Python Program:

Example:

num=int(input("Enter any +ve number : "))

def prime(n) :
  if n<2 : return 0
  for i in range(2,int(n/2)+1) :
    if n%i==0 : return 0
  return 1

print("Your prime numbers between 1 & ",num," are :",end=" ")
for i in range(2,num) :
  if(prime(i)) :
    print(i,end=" ")

Output:

  Enter any +ve number : 20
  Your prime numbers between 1 & 20 are :
  2 3 5 7 11 13 17 19

You should know first :

Prime numbers: Numbers greater than 1 & have only 2 factors ( 1 and itself )

Main Logic to check prime :

def prime(n) :
  if n<2 : return 0
  for i in range(2,int(n/2)+1) :
    if n%i==0 : return 0
  return 1

We are running the loop n/2 Since, any number can have only factors under its half, except itself

To print all primes :

for i in range(2,num) :
  //check every number for prime [ if prime(i) : ], if prime then print the number
}