get next operator or numeric operand

#include <ctype.h>

int getch(void);
void ungetch(int);

/*getop: get next operator or numeric operand */
int getop(char s[]){
    int i, c;

    while ((s[0] = c =getch()) == ' ' || c =='\t')
            ;
    s[1] ='\0';
    if (!isdigit(c) && c != '.')
         return c;            /*not a number */
    i =0;
    if (isdigit(c))    /*collect integer part */
       while(isdigit(s[++i] =c=getch()))
        ;
    if (c == '.')   /*collect fraction part */
        while (isdigit(s[i++] =c=getch()))
              ;
    s[i] ='\0';
    if (c != EOF)
        ungetch(c);
    return NUMBER;
}