Evaluation of Postfix Expression

Procedure to evaluate a postfix expression using stack.

Procedure :

  • Iterate through each character of the given postfix expression.
  • If the character is an operand (0 to 9), push it to stack after properly typecasting into integer value from char value by subtracting 48.
  • If the character is an operator, pop 2 items from the stack. The element to the popped will be num2 and the next element will be num1.
  • Evaluate the operation using switch case.
  • Push the result of evaluation to stack.
  • The result will be at stack top after the whole process.

C Program :

int isOperand(char);
​
// Main function
int main() {
​
char postfix[30];
int stack[30];
int num1, num2, res, i, top=-1;
​
printf("Enter a postfix expression : \n");
scanf("%s",postfix);
​
// Iterate through each character of postfix expression
for(i=0;postfix[i]!='\0';i++) {
​
// If character is an operand, push it to stack
if(isOperand(postfix[i])==1) {
top++;
stack[top] = (int)(postfix[i]-48);
}
​
// If character is an operator, pop 2 items from stack and perform operation
else {
// Popping two items from stack
num2 = stack[top];
top--;
num1 = stack[top];
top--;
​
// Switch case to choose operation
switch(postfix[i]) {
​
case '+' : res = num1+num2;
break;
​
case '-' : res = num1-num2;
break;
​
case '*' : res = num1*num2;
break;
​
case '/' : res= num1/num2;
break;
​
}
​
// Add result to stack top
top++;
stack[top] = res;
​
}
}
​
// Getting result
printf("Answer of expression : %d\n",stack[top]);
return 0;
​
}
​
// isOperand function
int isOperand(char x) {
​
if(x>='0' && x<='9') {
return 1;
}
else {
return 0;
}
​
}
Contributed by Nitin Ranganath