
if satisfies_your_conditions(num):
break
else:
# explain why not good
except ValueError:
# print error message
```

**Looping over the integers**

First thing to know, you don't need to go further than the square root of `num_to_test`

.

```
sr = int(math.sqrt(num_to_test))+1
```

And you'll do the test `for d in range(2, sr)`

. Since we don't have a list of previously verified primes, we'll have to do it for all integers. To check whether `d`

divides `num_to_test`

, use the modulo operation:

```
if num_to_test % d == 0:
# d does divide num_to_test
else:
# d does not divide num_to_test
```

**Ending**

One cool feature is the `for...else...`

:

```
for ... :
# do your tests, with possibility of breaking the loop
else:
# this part of the code will only be reached if the loop did not break
```

With this, you'll be able to set a boolean to `True`

or `False`

and then `print`

the result to the user

You could use the ternary operator for this:

```
print('some string' if is_prime else 'another string')
```

This is only a possibility of implementation among others.

```
import math
def main():
while True:
try:
num_to_test = int(input('What\'s your number? '))
if num_to_test == int(num_to_test) and num_to_test > 2:
break
else:
print('Must be an integer larger than 1. Try again.')
except (NameError, ValueError):
print('Not a number. Try again.')
sr = int(math.sqrt(num_to_test))+1
for d in range(2, sr):
if num_to_test % d == 0:
is_prime = False
break
else:
is_prime = True
print('%d is a prime number'%num_to_test if is_prime else '%d is not prime'%num_to_test)
if __name__ == '__main__':
main()
```

Source (Stackoverflow)

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