Thread: *Need help with some corrections on this Program.*

Hello everyone, the following program is supposed to allow the user to calculate all the prime numbers up into a certain maximum value, given by the user on the line of commands. After finishing said program, all that's left is to add a couple of tests, just to make sure everything works just fine, in case the user goes a little wild while trying this out.
The main upsets are the following:

1-I still haven't been able to terminate the program in the event that the given argument is a character, for instance 'a' (or any other one);
2- I also don't know how to eliminate the characters present in the string, in case the user writes a string like '2134sabb'. That is to say, i want to only take into account the numbers and discard the rest of the string.

One more thing, if there is any clear mistake i am doing, or test that needs to be added, please let me know. Thank you very much, and here's the code:

Code:

 
#include <stdio.h>
#include <stdlib.h>
#include <math.h>


int
main(int argc, char **argv)
{
    if( argc != 2)
    {
        printf("Error! Input a positive integer for the value of N!\n");
        return 1;
    }


  int n,a,j,i,b,t;
  int *v;
  FILE *f1;
  f1 = fopen("Prime.txt","wb");


  if( f1 == NULL)
  {
      printf("Error! Coulnd't open file!");
      return 1;
  }


  sscanf(argv[1],"%d",&a);


  if( a < 2 )
  {
        printf("Error! Input a positive integer for the value of N!\n");
        return 1;
  }




  v =(int*) malloc((a+1)*sizeof(int));
  n = sqrt(a);


  for(i=0;i<a+1;++i)
    v[i] = 1;


  i=2;




  while(i<n)
    {
      if(v[i] != 0)
    {
      j = i;
      b = 0;


      while(i*j <= a)
        {
          b = i*j;


          v[b] = 0;


          ++j;




        }


    }
      i++;


    }


  printf("The prime numbers are:\n");
  t=1;
  for(i=2; i<a+1; ++i)
    {
      if(v[i] ==1 )
    {
      printf("%d\t",i);
      if((t%6)== 0)
        printf("\n");
      ++t;
      fwrite(&i,sizeof(int),1,f1);
    }
    }
  printf("\n");


  fclose(f1);


printf("The numbers read from the file are as follows:\n");
  /* Ler o ficheiro*/
  f1 = fopen("Primos.txt","rb");
  for(i=1;i<t;++i)
    {
      fread(&a,sizeof(int),1,f1);
      printf("%d\n",a);
    }


  return 0;
}

For your error output, I would recommend either perror("enter your msg here\n") or fprintf(stderr, "enter your msg here\n")

Another recommendation would be to rename your variables into something anyone can understand what it's associated with instead of just "int a, b, c, d...".

As for checking for valid input, use something like getc or fgetc. I can't remember exactly how to do it though. Still new myself. (s)scanf is pretty dumb (not as an insult, it's just literally dumb).

Are you using files to store the output of the prime numbers? If not, what are you doing with files?

And last, I don't think you need to add "v = (int *) malloc(...)", just use "v = malloc(...)". Same with "sizeof(int)", just use "sizeof(...)".