Skip to main content

Postfix Expression Evaluation

With a given Postfix Expression, we will show you how to evaluate a postfix expression using stack.
 

Algorithm to evaluate postfix expression:

In this algorithm, we will use stack to store operands. The step are as follows:
  • Get the Postfix Expression String
  • While the end of Postfix Expression string
    • If the current character is operand
      • Push it into Stack
    • End If
    • If the current character is operator
      • Pop stack for second_operand
      • Pop stack for first_operand
      • Evaluate the expression (first_operand)(operator)(second_operand) and push the result into stack
    • End If
  • End While
  • Pop stack for result and return it
 
 


 

Function to Evaluate Postfix Expression:


double evaluate_postfix_exp(char *poststr){
 STACK stk; //stack for pushing operands
 int i = -1;
 double op1, op2;
 stk.top = -1; //top of stack
 while(poststr[++i]){
  if(poststr[i] == ' '){ //escape space
   continue;
  }
  if(isdig(poststr[i])){
   push( &stk, (double)(poststr[i]-'0')/*char to int*/ );
  }
  else{
   op2 = pop(&stk); // first poped number will be used as 2nd operand
   op1 = pop(&stk);
   push( &stk, operation( poststr[i], op1, op2 ) );
  }
 }
 return pop( &stk ); //answer of evaluation
}
 

Program to Evaluate Postfix Expression:


#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define STACK_SIZE 50

typedef struct{
 int top;
 double stack[STACK_SIZE];
} STACK;

double evaluate_postfix_exp(char *);
void push(STACK *, double );
double pop(STACK *);
int isdig(char);
double operation(char, double, double);

int main(){
 double answer;
 char *postfix_str = "1 2 + 3 ^ 4 *"; //108
 answer = evaluate_postfix_exp(postfix_str);
 printf("The answer of postfix expression \"%s\" =   %.2f", postfix_str, answer);
 return 0;
}

double evaluate_postfix_exp(char *poststr){
 STACK stk; //stack for pushing operands
 int i = -1;
 double op1, op2;
 stk.top = -1; //top of stack
 while(poststr[++i]){
  if(poststr[i] == ' '){ //escape space
   continue;
  }
  if(isdig(poststr[i])){
   push( &stk, (double)(poststr[i]-'0')/*char to int*/ );
  }
  else{
   op2 = pop(&stk); // first poped number will be used as 2nd operand
   op1 = pop(&stk);
   push( &stk, operation( poststr[i], op1, op2 ) );
  }
 }
 return pop( &stk ); //answer of evaluation
}

int isdig(char c){ //whether the c is number
 return ( (c >= '0') && (c <= '9') );
}

void push(STACK *s, double num){
 if(s->top == STACK_SIZE-1){
  printf("Stack Overflow\n");
 }
 else{
  s->stack[++s->top] = num;
 }
}

double pop(STACK *s){
 if(s->top == -1){
  printf("Stack Underflow\n");
  exit(2);
 }
 else{
  return s->stack[s->top--];
 }
}

double operation(char c, double op1, double op2){
 switch(c){
  case '+' : return op1+op2;
  case '-' : return op1-op2;
  case '*' : return op1*op2;
  case '/' : return op1/op2;
  case '^' : return pow(op1, op2);
  default  : printf("Invalid operator %c used", c);
      exit(1);
 }
}
 
The answer of postfix expression "1 2 + 3 ^ 4 *" =   108.00
 

Related topics:

Labels:

Comments

Post a Comment

Popular posts from this blog

Prefix to Infix Conversion

With a given Prefix Expression, we will see how to convert Prefix Expression into Infix Expression using stack.   Algorithm to convert Prefix Expression to Infix Expression: In this algorithm, we will use stack to store operands during the conversion. The step are as follows: Read the prefix string While the end of prefix string scanned from right to left symb = the current character If symb is an operator poped_sym1 = pop the stack poped_sym2 = pop the stack concat the string  STR = ( poped_sym1 )+ ( operator )+( poped_sym2 ) push the string STR into stack Else push the operand symb into stack End If End While infix_str = pop the stack   Function to convert Prefix Expression to Infix Expression: void prefix_to_infix(char prefix[], char infix[]){ char op[2]; //operator string char poped1[MAX]; char poped2[MAX]; char temp[MAX]; int i = strlen(prefix); op[1] = '\0'; while(--i != -1){ if(prefix[i] == ' '){ continue; } if(isoper...
Post a Comment
Read more

Concatenating two link list

Here we will see how to concat two single link list into other link list. First we copy the content of first list in third list and then the content of second list into third list. Algorithm for concatenation of two link list: This algorithm will use three list. List 1 and List 2 will be concatenated into List 3. The step below: Copy the list 1 into list 3 Copy the list 2 at the last of list 3 Function to concat two link list: void concatLists(NODE **start1, NODE **start2, NODE **mergeinto){ NODE *temp; temp = *start1; while(temp != NULL){ insertAtLast(&*mergeinto, temp->info); temp = temp->next; } temp = *start2; while(temp != NULL){ insertAtLast(&*mergeinto, temp->info); temp = temp->next; } }   Program to concat two link lists: #include <stdio.h> #include <malloc.h> struct node{ int info; struct node *next; }; typedef struct node NODE; void insertAtLast(NODE **, int); void traverse(NODE **); void concatLists(NODE **,...
Post a Comment
Read more

Implementation of Stack to store different data types

Here we will see how to implement stack using structure and union to store different datatypes in stack. First we define the basic structure for elements/items to be stored in the stack. struct Items{ int ele_type; union { int ivalue; float fvalue; char *strvalue; } stack_element; }; In the above code, we have defined a Items structure to store the info about the type of items in ele_type variable and a union variable named stack_element to store the actual value of stack items. In union the, ivalue is used to store the integer value, fvalue to store floating value and strvalue to store the string value. If you wish to add other datatype, you can add any datatype within the union. Now we will define the structure to implement the stack. The code below: struct Stack{ int top; struct Items element[MAX]; }; In the above code, struct Stack is defined to hold the top of stack and the array of Items structure named element to hold the actual Items defined above. MA...
Post a Comment
Read more