Good evening. I'm studying the reverse polish calculator of KnR but there are some lines I'm not understanding very well.

Code:
#include <stdio.h> 
#include <stdlib.h>

#ifndef MAXOP
  #define MAXOP 100
#endif   

#ifndef NUMBER
  #define NUMBER '0'
#endif  

int getop(char[]);
void push(double);
double pop(void);

int main(void) 
{ 
  int type; 
  double op2;
  char s[MAXOP];
  
  while( (type = getop(s)) != EOF)
  { 
    switch(type)
    { 
     case NUMBER: 
        push(atof(s));
        break;
     case '+': 
        push(pop() + pop());
        break;
     case '-': 
        op2 = pop();
        push(pop() - op2);
        break;
     case '*': 
        push(pop() * pop());
        break;
     case '/': 
        op2 = pop();
        if(op2 != 0.0) 
          push(pop() / op2);
        else 
          printf("error: zero divisor\n");
        break;
     case '\n': 
        printf("\t%.8g\n", pop());       
        break;
     default: 
        printf("error: unknown command %s\n", s);
        break;                   
    }
  }               
 return 0;
}

#define MAXVAL 100

int sp = 0; 
double val[MAXVAL];

void push(double f) 
{ 
  if(sp < MAXVAL) 
    val[sp++] = f;
  else
    printf("error: stack full, can't push %g\n", f);          
}     

double pop(void) 
{ 
   if(sp > 0) 
   { 
     return val[--sp];  
   }
   else  { 
     printf("error: stack empty\n");
     return 0.0;       
   }       
}

#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;   
}

#define BUFSIZE 100

char buf[BUFSIZE]; /* buffer for ungetch */
int bufp = 0; /* next free position in buf */     

int getch(void)
{ 
  return (bufp > 0)? buf[--bufp] : getchar();    
} 

void ungetch(int c) 
{ 
   if(bufp >= BUFSIZE) 
     printf("Ungetch: too many characters\n");
   else 
     buf[bufp++] = c;      
}

this section

Code:
 
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()))
Why did he insert the null terminator in s[1] ? to stop getting blanks ? after that, did he reset i to overwrite '\0' with an input ? (isdigit(s[++i]) ) ... if so, why did he insert the null terminator in the first place? I don't get it ...