CODEWITHSUNDEEP
pythonfunctionbooleanprimes
Olivia Scruton
Olivia Scruton

Reputation: 3

Need to create function that returns a boolean when asked, "Is this a prime number?"

I am trying to create a function that will test whether a number is a prime number and then return True or False. I am an extreme beginner at Python, so please make your code as simple as possible. Here's what I tried so far (only returns True):

def isPrime(x):

    x = int(x)
    for i in range(2, x):
        if(x % i == 0):
            x == False
        else:
            x == True

    return x
                
print(isPrime(input("Enter a prime number.")))

Upvotes: 0

Views: 3873

Answers (3)

Rfroes87
Rfroes87

Reputation: 678

You almost made it, just need to change it like this:

def isPrime(x):
    x = int(x)
    for i in range(2, x):
        if x % i == 0:
            return False
    return x >= 2
                
print(isPrime(input("Enter a prime number.")))

EDIT: As pointed by @MarkRansom, when the i variable reaches the square root of x, it is safe to assume that there are no more possible divisors for it:

from math import sqrt

def isPrime(x):
    x = int(x)
    for i in range(2, int(sqrt(x)) + 1):
    ...

Upvotes: 2

user10881750
user10881750

Reputation:

When you typed this, I believe you meant as follows below:

def isPrime(x):
    x = int(x)
    for i in range(2, x):
        if (x % i) == 0:
            x = False
        else:
            x = True
    return x

print(isPrime(input("Enter a prime number.")))

## Which runs as follows in python IDLE:

>>> def isPrime(x):
        x = int(x)
        for i in range(2, x):
            if (x % i) == 0:
                x = False
            else:
                x = True
        return x
>>> print(isPrime(input("Enter a prime number.")))
Enter a prime number.11
True
>>> print(isPrime(input("Enter a prime number.")))
Enter a prime number.100
False

I know when I started out, I made many typing errors. By rereading my code and lots of practice, I slowly got better.

Here's a how I would write out your task.

def isPrime(h):
    h = int(h)
    test = None
    for x in range(2, h):
        if (h%x) == 0:
            test = False
            print(f"{test}, the number {h} is not a prime!")
            break
    else:
        test = True
        print(f"{test}ly, the number {h} is a prime!")

## Then I'll run the call to the function in python IDLE as:
>>> 
>>> isPrime(input("Enter a number.\n"))
Enter a number.
11
Truely, the number 11 is a prime!
>>> 
>>> isPrime(input("Enter a number.\n"))
Enter a number.
110
False, the number 110 is not a prime!
>>>

One of most important things, that really helped me was:

  • reading others code (and the docs!!)
  • then play with their code in IDLE
  • repeat the above <--------(I still do this all the time!!)

Upvotes: 0

cdlane
cdlane

Reputation: 41925

I believe the simplest form of your program would be along the lines:

def isPrime(x):
    if x > 1:
        for i in range(2, x):
            if x % i == 0:
                return False

        return True

    return False

print(isPrime(int(input("Enter a number: "))))

But this is not as efficient as it could be. To do better, we would treat 2 (and all even numbers) as a special case and only allow the divisor to cover the odd numbers from 3 to the square root of the number we're testing. The more divisions we can avoid, the faster it goes.

Ultimately using a Sieve of Eratosthenes will beat this approach, even with optimizations.

Upvotes: 1

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