计算器--K & R C

来自K & R C第四章的逆波兰计算器

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

#define MAXOP 100
#define NUMBER '0'
#define MAXVAL 100
#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

int sp = 0;
double val[MAXVAL];

int getop(char s[]);
int getch();
void ungetch(int c);

void push(double f);
double pop ();

int main(){
  int type = 0;
  double op2 = 0;
  char s[MAXOP];

  while( EOF != (type = getop(s)) ){
    switch(type){
    case NUMBER:{
      push(atof(s));
      break;
    }
    case '+':{
      push(pop() + pop());
      break;
    }
    case '*':{
      push(pop() * pop());
      break;
    }
    case '-':{
      op2 = pop();
      push(pop() - op2);
      break;
    }
    case '/':{
      op2 = pop();
      if( 0.0 != op2 ){
	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);
    }
      
    }
  }

  return 0;
}


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


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

int getop(char s[]){
  int i = 0;
  int c = 0;

  while( (s[0] = c = getch()) == ' ' || c == 't') ;
  s[1] = '\0';
  if(!isdigit(c) && c != '.' ){
    return c;
  }

  i = 0;
  if( isdigit(c) ){
    while( isdigit( s[++i] = c = getch())) ;
  }

  if( '.' == c ){
    while( isdigit(s[++i] = c = getchar())) ;
  }

  s[i] = '\0';

  if( c != EOF ){
    ungetch(c);
  }

  return NUMBER;
}

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

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

运行结果：

1 2 -
	-1
1 2 - 4 5 + *
	-9