Use the screening method to find prime numbers within 100 (screening method !!!)

#include <stdio.h> 
#include <math.h>
 int main () 
{ 
    int a [ 101 ], i, j; 

    for (i = 1 ; i <= 100 ; i ++ ) 
    { 
        a [i] = i;    // Assign the initial value to the array 
    } 
    a [ 1 ] = 0 ; // Remove a [1] 

    for (i = 2 ; i <sqrt ( 100 ); i ++)   // If you need to find prime numbers in the range of 1-n The table only needs to be divided by the root number n (take an integer) 
    {
         for (j = i + 1 ; j <= 100 ; j ++ )
            if( a[i]!=0 && a[j]!=0 )
                if(a[j]%a[i] == 0)
                {
                    a[j] = 0;
                }

    }

    printf("\n");
    int n;

    for(i=2,n=0; i<=100; i++)
    {
        if(a[i]!=0)
        {
            printf("%5d",a[i]);
            n++; 
        } 

        if (n == 10 ) 
        { 
            printf ( " \ n " ); 
            n = 0 ; // Initialize after one completion 
        } 
    } 


    return  0 ; 
}

 

The so-called screening method is the "Eratosene sieve method". A set of data is judged whether they are prime numbers one by one, and if a non-prime number is found, it will be dug out.

The algorithm can be expressed as;

(1) Dig out 1;

(2) Divide the next number p by the next undigged number p, and dig out multiples of p

(3) Check whether p is less than the integer part of the root number n, if it is, then return (2) to continue execution, otherwise it ends

(4) The rest are prime numbers