Question

I’m working on C programming assignment to implement Sieve of Eratosthenes without using the square root function of C. Below is my output and my professors output, I’m not sure what in my code is causing it to be wrong. Any ideas?

Here’s the expected output

Program initiated
   1   2   3   5   7  11  13  17  19  23  29  31
  37  41  43  47  53  59  61  67  71  73  79  83
  89  97 101 103 107 109 113 127 131 137 139 149
 151 157 163 167 173 179 181 191 193 197 199 211
 223 227 229 233 239 241 251 257 263 269 271 277
 281 283 293 307 311 313 317 331 337 347 349 353
 359 367 373 379 383 389 397 401 409 419 421 431
 433 439 443 449 457 461 463 467 479 487 491 499
 503 509 521 523 541 547 557 563 569 571 577 587
 593 599 601 607 613 617 619 631 641 643 647 653
 659 661 673 677 683 691 701 709 719 727 733 739
 743 751 757 761 769 773 787 797 809 811 821 823
 827 829 839 853 857 859 863 877 881 883 887
Program terminated

Here’s my output:

Program initiated
   1  37  41  43  47  53  59  61  67  71  73  79
  83  89  97 101 103 107 109 113 127 131 137 139
 149 151 157 163 167 173 179 181 191 193 197 199
 211 223 227 229 233 239 241 251 257 263 269 271
 277 281 283 293 307 311 313 317 331 337 347 349
 353 359 367 373 379 383 389 397 401 409 419 421
 431 433 439 443 449 457 461 463 467 479 487 491
 499 503 509 521 523 541 547 557 563 569 571 577
 587 593 599 601 607 613 617 619 631 641 643 647
 653 659 661 673 677 683 691 701 709 719 727 733
 739 743 751 757 761 769 773 787 797 809 811 821
 823 827 829 839 853 857 859 863 877 881 883 887
Program terminated

Here is my code:

#include <stdio.h>

void zap(int data[], int divisor)
{
    for(int i=0;i<900;i++)
    {
        if(data[i]%divisor==0) // if mod is not 0, 0 out the index. 
        {
            data[i] = 0; 
        }
    }
}
// the display method 
void display(int data[])
{
    int count = 0; // init counter on the out side
    for(int i=0;i<900;i++)
    {
        if(data[i]>0)// don't print 0s
        {
            printf("%4d",data[i]);// print the data in a column 

            count++;// increment count 

            if(count==12) // print rows and columns 
            {
                count=0; // reset count
                printf("\n"); // print new line 
            }
        }
    }
    if(count<12)// we terminate loop and we now need print a new line 
    {
        printf("\n"); 
    }
}

int main()
{   
    // start the program, with a message
    printf("Program initiated\n"); 

    // needs to be 900 long 
    int primes[900];

    // populate the array 
    for(int i=1; i <= 900; i++)
    {
            primes[i] = i; 
    }

    // elminate the bad numbers 
    for(int i=2; i < 35; i++)
    {
        zap(primes,i); 
    }

    // display the array. 
    display(primes);

    // print the end message    
    printf("Program terminated\n"); 

    return 0; 
}

Answer 1

well you can can use something like this:

(initialize sieve to be a big enough bool-array with each entry set to true – as I want to keep it simple set sieve[0] = false; sieve[1] = false;

for(int i = 2; i < endOfNumbers; i++)
{
   if (sieve[i] == false) continue;
   for (int m = 2*i; m < endOfNumbers; m += i)
      sieve[m]=false;
}