What Does the Percent Sign (%) Do in Python?

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#1

What does the percent sign mean in Python?

In a tutorial, I came across an example for finding prime numbers:

for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print(n, 'equals', x, '*', n // x)
            break
    else:
        # loop fell through without finding a factor
        print(n, 'is a prime number')

I understand that the double == is used to test for equality, but I’m confused about the if n % x part. I can explain what the statement does for the example, but I don’t fully understand the purpose of the percent sign in n % x.

What exactly does the percent sign Python operator do in this context?

#2

Ah, I see what you’re asking! So, the percent sign Python operator is actually called the modulus operator. It’s used to calculate the remainder when one number is divided by another. Here’s a quick example:

python

Copy code

n = 10
x = 3
remainder = n % x  # 10 % 3 = 1
print(remainder)  # Output: 1

In this case, n % x gives us the remainder (1), because 10 divided by 3 gives 3 with a remainder of 1.

Now, let’s tie this to the prime number example you mentioned. In this code:

if n % x == 0:

The percent sign Python checks if n is divisible by x with no remainder. If n % x == 0, then n isn’t prime because it has a divisor other than 1 and itself. Essentially, the modulus helps us find out if there’s any remainder when dividing n by any number between 2 and n-1. If there’s no remainder (i.e., n % x == 0), it means n is divisible by x and, therefore, not prime.

#3

Right! Building on what Priyanka said, the percent sign Python operator isn’t just for checking divisibility in prime number tests—it’s also useful for checking whether a number is even or odd. It’s a very common pattern!

For instance, here’s how you can use the modulus operator to check if a number is even or odd:

number = 7
if number % 2 == 0:
    print(f"{number} is even.")
else:
    print(f"{number} is odd.")

Output:

7 is odd.

In this case, 7 % 2 returns 1 (because when you divide 7 by 2, the remainder is 1). If it had returned 0, that would indicate the number is even. So, the percent sign Python is a quick and efficient way to classify numbers based on their divisibility.

#4

Great points! Just to build on what Akansha said, the percent sign Python also plays a crucial role in more complex scenarios, like loops for prime number detection, which is what you were initially asking about. Let me walk you through the prime number example, but with extra detail:

for n in range(2, 10):  # Check numbers from 2 to 9
    for x in range(2, n):  # Check divisors from 2 to n-1
        if n % x == 0:  # If remainder is 0, n is divisible by x
            print(f"{n} equals {x} * {n // x}")
            break
    else:
        # If no divisor was found, n is prime
        print(f"{n} is a prime number")

Here, the percent sign Python (i.e., n % x) checks whether n has any divisors (other than 1 and itself). When we find that n % x == 0, it means that n is divisible by x, so it isn’t a prime number. If the loop runs without finding any divisors, the else block gets triggered, and n is printed as a prime number. The modulus operator helps us identify the key point here: whether there’s a remainder or not when dividing n by x.

This is a perfect use case of the percent sign Python, as it allows us to detect divisibility efficiently in loops, which is essential for prime number algorithms and many other mathematical checks.