How do you get a prime number in a while loop in python?

Question

In this post, we will write a program in Python to check whether the input number is prime or not using while loop.

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How to write Prime number Program in Python?
Example 1: Using a flag variable
Example 2: Using a for...else statement
How do you find a prime number in a while loop?
How do you get primes in Python?
How do you print a prime number in Python?
Is prime () in Python?

How to write Prime number Program in Python?

Prime number is a number that is greater than 1 and divided by 1 or itself.
In other words, prime numbers can’t be divided by other numbers than itself or 1.
For example- 2, 3, 5, 7, 11, 13, 17, 19, 23…. are the prime numbers.
Let’s see the prime number program in Python.

In this Python program, we will take an input from the user and check whether the number is prime or not.

num=int(input("Enter a number ( greater than 1)"))

f=0

i=2

whilei<= num/2:

ifnum%i==0:

f=1

break

i=i+1

iff==0:

print("The entered number is a PRIME number")

else:

print("The entered number is not a PRIME number")

Output:

Enteranumber:19

TheenterednumberisaPRIMEnumber

Example to check whether an integer is a prime number or not using for loop and if...else statement. If the number is not prime, it's explained in output why it is not a prime number.

To understand this example, you should have the knowledge of the following Python programming topics:

Python if...else Statement
Python for Loop
Python break and continue

A positive integer greater than 1 which has no other factors except 1 and the number itself is called a prime number. 2, 3, 5, 7 etc. are prime numbers as they do not have any other factors. But 6 is not prime (it is composite) since, 2 x 3 = 6.

Example 1: Using a flag variable

# Program to check if a number is prime or not

num = 29

# To take input from the user
#num = int(input("Enter a number: "))

# define a flag variable
flag = False

# prime numbers are greater than 1
if num > 1:
    # check for factors
    for i in range(2, num):
        if (num % i) == 0:
            # if factor is found, set flag to True
            flag = True
            # break out of loop
            break

# check if flag is True
if flag:
    print(num, "is not a prime number")
else:
    print(num, "is a prime number")

In this program, we have checked if num is prime or not. Numbers less than or equal to 1 are not prime numbers. Hence, we only proceed if the num is greater than 1.

We check if num is exactly divisible by any number from 2 to num - 1. If we find a factor in that range, the number is not prime, so we set flag to True and break out of the loop.

Outside the loop, we check if flag is True or False.

If it is True, num is not a prime number.
If it is False, num is a prime number.

Note: We can improve our program by decreasing the range of numbers where we look for factors.

In the above program, our search range is from 2 to num - 1.

We could have used the range, range(2,num//2) or range(2,math.floor(math.sqrt(num)+1)). The latter range is based on the fact that a composite number must have a factor less than or equal to the square root of that number. Otherwise, the number is prime.

You can change the value of variable num in the above source code to check whether a number is prime or not for other integers.

In Python, we can also use the for...else statement to do this task without using an additional flag variable.

Example 2: Using a for...else statement

# Program to check if a number is prime or not

num = 407

# To take input from the user
#num = int(input("Enter a number: "))

# prime numbers are greater than 1
if num > 1:
   # check for factors
   for i in range(2,num):
       if (num % i) == 0:
           print(num,"is not a prime number")
           print(i,"times",num//i,"is",num)
           break
   else:
       print(num,"is a prime number")
       
# if input number is less than
# or equal to 1, it is not prime
else:
   print(num,"is not a prime number")

Output

407 is not a prime number
11 times 37 is 407

Here, we have used a for..else statement to check if num is prime.

It works on the logic that the else clause of the for loop runs if and only if we don't break out the for loop. That condition is met only when no factors are found, which means that the given number is prime.

So, in the else clause, we print that the number is prime.

How do you find a prime number in a while loop?

In a while loop, we will perform num modulus "i" where i is less than num. If num%i comes to zero, it means that the number entered by the user is not a prime number. If the value of num is equal to "i" then it is a prime number.

How do you get primes in Python?

To find a prime number in Python, you have to iterate the value from start to end using a for loop and for every number, if it is greater than 1, check if it divides n. If we find any other number which divides, print that value.

How do you print a prime number in Python?

Step 1: Loop through all the elements in the given range. Step 2: Check for each number if it has any factor between 1 and itself. Step 3: If yes, then the number is not prime, and it will move to the next number. Step 4: If no, it is the prime number, and the program will print it and check for the next number.

Is prime () in Python?

Python Function to Check for Prime Number The above function is_prime() takes in a positive integer n as the argument. If you find a factor in the specified range of (2, n-1), the function returns False —as the number is not prime. And it returns True if you traverse the entire loop without finding a factor.

programming python

How do you get a prime number in a while loop in python?

How to write Prime number Program in Python?

Example 1: Using a flag variable

Example 2: Using a for...else statement

How do you find a prime number in a while loop?

How do you get primes in Python?

How do you print a prime number in Python?

Is prime () in Python?

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