Thread: Show prime numbers using Brute Force

I am trying to make a c program that displays prime numbers 2 through the entered number, show runtime when 0 is entered and when a one is entered show the runtime plus display the prime numbers.
I can get it to display the runtime but for some reason I can't have it output the prime numbers. I have supplied my code for reference and any input would be greatly appreciated thank you so much.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>


int main(int argc, char **argv)
{
        int max, i, show, prime, j;
        struct timeval time_start;                                                                                                                                       // starting time
        struct timeval time_end;                                                                                                                                         // ending time




        if (argc < 3)                                                                                                                                                   //if arguments are less $
        {
                fprintf(stderr, "Not Enough Arguments!\n");
                exit(1);
        }
        int arg1 = atoi(argv[1]);


        max  = atoi(argv[1]);                                                                                                                                           //shows max
        show = atoi(argv[2]);                                                                                                                                           //SHOWS PRIMES




        gettimeofday(&time_start, 0);
        if (show==1)                                                                                                                                                    //if show ='s 1 then
        {
                for (i = argc + 1; i < argc; i++)
            {
                prime = 0;
                for (j = 2; j <= i/2; j++)
                {
                        if(i%j==0)
                        {
                                prime=1;
                                break;
                        }
                }
                        if(prime==0)
                        printf("%d\n", i);
                        fprintf(stderr, "%10.6lf\n", time_end.tv_sec - time_start.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
           }
        }
        gettimeofday(&time_end, 0);


        if (show == 0)
        {
                fprintf(stderr, "%10.6lf\n", time_end.tv_sec - time_start.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
        }


        return 0;
}

Hi all..

1 possible issue...
The if statement "if(prime==0)" line 43, should have opening and closing curly braces as the if contains more than 1 statement.

John

I think you need to work your algorithm in line 29 to line 48.

Check if your program enters the loop in line 32 (conditional expression). Do you need the value of the argument count variable (argc) in the for statement?
Does you algorithm use the entered number (max) and does it terminate if max is reached?

Do you assign proper values to the members of the struct time_end before you access them (line 45 & line 48)?

Originally Posted by dextre

I think you need to work your algorithm in line 29 to line 48.

Check if your program enters the loop in line 32 (conditional expression). Do you need the value of the argument count variable (argc) in the for statement?
Does you algorithm use the entered number (max) and does it terminate if max is reached?

Do you assign proper values to the members of the struct time_end before you access them (line 45 & line 48)?

I just checked and saw that on line 32, it doesn't not enter the loop. I think my math is messed up somewhere in the loop. I need it to reach 2 through the max number(showing primes) I need it to have argv[2] count for the prime number. Just trying to work out where exactly I went wrong in the loop.

I did an edit of my code and assigned it to my max value. Still can't get any output.. Any ideas? Possibly include it outside of my for loop?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>


int main(int argc, char **argv)
{
        int max, i = 2, show, prime, j;
        struct timeval time_start;                                                                                                                                       // starting time
        struct timeval time_end;                                                                                                                                         // ending time




        if (argc < 3)                                                                                                                                                   //if arguments are less $
        {
                fprintf(stderr, "Not Enough Arguments!\n");
                exit(1);
        }
        int arg1 = atoi(argv[1]);


        max  = atoi(argv[1]);                                                                                                                                           //shows max
        show = atoi(argv[2]);                                                                                                                                           //SHOWS PRIMES




        gettimeofday(&time_start, 0);
        if (show == 1)
        {
                for (i = 2; i < max -2; i++)
          {
                prime = 0;
                for (j = 2; j <= i/2; j++)
                {
                        if (i%j==0)
                        {
                                prime=1;
                                break;




                                if(prime==0)
                                printf("%d\n", i);
                                fprintf(stderr, "%10.6lf\n", time_end.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
        }
   }
        gettimeofday(&time_end, 0);


        if (show == 0)
        {
                fprintf(stderr, "%10.6lf\n", time_end.tv_sec - time_start.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
        }


        return 0;
}

Please ensure your code is neatly and consistently formatted/indented.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>

int main(int argc, char **argv)
{
    int max, i = 2, show, prime, j;
    struct timeval time_start;
    struct timeval time_end;

    if (argc < 3)
    {
        fprintf(stderr, "Not Enough Arguments!\n");
        exit(1);
    }
    
    int arg1 = atoi(argv[1]);

    max  = atoi(argv[1]);
    show = atoi(argv[2]);
 
    gettimeofday(&time_start, 0);
    if (show == 1)
    {
        for (i = 2; i < max -2; i++)
        {
            prime = 0;
            for (j = 2; j <= i/2; j++)
            {
                if (i%j==0)
                {
                    prime=1;
                    break;
 
                    if(prime==0)
                    printf("%d\n", i);
                    fprintf(stderr, "%10.6lf\n", time_end.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
                }
            }
        
            gettimeofday(&time_end, 0);
 
            if (show == 0)
            {
                fprintf(stderr, "%10.6lf\n", time_end.tv_sec - time_start.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
            }
 
            return 0;
        }

Note that some closing braces are missing, and as such this will not compile.

I went through and changed the brackets all should be well.

Code:

#include <stdio.h>
    #include <stdlib.h>
#include <sys/time.h>
 
int main(int argc, char **argv)
{
    int max, i = 2, show, prime, j;
    struct timeval time_start;
    struct timeval time_end;
 
    if (argc < 3)
    {
        fprintf(stderr, "Not Enough Arguments!\n");
        exit(1);
    }
     
    int arg1 = atoi(argv[1]);
 
    max  = atoi(argv[1]);
    show = atoi(argv[2]);
  
    gettimeofday(&time_start, 0);
    if (show == 1)
    {
        for (i = 2; i < max -2; i++)
        {
            prime = 0;
            for (j = 2; j <= i/2; j++)
            {
                if (i%j==0)
                {
                    prime=1;
                    break;
  
                    if(prime==0)
                    printf("%d\n", i);
                    fprintf(stderr, "%10.6lf\n", time_end.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
                }
    }    }    }
         
            gettimeofday(&time_end, 0);
  
            if (show == 0)
            {
                fprintf(stderr, "%10.6lf\n", time_end.tv_sec - time_start.tv_sec + ((time_end.tv_usec - time_start.tv_usec) / 1000000.0));
            }
  
       return 0;
}

Cramming three closing braces on a single line is not proper formatting. I also suspect some of those actually belong elsewhere, as per your original code.

I would suggest you start a new project, and write only the code for the brute-force calculations.

Code:

#include <stdio.h>

int main(void)
{
    int max = 100;

    /* brute-force code here */

    return 0;
}

Test this and get it to work before adding additional features.

can you can eliminate any even number, other than 2. if (i % 2) == 0 then it is not a prime.