EVALUATION OF POSTFIX EXPRESSION USING STACK IN C PROGRAM

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CODING:
#include<stdio.h>
#include<conio.h>
#define max 50
float st[max];
int top=-1;
void  push(float st[],float num);
float pop(float st[]);
float postex(char exp[]);
void main()
{
float num;
char exp[50];
clrscr();
printf("ENTER THE POSTFIX EXPRESSION:\n");
fflush(stdin);
gets(exp);
num=postex(exp);
printf("\nANSWER OF THE POSTFIX EXPRESSION:%f",num);
getch();
}
float postex(char exp[])
{
int i=0;
float no1,no2,num;
while(exp[i]!='\0')
{
 if(isdigit(exp[i]))
   push(st,(float)(exp[i]-'0'));
 else
 {
  no1=pop(st);
  no2=pop(st);
 switch(exp[i])
  {
   case '+':
   num=no1+no2;
   break;
   case '-':
   num=no1-no2;
   break;
   case '*':
   num=no1*no2;
   break;
   case '/':
   num=no1/no2;
   break;
   case '%':
   num=(int)no1%(int)no2;
   break;
   }
    push(st,num);
  }
   i++;
 }
  return (pop(st));
}
void push(float st[],float num)
{
 if(top==max-1)
  printf("\nSTACK IS OVERFLOW...");
 else
 {
  top++;
  st[top]=num;
 }
}
float pop(float st[])
{
float num;
 if(top==-1)
  printf("\nSTACK IS UNDERFLOW...");
 else
 {
  num=st[top];
  top--;
 }
 return num;
}