表达式求值(以@结尾的中缀算术表达式)

[c++]代码库
#include<iostream.h>
#include<stdlib.h>
#include<ctype.h>
#include<PROCESS.H>
#include<STRSTREAM.H>
typedef float elemtype;
struct stack
{
    elemtype *stack;
    short top;
    short stackmaxsize;
};
 
static void initstack ( stack &s,int ms )
{
    s.stack=new elemtype[ms];
    if ( !s.stack )
    {
        cerr<<"memory full!"<<endl;
        exit ( 1 );
    }
    s.top=-1;
    s.stackmaxsize=ms;
}
 
static void  clearstack ( stack &s )
{ s.top=-1;}
 
static void  deletestack ( stack &s )
{
    delete []s.stack;
    s.top=-1;
    s.stackmaxsize=0;
}
 
static int  stackempty ( stack &s )
{ return s.top==-1;}
 
static elemtype  peek ( stack &s )
{
    if ( s.top==-1 )
    {
        cerr<<"stack empty!"<<endl;
        exit ( 1 );
    }
    return s.stack[s.top];
}
 
static void push ( stack &s,elemtype itim )
{
    if ( s.top==s.stackmaxsize-1 )
    {
        cerr<<"stack ovflw!"<<endl;
        exit ( 1 );
    }
    s.top++;
    s.stack[s.top]=itim;
}
 
static elemtype pop ( stack &s )
{
    if ( s.top==-1 )
    {
        cerr<<"stack empty!"<<endl;
        exit ( 1 );
    }
    s.top--;
    return s.stack[s.top+1];
}
 
static int stackfull ( stack &s )
{
    return s.top==s.stackmaxsize-1;
}
 
int precedence ( char op )
{
    switch ( op )
    {
    case'+':
    case'-':
        return 1;
    case'*':
    case'/':
        return 2;
    case'(':
    case'@':
    default:
        return 0;
    }
}
 
 
static float compute ( char *str )
{
    typedef float elemtype;
    int sm=20;
    stack s;
    initstack ( s,sm );
    istrstream ins ( str );
    char ch;
    float x;
    ins>>ch;
    while ( ch!='@' )
    {
        switch ( ch )
        {
        case'+':
            x=pop ( s ) +pop ( s );
            break;
        case'-':
            x=pop ( s );
            x=pop ( s )-x;
            break;
        case'*':
            x=pop ( s ) *pop ( s );
            break;
        case'/':
            x=pop ( s );
            if ( x!=0 )
                x=pop ( s ) /x;
            else
            {
                cerr<<"divide by 0!"<<endl;
                exit ( 1 );
            }
            break;
        default:
            ins.putback ( ch );
            ins>>x;
        }
        push ( s,x );
        ins>>ch;
    }
//   x=pop(s);
//   return x;
    if ( !stackempty ( s ) )
    {
        x=pop ( s );
        if ( !stackempty ( s ) )
        {
            cerr<<"expression error!"<<endl;
            exit ( 1 );
        }
    }
    else
    {
        cerr<<"stack is empty!"<<endl;
        exit ( 1 );
    }
    return x;
}
 
static void change ( char *s1,char  *s2 )
{
    typedef char elemtype;
    int ms=20;
    stack r;
    initstack ( r,ms );
    push ( r,'@' );
    int i,j;
    i=0;
    j=0;
    char ch=s1[i];
    while ( ch!='@' )
    {
        if ( ch==' ' )
            ch=s1[++i];
        else if ( ch=='(' )
        {
            push ( r,ch );
            ch=s1[++i];
        }
        else if ( ch==')' )
        {
            while ( peek ( r ) !='(' )
                s2[j++]=pop ( r );
            pop ( r );
            ch=s1[++i];
        }
        else if ( ch=='+'||ch=='-'||ch=='*'||ch=='/' )
        {
            char w=peek ( r );
            while ( precedence ( w ) >=precedence ( ch ) )
            {
                s2[j++]=w;
                pop ( r );
                w=peek ( r );
            }
            push ( r,ch );
            ch=s1[++i];
        }
        else
        {
            while ( isdigit ( ch ) ||ch=='.' )
            {
                s2[j++]=ch;
                ch=s1[++i];
            }
            s2[j++]=' ';
        }
    }
    ch=pop ( r );
    while ( ch!='@' )
    {
        if ( ch=='(' )
        {
            cerr<<"expression error!"<<endl;
            exit ( 1 );
        }
        else
        {
            s2[j++]=ch;
            ch=pop ( r );
        }
    }
    s2[j++]='@';
    s2[j++]='\0';
}
 
void main()
{
    char a[50],b[50];
    cout<<"input in @ end :"<<endl;
    cin.getline ( a,sizeof ( a ) );
    change ( a,b );
    cout<<"dui ying de hou zhui biao da shi:"<<endl;
    cout<<b<<endl;
    cout<<"qiu zhi jie guo shi:"
        <<compute ( b ) <<endl;
}
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