Reputation: 55
Trying to write a program that checks if a number is prime. Wrote the below code, but do not understand why do I have an output of 2 lines:
num = int(input("Provide number to check if prime: "))
if num <=1:
print("Invalid choice, try again")
num = int(input("Provide number to check if prime: "))
for i in range(2,num):
if num% i ==0:
print("Number is not prime")
break
if num %i !=0:
print("Number is prime")
My output is :
Provide number to check if prime: 15
Number is prime
Number is not prime
Upvotes: 0
Views: 2780
Reputation: 462
The sympy.isprime() is a built-in function under the SymPy module and can be utilized for checking of possible prime numbers. It is a direct function and returns True if the number to be checked is prime and False if the number is not prime.
>>> import sympy
>>> sympy.isprime(8)
False
>>> sympy.isprime(11)
True
or else define a function like this
>>> def isPrime(k):
# 1 is not prime number
if k==1:
return False
# 2, 3 are prime
if k==2 or k==3:
return True
# even numbers are not prime
if k%2==0:
return False
# check all numbers till square root of the number ,
# if the division results in remainder 0
# (skip 2 since we dont want to divide by even numbers)
for i in range(3, int(k**0.5)+1, 2):
if k%i==0:
return False
return True
>>> print(isPrime(13))
True
>>> print(isPrime(18))
False
Upvotes: 5
Reputation: 1
# Prime number between 100 and 200
def isprime(n):
if n<2:
return False
for i in range(2,int(n*.5)+1):
if n%i==0:
return False
else:
return True
primenum=[x for x in range(100,201) if isprime(x)]
print(primenum)
Upvotes: -1
Reputation: 1476
If you want to just check whether a number is prime or not just do the following:
num = int(input("Provide number to check if prime: "))
flagNotPrime = False
if num > 1:
for i in range(2, num):
if (num % i) == 0:
flagNotPrime = True
break
if flagNotPrime:
print("Number is not prime")
else:
print("Number is prime")
Firstly, numbers that are <= 1
are not prime numbers. Therefore, the above code only proceeds if the num
is greater than 1.
Secondly, the code checks if num
is exactly divisible by any number from 2 to num - 1
. If there is a factor in that range, the number is not prime, so the flag is set to True
and the loop is broken using break
.
Lastly, outside the loop, if the flag is True
then num
is not prime.
Upvotes: 0
Reputation: 2993
You have issues in output, not only for the case of 15
, but also for cases smaller than 1
. The following code should work. It has two improvements.
else
block to align with the for
loop.num = int(input("Provide number to check if prime: "))
while num <=1: #you need to use while loop
print("Invalid choice, try again")
num = int(input("Provide number to check if prime: "))
if num > 1: #only evaluate number is prime or not if it is greater than 1
for i in range(2,num):
if num% i ==0:
print("Number is not prime")
break
else: #to move the `else` block to align with the `for` loop.
print("Number is prime")
break #add a break here
Output:
What is a while loop?
A while loop tests the input condition. Every time the loop finishes, the condition is reevaluated (only evaluate number is prime or not if it is greater than 1). As long as the the number entered is <=1
, the loop keeps executing (keep asking users for input).
Upvotes: 1
Reputation: 4092
As the first thing, you should remember that 1
isn't a prime number by definition, even if it can't be divided by any other number:
if (num == 1):
print("The number is NOT prime")
else:
for i in range(2, num):
if (num%i == 0): # If the number has a divisor
print("The number is NOT prime")
break
else: # If the for loop ends without reaching any break
print("The number IS prime")
The else
branch of a for
loop is reached when the loop ends without reaching any break
AND the loop executes at least one time.
To better understand my answer, I would suggest to read this.
The error with your solution is caused by the loop printing that the number is prime for each time num%i == 0
, so taking num = 6
:
6%4 != 0 # The number is prime
6%5 != 0 # The number is prime
As Rajarshi Ghosh
suggested, you should know that while programming it's a good idea to use import
ed functions to do this simple operations, in order to avoid long operations for such a simple job.
If you don't want to use an imported function, I would suggest you to read this article where they explained 6 ways of finding if a number is prime without using functions made by others.
Upvotes: 1