利用堆栈求后缀表达式的值，运算数为实数。实数获取和堆栈问题

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

#define MAXOP 100
#define INFINITY 1e9
#define ERROR -1e9
typedef double ElementType; /*堆栈元素类型*/
/*类型依次为运算数、运算符、和结尾*/
typedef enum {
	num, opr, end
} Type;

//堆栈的定义
typedef int Position;
typedef struct SNode * PtrToSNode;
struct SNode{
	ElementType * Data;
	Position Top;
	int MaxSize;
};
typedef PtrToSNode Stack;


Stack CreateStack( int MaxSize )
{
	Stack S = ( Stack ) malloc( sizeof( struct SNode ) );
	S->Data = ( ElementType* ) malloc ( MaxSize * sizeof ( ElementType ) );
	S->Top = -1;
	S->MaxSize = MaxSize;
	return S;
}


bool IsFull( Stack S )
{
	return( S->Top == S->MaxSize - 1 );
}


bool Push( Stack S, ElementType X )
{
	if( IsFull(S) ){
		printf(" 堆栈满 ");
		return false;
	}
	else{
		S->Data[++(S->Top)] = X;
		return true;
	}
}


bool IsEmpty( Stack S )
{
	if(S->Top == -1)
		return true  ;
	else
		return false;
}


ElementType Pop( Stack S )
{
	if( IsEmpty ){
		printf( " 堆栈空 " );
		return ERROR;
	}
	else
		return (S->Data[ (S->Top)-- ]);
}

//读取Expr字符串内的对象保存到字符串str中
Type GetOp( char *Expr, int * start, char * str )
{
	int i = 0;
	while(Expr[ (*start) ] == ' ' )
		str[0] = Expr[ (*start)++ ];
//	printf( "%d", *start );
	
	while( str[i] != ' ' && str[i] != '\0' )
		str[++i] = Expr[ (*start)++ ];
	
	if( str[i] == '\0' )
		(*start)--;
	str[i] = '\0';
//	printf( "%d", *start );
	if( i == 0 )
		return end;
	else if( isdigit( str[0] ) || isdigit( str[1] ) )
		return num;
	else
		return opr;
}

//总体程序运算过程
ElementType PostfixExp( char * Expr )
{
	Stack S;
	Type T;
	ElementType Op1, Op2;
	char str[MAXOP];
	int start = 0;
	
	S = CreateStack( MAXOP );
	
	Op1 = Op2 = 0;
	while(( T = GetOp(Expr, &start, str) ) != end){
		
		if( T == num ){
			Push( S, atof( str ) );
			puts(str);
			printf( "%.4f\n", atof( str ));
			printf( "%.4f\n", S->Data );
			printf("%d\n", S->Top);
		}
		else{
			if(!IsEmpty(S)) 
				Op2 = Pop(S);
			else
				Op2 = INFINITY;
			if(!IsEmpty(S))
				Op1 = Pop(S);
			else
				Op2 = INFINITY;
			switch(str[0]){
				case'+': Push( S, Op1 + Op2 ); break;
				case'-': Push( S, Op1 - Op2 ); break;
				case'*': Push( S, Op1 * Op2 ); break;
				case'/':
					if( Op2 != 0.0 )
						Push( S, Op1 / Op2 );
					else{
						printf( "错误：分母为零\n" );
						Op2 = INFINITY;
					}
					break;
				default:
					printf( "错误：未知运算符\n" );
					Op2 = INFINITY;
					break;
			}
			if(Op2 >= INFINITY)
		 		break;
		}
	}
	if( Op2 < INFINITY )
		if(!IsEmpty(S))
			Op2 = Pop(S);
		else
			Op2 = INFINITY;
	free(S);
	return Op2;
}
//主函数
int main(void)
{
	int i, j;	
	char Expr[MAXOP];
	ElementType f;
	gets( Expr );
//	j = strlen(Expr);
//	for( i = 0; i < j; i++ )
//		printf( "%.4c\n", Expr[i] );
	f = PostfixExp(Expr);
	if(f < INFINITY)
		printf( "%.4f\n", f );
	else
		printf( "表达式错误\n" );
	
	return 0; 
}

程序主体是照搬陈越、何钦铭数据结构第二版第三章
错误就如图中所看到的一样，读取不到正确的数，堆栈判断也存在问题。自己琢磨了半天还是搞不明白。