CODEWITHSUNDEEP
python-3.x
String Ana
String Ana

Reputation: 21

python 3. logical error in checking if a number is prime

in my program for finding a prime number, only the else statement gets displayed,even if the user enters a prime number. is there a problem in my if else statement? or in some mathematical operator? i have tried to indent it properly by python syntax. and please rectify the code.

    num=int(input("enter a number  :    "))
    ctr=0
    i=1
    for i in range(1,num):
        if(num%i==0):
             ctr+=1

    if(ctr==2):
        print(num," is prime")

    else:
        print(num," is not prime")

Upvotes: 0

Views: 79

Answers (2)

Richie Bendall
Richie Bendall

Reputation: 9192

If you're still having trouble, try using the following function:

def isprime(number):
    if number == 1:
        return False
    for i in range(2,int(number**0.5)+1):
        if number%i==0:
            return False
    return True

To use it type isprime(number) and replace number with the number which you want to check.

Upvotes: 0

Scott Hunter
Scott Hunter

Reputation: 49813

When the user enters a prime for n, it is only divisible by 1 and itself. But range(1,n) never gets to n, so ctr is 1 for prime numbers, not 2.

Upvotes: 2

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