# How do you calculate the amount of prime numbers less than or equal to N?

## Question:

I want to write a program that determines the amount of prime numbers less than or equal to N (variable containing user input).
This is what I’ve got:

``````N = int(input("Enter number: "))
count = 0
primes = []
for i in range(2, N+1):
for j in range(2, int(i/2)+1):
if (i % j) != 0:
primes += [i]
count += 1

print(count)
print(primes)
``````

But when I run the code, it doesn’t take the numbers 2 and 3 to be prime. How can I fix this program?

You can try these changes:

``````N = int(input("Enter number: "))
count = 0
primes = []
for i in range(2, N+1):
is_prime = True
for j in range(2, int(i/2)+1):
if (i % j) == 0:
is_prime = False
if is_prime:
primes += [i]
count += 1
print(count)
print(primes)
``````

To check if the number is prime, you need it to check if for ALL smaller numbers the remainder is different than 0.

Also, `primes += [i]` should be in external loop as you want to count every number at most once.

Note that this solution is far from optimal in terms of efficiency. Some improvements may be:

• iterate `j` only up to `math.sqrt(i)+1`
• break the inner loop as soon as you find the number is not prime
• use Sieve of Eratosthenes

You can try the following code:

``````primes = []
#Check if given number is prime
def num_is_prime(n):
if n <= 1 :
return False

for i in range(2, n):
if n % i == 0:
return False

return True

#Print prime number less than given number
def print_prime_num(n):
for i in range(2, n + 1):
if num_is_prime(i):
primes.append(i)
print(i)

print("Number of prime numbers less than or equal to {} is {}.".format(n,len(primes)))
``````

Then call the function:

``````print_prime_num(YOUR_NUM)
``````

The ‘num_is_prime’ function returns true if the given number is prime and false otherwise.
Then we print the prime numbers from 2(first prime number) till the given number and add them to a list in the ‘print_prime_num’ function. We also print the length of list containing the prime numbers.

Your starting point should be to write an efficient prime number validator.

For example:

``````primeset = {2, 3}

def isprime(n):
if n in primeset:
return True
if n < 2 or n % 2 == 0 or n % 3 == 0:
return False
f = 5
while f * f < n:
if n % f == 0 or n % (f + 2) == 0:
return False
f += 6
return True
``````

Then, to handle this particular case:

``````N = int(input('Enter number: '))

primes = []

for p in range(2, N+1):
if isprime(p):
primes.append(p)

print(len(primes))
print(primes)
``````

Note: The sympy module has an isprime() function that will be much faster than this. This can be improved further with some more code. 2 is the only even prime number. Therefore you can check for that and subsequently (if necessary) start your range from 3 with a step of 2

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