Evaluation of Postfix Expression

AIM:

          To write a C program to evaluate the postfix expression

ALGORITHM:

            Step 1: Start the program.

Step 2: Read the postfix expression from left to right

Step 3: If the symbol read is an operand then push it onto the stack

Step 4: If the operator is read POP two operands and perform arithmetic 

            operations if operator is 

            + then result=operand 1 + operand 2

            -  then result=operand 1 - operand 2

            * then result=operand 1 * operand 2

            /  then result=operand 1 / operand 2

Step 5: Push the result onto the stack

Step 6: Repeat steps 2-5 till the postfix expression is not over

Step 7: Stop the program.

PROGRAM

#include <stdio.h>

#include <string.h>

int top = -1;

int stack[100];

/* push the given data into the stack */

void push (int data) {

stack[++top] = data;

}

/* Pop the top element from the stack */

int pop () {

int data;

if (top == -1)

return -1;

data = stack[top];

stack[top] = 0;

top--;

return (data);

}

int main()

 {

char str[100];

int i, data = -1, operand1, operand2, result;

/* Get the postfix expression from the user */

printf("Enter ur postfix expression:");

fgets(str, 100, stdin);

for (i = 0; i < strlen(str); i++)

 {

if (isdigit(str[i]))

{

/*

* if the i/p char is digit, parse

* character by character to get

* complete operand

*/

data = (data == -1) ? 0 : data;

data = (data * 10) + (str[i] - 48);

continue;

}

if (data != -1)

 {

/* if the i/p is operand, push it into the stack */

push(data);

}

if (str[i] == '+' || str[i] == '-'|| str[i] == '*' || str[i] == '/')

{

/*

* if the i/p is an operator, pop 2 elements

* from the stack and apply the operator

*/

operand2 = pop();

operand1 = pop();

if (operand1 == -1 || operand2 == -1)

break;

switch (str[i])

{

case '+':

result = operand1 + operand2;

/* push the result into the stack */

push(result);

break;

case '-':

result = operand1 - operand2;

push(result);

break;

case '*':

result = operand1 * operand2;

push(result);

break;

case '/':

result = operand1 / operand2;

push(result);

break;

}

}

data = -1;

}

if (top == 0)

printf("The answer is:%d\n", stack[top]);

else

printf("u have given wrong postfix expression\n");

return 0;

}

OUTPUT

Enter you postfix expression: 10 20 * 30 40 10         /-+

The answer is: 226

Result: Thus a C program is written to evaluate postfix expression and executed

              successfully