Code for implementing Postfix Evaluation
This code is designed for converting an infix equation(expression) into postfix equation(expression) means Infix to Postfix and also for implementing Postfix Evaluation. In this code you have to pass an equation or expression in infix form as string (character array) format, the code will convert the infix expression into postfix expression. In this code precedence of the operator is also considered. This is the easiest way to implement infix to postfix conversion concept in C. Then the postfix expression is evaluated for getting the result. For Such more codes click here and for video tutorial click here.
#include<stdio.h>
#include<stdlib.h>
#define max 50
typedef struct stack
{
char ar[max];
int top;
}st;
typedef struct stack1
{
float res[max];
int top;
}st1;
void push(st *,char);
void push1(st1 *,float);
char pop(st *);
float pop1(st1 *);
char peek(st);
int isfull(st);
int isempty(st);
float pfixeval(char []);
void intopost(char [],char []);
int getpriority(char);
int main()
{
char expr[max],res[max];
float p;
printf("\nEnter the expression (in digits only) to be evaluated in Infix form and rest is our responsibility : ");
fgets(expr,max,stdin);
intopost(expr,res);
printf("\nThe postfix expression is : ");
puts(res);
p=pfixeval(res);
printf("\nEvaluated result is : %f",p);
}
void intopost(char expr[],char res[])
{
st s;
char ch;
int i,j;
s.top=-1;
for(i=0,j=0;expr[i]!='\0';i++)
{
if(expr[i]==' ')
continue;
else if(expr[i]=='(' || expr[i]=='{' || expr[i]=='[')
push(&s,expr[i]);
else if((expr[i]>=48 && expr[i]<=57) || (expr[i]>=65 && expr[i]<=90) || (expr[i]>=97 && expr[i]<=122))
res[j++]=expr[i];
else if(expr[i]=='+' || expr[i]=='-' || expr[i]=='*' || expr[i]=='/')
{
while(!isempty(s) && getpriority(expr[i])<=getpriority(peek(s)) && peek(s)!='(')
{
res[j++]=pop(&s);
}
push(&s,expr[i]);
}
else if(expr[i]==')' || expr[i]=='}' || expr[i]==']')
{
while(!isempty(s) && (ch=pop(&s))!='(')
{
res[j++]=ch;
}
}
else
{
printf("Expression not ok");
break;
}
}
if(!isempty(s))
{
while(!isempty(s) && (ch=peek(s))!='(')
res[j++]=pop(&s);
}
res[j]='\0';
}
void push(st *s,char ch)
{
s->ar[++s->top]=ch;
}
char pop(st *s)
{
return s->ar[s->top--];
}
char peek(st s)
{
return s.ar[s.top];
}
int isempty(st s)
{
if(s.top==-1)
return 1;
else
return 0;
}
int isfull(st s)
{
if(s.top==max-1)
return 1;
else
return 0;
}
int getpriority(char ch)
{
switch(ch)
{
case '+':
case '-': return 0;
case '*':
case '/':return 1;
}
}
//postfix evaluation taking place
float pfixeval(char str[])
{
float val1,val2,res;
int i;
st1 s;
s.top=-1;
for(i=0;str[i]!='\0';i++)
{
if(str[i]>=48 && str[i]<=57)
push1(&s,str[i]-'0');
else
{
val1=pop1(&s);
val2=pop1(&s);
switch(str[i])
{
case '+':res=val2+val1;
break;
case '-':res=val2-val1;
break;
case '*':res=val2*val1;
break;
case '/':res=(val2*1.0)/val1;
break;
}
push1(&s,res);
}
}
return pop1(&s);
}
float pop1(st1 *s)
{
return s->res[s->top--];
}
void push1(st1 *s,float p)
{
s->res[++s->top]=p;
}
