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Python Programs - Looping Statements

Print all prime numbers in a given range - Python Program

Example 1 :

Python 
#  Prime numbers in a range

start = int(input("Enter start: "))
end = int(input("Enter end: "))

print(f"Prime numbers between {start} and {end}:")

for num in range(start, end + 1):
    if num > 1:
        is_prime = True
        for i in range(2, int(num ** 0.5) + 1):
            if num % i == 0:
                is_prime = False
                break
        if is_prime:
            print(num, end=" ")

Output

 
OUTPUT  :
Enter start: 5
Enter end: 25
Prime numbers between 5 and 25:
5 7 11 13 17 19 23

Example 2 : Advanced Program

Python 
def find_primes_in_range(lower_limit, upper_limit):
    """
    Prints all prime numbers within a given range (inclusive).

    Args:
        lower_limit (int): The lower bound of the range.
        upper_limit (int): The upper bound of the range.
    """
    if lower_limit > upper_limit:
        print("Error: The lower limit cannot be greater than the upper limit.")
        return

    print(f"Prime numbers between {lower_limit} and {upper_limit} are:")
    
    for num in range(lower_limit, upper_limit + 1):
        # All prime numbers are greater than 1
        if num > 1:
            is_prime = True
            # Check for factors from 2 up to the square root of num
            # We only need to check up to the square root because if a number
            # has a factor greater than its square root, it must also have a
            # factor smaller than its square root.
            for i in range(2, int(num**0.5) + 1):
                if (num % i) == 0:
                    is_prime = False
                    break  # Not prime, no need to check further
            if is_prime:
                print(num)

# Example Usage:
lower = 10
upper = 50
find_primes_in_range(lower, upper)

print("\nAnother example:")
lower = 1
upper = 10
find_primes_in_range(lower, upper)

Output

 
OUTPUT  :
Prime numbers between 10 and 50 are:
11
13
17
19
23
29
31
37
41
43
47

Another example:
Prime numbers between 1 and 10 are:
2
3
5
7

Explanation:

find_primes_in_range(lower_limit, upper_limit) function:

This function takes two integer arguments: lower_limit and upper_limit, representing the inclusive range within which to find prime numbers.

Input Validation:

It first checks if lower_limit is greater than upper_limit. If so, it prints an error message and exits, as an invalid range is provided.

Iteration through Range:

The program then iterates through each number (num) from lower_limit to upper_limit (inclusive) using a for loop and the range() function.

 

Prime Number Check:

  • It checks if numis greater than 1, as 1 is not considered a prime number.
  • A boolean variable is_primeis initialized to True for each num, assuming it's prime until proven otherwise.
  • An inner forloop iterates from 2 up to the integer part of the square root of num. This optimization is based on the property that if a number num has a divisor, it must have one less than or equal to its square root.
  • Inside the inner loop, it checks if numis divisible by i (i.e., num % i == 0). If it is, num is not prime, so is_prime is set to False, and the inner loop is break
  • If the inner loop completes without finding any divisors, is_primeremains True, and num is printed as a prime number.

 

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