how do I know what n(positive integer) is when user types ./a.out -n?

Question

How do I make an command line argument a variable?

That is, for example, how do I know what n(positive integer) is when user types ./a.out -n?

I know I can use strcmp(argv[1], "-1") strcmp(argv[1], "-100") but is there a simpler way to implement this?

Answer 1

You can use

strtol(argv[1]) or atoi(argv[1])

to get the integer no need to compare it as a string and see.

PS: atoi(NULL) will cause UB.

Check the code below:

int main(int argc, char **argv)
{
   int n;
   if( argc >2)
   return 1;

   if(argv[1][0] == '-')
     n = atoi(argv[1] +1);
   else
      n= atoi(argv[1]);
   printf("%d\n",n);
   return 0;
}

Answer 2

Assuming that -n is a command line optional argument, It is always suggested using getopt or Argp , and use case to check if it its a positive integer.

Refer the following links for further information about using them http://www.gnu.org/software/libc/manual/html_node/Parsing-Program-Arguments.html

For more information about command line arguments, the following link will help you http://courses.cms.caltech.edu/cs11/material/c/mike/misc/cmdline_args.html