为什么我的代码不打印素数？

Question

I found some codes to print prime numbers on google but it was way to advanced and couldn't understand it so, I wanted to try it myself but my code is not working. It is just printing 2 but i want to print all prime numbers from 1 to 50. Somebody help!!

 #include <stdio.h>

    int main(){
        //prime num from 1 to 50
        int k = 0;

        for(int i = 1; i <=50 ; i++){
                for(int j = 1; j <= i ; j++){
                    if(i % j == 0){
                        k++;
                    }
                    if(k == 2){
                        printf("%d\n",i);
                    }
                }
        }
        return 0;
    }

Answer 1

A few problems here

• x % 1 will give 0 ( 1 is not a prime number )
• x % x will also give 0
• k needs to be reset at the beginning of the i loop
• no need of a complex count, start with 2 and check if k is zero after the j loop
• then exit the j loop if k is incremented since the number will not be a prime, no need to keep testing

Corrected version:

 for(int i = 2; i <=50 ; i++) {
      int k = 0;
      for(int j = 2; j < i ; j++){
            if(i % j == 0){
                 k++;
                 break; // No need to go on
            }
      }
      if(k == 0){
            printf("%d\n",i);
      }
 }

Answer 2

There are three bugs in this code.

First, you are never resetting the value of k , so after it is equal to 2 and is subsequently increased, it will never again be equal to 2. (Unless of course, you run this program so long that it overflows back to 2.) Fix this by resetting k between every iteration of i .

Second, once k is equal to 2 in the inner loop, it will continue to print every time the inner loop iterates until it either increments k again or the loop completes, which will lead to both repetitions and false positives. Fix this by moving the check out of the inner loop and into the outer loop.

Third, you start i at 1. Not only will this lead to a false positive, it is unnecessary as 1 is not a prime number (nor is it non-prime). Fix this by starting i at 2.

Working program:

#include <stdio.h>

int main(){
  //prime num from 1 to 50
  int k = 0;

  for(int i = 2; i <=50 ; i++){
    k = 0;
    for(int j = 1; j <= i ; j++){
      if(i % j == 0) {
        k++;
      }
    }
    if(k == 2){
      printf("%d\n",i);
    }
  }
  return 0;
}

Notice, however, that while this will print primes, it is a brute force method that is not very efficient. There are many more elegant and efficient ways of finding primes, the most common of which to start out with is the Sieve of Eratosthenes .

Answer 3

• Your if statement should be outside the first but inside the second for loop.

• You need to reset k after the prime test is over for each number.

   int main(){ //prime num from 1 to 50 int k = 0; for(int i = 1; i <=50; i++){ for(int j = 1; j <= i; j++){ if(i % j == 0){ k++; } } if(k == 2){ printf("%d\n",i); } k=0; } return 0; }