20)Write a ‘C’ program to read a postfix expression, evaluate it and display the result. (Using two variable).

#include<stdio.h>
#include<math.h>
#define MAX 20
typedef struct stack
{
int stk[MAX],top;
}STACK;
void init_stack(STACK *sp)
{
sp->top = -1;
}
void push(STACK *sp, int val)
{
sp->stk[++sp->top]=val;
}
int pop(STACK *sp)
{
return sp->stk[sp->top--];
}
int is_empty(STACK *sp)
{
return sp->top==-1;
}
void disp(STACK *sp)
{
int i;
for(i=0;i<=sp->top;i++)
printf("%d,",sp->stk[i]);
printf("\b ");
}
int eval(int a, int b, char c)
{
int ans;
switch(c)
{
case '+':
ans = a+b;
break;
case '-':
ans = a-b;
break;
case '*':
ans = a*b;
break;
case '/':
ans = a/b;
break;
case '^':
ans = pow(a,b);
break;
}
return(ans);
}

void main()
{
char postfix[MAX], symb;
int i, opd1=0, opd2=0, result=0, values[2];
STACK opndstk;
init_stack(&opndstk);
clrscr();
printf("\nEnter two variable postfix string: ");
scanf("%s",postfix);
printf("Enter value of a & b:");
scanf("%d %d",&values[0],&values[1]);
printf("symb\topnd1\topnd2\tvalue\topndstk\n");
for(i=0;postfix[i]!='\0';i++)
{
symb = postfix[i];
if(symb=='a' || symb=='b')
push(&opndstk,values[symb-'a']);
else
{
opd2 = pop(&opndstk);
opd1 = pop(&opndstk);
result = eval(opd1, opd2, symb);
push(&opndstk,result);
}
printf("%c\t%d\t%d\t%d\t",symb,opd1,opd2,result);
disp(&opndstk);
printf("\n");
}
printf("Result = %d",pop(&opndstk));
getch();
}

Output:
Enter two variable postfix string: 2
Enter value of a & b:4 8
symb opnd1 opnd2 value opndstk
2 2106 2088 245
Result = 245
 /

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