given an integer, write code to list all primes preceding it.

Question

Toseef · Accepted Answer

This solution can be optimized further. No need to check for even numbers (except 2, 2 is the only prime even number). You can find if 'n' is prime when it is not divisible by any number between 2 to n/2. More optimizaton can be achieved if the range is reduced further from 2 to sqrt(n). But finding a sqrt will be tedious and might require inclusion of math.h library.

unemployed · Answer

there is a much more optimal solution, think about what you already know ;-) of course if this was an interview this would be my answer too (unless they asked for more performance.)

googler · Answer

void printPrime(int n) {
    bool not_prime = false;

    for (int i=2; i

CazInt · Answer

void printPrimes (int N)
{
  int i;
  int * isNotP = malloc(sizeof(int) * N);
  if (isNotP == NULL) return;

  if (N < 2)
  {
    printf("Error in input %d
", N);
    return;
  }
  if (N == 2)
  {
    printf("- 2 -
");
    return ;
  }


  memset(isNotP, 0, sizeof(int) * N);
  for (i = 3; i <= N; i+=2)
  {
    int k = i*i;
    isNotP[i-1] = 1;
    while (k <= N)
    {
      isNotP[k] = 1;
      k += i;
    }
  }

  printf("- 2 -
");
  for (i = 3; i <= N; i+=2)
    if (isNotP[i] == 0) printf("- %d -
", i);
}

Raviteja Ainampudi · Answer

#!usr/bin/python 

def PrintPrecedingPrimes(x):
    for i in range(1, x):
        if i == 1 or i <= 0:
            pass
        elif i == 2:
            print(2)
        else:
            if all(i%j != 0 for j in range(2,int(i/2) + 1)):
                print(i)
            else:
                pass
try:
    inp = int(input("Enter Value:"))
    PrintPrecedingPrimes(inp)
except Exception as e:
    print(e)

Swift 5 · Answer

var primes = [Int]()
for i in 2...100 {
  print (i, terminator: ": ")
  var primeCount = 0
  findPrimes: for prime in primes {
    if prime > i / 2 { break findPrimes }
    if i % prime == 0 { 
      primeCount += 1
      print (prime, terminator: " ") 
    } 
  }
  if primeCount == 0 {
    primes.append(i)
  }
  print()
}
print("[", terminator:" ")
for prime in primes {
  print(prime, terminator: " ")
}
print()

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