#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <ctype.h>//用來判斷字元是不是數字 
#define OK 1
#define ERROR 0
#define MAX_INIT_SIZE 100
#define STACKINCREMENT 10
#define MAXBUFFER 10
typedef double ElemType;
typedef int Status;
typedef struct{
    ElemType *top;
    ElemType 
 
*base;
    int StackSize;
}SqStack;
//建立棧
 Status InitStack(SqStack *S){
     S->base=(ElemType *)malloc(MAX_INIT_SIZE*sizeof(ElemType));\
     if(!S->base) return ERROR;
     S->top=S->base;
     S->StackSize=MAX_INIT_SIZE;
     return OK;
 }
 Status Push(SqStack *S,ElemType e){
     if
 
(S->top-S->base>=S->StackSize){
         S->base=(ElemType *)realloc(S->base,(S->StackSize+STACKINCREMENT)*sizeof(ElemType));
         if(!S->base) exit(0);
         S->top=S->StackSize+S->base;
         S->StackSize+=STACKINCREMENT;
     }
     *(S->top)=e;
     S
 
->top++;
 }
 void Pop(SqStack *S,ElemType *e){
     if(S->top==S->base) return;
     *e=*--(S->top);
 }
 int StackLen(SqStack S){
     return S.top-S.base;
 }
 void ClearStack(SqStack *S){
     S->top=S->base;
 }
 void DestroyStack(SqStack *S){
     int i;
     int len=S->StackSize;
     for(i=0;i<len;i++){
         free(S->top);
         S->top--;
     }
     S->top=S->base=NULL;
     S->StackSize=0;
 }
 int main(){
     SqStack S;
     int i=0;
     ElemType d,e;
     char c,str[MAXBUFFER]; 
     InitStack(&S);
     printf("請輸入逆波蘭式以#結束：\n");
     scanf("%c",&c);
     while(c!='#'){
         while(isdigit(c)||c=='.'){
             str[i++]=c;
             str[i]=0;//給字串新增結束符，為呼叫atof函式做準備 
             if(i>=10){
                 printf("出錯：輸入的單個數據過大！");
                return -1; 
             }
             scanf("%c",&c);
             if(c==' '){
                 d=atof(str);//將字串轉換成雙精度浮點型 
                 Push(&S,d);
                 i=0;
                 break;
             }
         }
         if(c=='+'){
             Pop(&S,&d);
             Pop(&S,&e);
             Push(&S,d+e);
         }
        else if(c=='-'){
             Pop(&S,&d);
             Pop(&S,&e);
             Push(&S,e-d);
        }
        else if(c=='*'){
             Pop(&S,&d);
             Pop(&S,&e);
             Push(&S,e*d);
        }
        else if(c=='/'){
             Pop(&S,&d);
             Pop(&S,&e);
             if(d==0){
                 printf("程式出錯，分母為0！");
                 return -1;
             }
             Push(&S,e/d);
        }
        scanf("%c",&c);
     }
     Pop(&S,&d);
     printf("%lf",d);
 } 

舉例：1 2 - 3 4 * *

過程：接收1後存如str陣列，再接收空格後，將1轉換成雙精度浮點型然後入棧S，接著接收2，同上操作，此時棧內有兩個數字1和2，繼續接收到了‘-’號，此時進行減法運算（操作是按順序出棧，則d=2，e=1，計算完後得-1，入棧，此時棧內只有-1），接著接收3，4，*，同上操作，此時結果為12入棧，此時棧內有兩個數字-1和12，接著接收*，然後運算，此時結果為-12入棧，棧內只有-12；

最後出棧列印；