range - print numbers in a range

# include <stdio.h>
/*
        This program, range, accepts 2 or 3 command line arguments
        and checks that they are integers. Range prints a range of
        numbers one per line on the standard output, starting with
        the first argument, incrementing (or decrementing) with the
        third argument until the next number printed would be greater
        (or lesser) than the second argument.
        If the third argument is missing, then the increment
        is set to 1 (similarly when the third argument is specified as
        zero). The absolute of the third argument is used as the
        increment, whereas the sign of the other arguments is never
        changed.

        Author:         Brett Jordan, CiTR
        Date created:   20 March 1991
*/

main(argc, argv)
int argc;
char *argv[];
{
        int     err, i, stnum, endnum, incr, *argi[3];

        err = 0;
        argi[0] = &stnum;  /* Make argis point to the final resting */
        argi[1] = &endnum; /* place of the command line arguments */
        argi[2] = &incr;

        /* if incorrect number of arguments ... */
        if (argc < 3 || argc > 4)
                err = 1;
        else {
                /* argc is the count of all words on the command line,
                   whereas argc-1 is the number of arguments, therefore
                   for all arguments ... */
                for (i=0; i<(argc-1); i++)
                        /* increment argv to point to next argument,
                           initially pointing to 'range', and pass it
                           by value, pass argi element so that sscanf
                           can convert text into integer and place the
                           integer arguments in their final resting
                           places.  If sscanf returns 0 then there was
                           an error. */
                        if (sscanf(*++argv, "%d", argi[i]) == 0) {
                                err = 1;
                                break;
                        }
        }

        /* if an error occurred, print usage info */
        if (err != 0) {
                usage();
                return(1);
        }
        else {
                /* if no increment specified, or set to 0, make it 1 */
                if (i==2 || incr==0)
                        incr = 1;
                /* ensure that increment is positive */
                if (incr < 0)
                        incr = 0 - incr;

                /* if decrementing... */
                if (stnum > endnum) {
                        for (i=stnum; i>=endnum; i-=incr)
                                printf("%d\n", i);
                }
                /* if incrementing... */
                else {
                        for (i = stnum; i <= endnum; i+=incr)
                                printf("%d\n", i);
                }
                return(0);
        }
}


usage()
{
        printf("usage: range start end [increment]\n");
        printf("\n");
        printf("start, end and increment must be integers.\n");
}