ok I have my problem entering in an expression in infix notation and
it outputs the postfix notation. I now need to evaluate the postfix
notation. I have some code written and there are comments as to what I
want to do but am unable to get it to work. So if someone could please
help me it would greatly be appreciated. Thanks for your help.
code:
#include <cstdio>
#include <cstdlib>
#include <iostream>
using namespace std;
const int SIZE=100;
class stack;
class stacknode
{
private:
char data;
stacknode *link;
stacknode(int d=0,stacknode *l=0):data(d),link(l){};
public:
friend class stack;
};
class stack
{
private:
stacknode *top;
public:
stack(){top=0;};
void push(const char);
char pop();
char empty();
};
void stack::push(const char y)
{
top=new stacknode(y,top);
}
char stack::pop()
{
if(top==0)
return 0;
char retvalue;
stacknode *x=top;
retvalue=top->data;
top=x->link;
delete x;
return retvalue;
}
char stack::empty()
{
if(top==0) return '1';
else return '\0';
}
//************
char isoprand(char symb)
{
if(symb=='+'||symb=='-'||symb=='*'||symb=='/'||symb=='^'|| symb=='('
||symb==')' )
return '\0';
else
return '1';
}
int isoperator(char symb)
{
int r=0;
if(symb=='+'||symb=='-'||symb=='*'||symb=='/'||symb=='^') r=1;
return r;
}
//************
int precedence(char op1,char op2)
{
char s[4][3]={"()","+-","*/","^^"};
int i=0,j,k1,k2,r=1;
for(i=0;i<4;i++)
for(j=0;j<2;j++)
{
if(op1==s[i][j])k1=i;
if(op2==s[i][j])k2=i;
}
if(k1<k2)r=0;
return r;
}
//************
void infix_postfix(char *infix,char *postfix)
{
int position, toppos=0;
char top_operator='\0',symb;
stack operator_stack;
for( position=0; (symb=infix[position])!='\0'; position++)
{
if(isoprand(symb))
{
postfix[toppos++]=symb;
postfix[toppos]='\0';
}
else
switch(symb)
{
case ')':
while( (top_operator=operator_stack.pop())
&& top_operator!='('
) postfix[toppos++]=top_operator;
break;
case '(':
operator_stack.push(symb);
break;
default :
while( (top_operator=operator_stack.pop())
&& precedence(top_operator,symb)
) postfix[toppos++]=top_operator;
if(top_operator)
operator_stack.push(top_operator);
operator_stack.push(symb);
}
}
while(!operator_stack.empty())
postfix[toppos++]=operator_stack.pop();
postfix[toppos]='\0';
}
//************
int isnotexp(char *infix)
{
int r=0, k1=0,k2=0,n=0;
char *p = infix;
char ch1;
while(*p)
{
if(*p=='(')k1++;
else if(*p==')')
k2++;
p++;
n++;
}
if(k1!=k2)
r=1;
p=infix;
ch1=*p++;
while(*p)
{
if(isoperator(ch1)&&isoperator(*p)) r=1;
if(ch1==')'&&isoprand(*p)) r=1;
if(ch1=='('&&isoperator(*p)) r=1;
if(isoprand(ch1)&& *p=='(') r=1;
ch1=*p++;
}
if(n>SIZE)r=n;
return r;
}
void evaluates(char *infix)
{
int position;
char symbol, numbers;
int number1, number2;
stack operator_stack;
for(position=0; infix[position] !='\0'; position++)
{
symbol=infix[position];
if(isoprand(symbol))
{
numbers=infix[position];
operator_stack.push(numbers);
}
else
{
//I want to get the top item from the stack_operator stack
and the next number and add them or subtract them and so on depending
on the sign. I'm not really sure what to do so if someone could please
help it would greatly be appreciated.
symbol=infix[position];
switch(symbol)
{
case '+':
break;
case '-':
break;
case '*':
break;
case '/':
break;
case '^':
break;
}
}
}
}
//************
int main()
{
char *infix=new char[SIZE];
char *postfix=new char[SIZE];
int k=0;
cout<<"Please enter an expression : "<<endl;
gets(infix);
k=isnotexp(infix);
if(!k)
{
infix_postfix(infix,postfix);
puts(postfix);
}
else
{
if(k==1) cout<<"Wrong expression!\n"<<endl;
else cout<<"infix is over flow.\n"<<endl;;
}
evaluates(postfix);
delete [] infix;
delete [] postfix;
return 0;
}
again thanks for the help
