Evaluation of Postfix Expression in C [Algorithm and Program]

Here you will get algorithm and program for evolution of postfix expression in C.

In postfix or reverse polish notation, every operator follows all of its operands.

For example: 5 3 2 * +

Also Read: Infix to Postfix Conversion in C [Program and Algorithm]

 

Algorithm for Evaluation of Postfix Expression

Create an empty stack and start scanning the postfix expression from left to right. 

  • If the element is an operand, push it into the stack.
  • If the element is an operator O, pop twice and get A and B respectively. Calculate BOA and push it back to the stack.
  • When the expression is ended, the value in the stack is the final answer.

Evaluation of a postfix expression using a stack is explained in below example:

Evaluation of Postfix Expression in C [Algorithm & Program]

 

Program for Evaluation of Postfix Expression in C

 

Output

Enter the expression(eg: 59+3*)
Single digit operand and operators only:74+5-

Value of expression=6

 

Image Credit: http://cis.stvincent.edu/html/tutorials/swd/stacks/stacks.html

7 thoughts on “Evaluation of Postfix Expression in C [Algorithm and Program]”

    1. #include
      #include

      int n, top = -1, *stack;

      void push(int x){
      if(top==n) return;
      stack[++top]=x;
      }

      int pop(){
      if(top==-1) return -1;
      return stack[top–];
      }

      int peek(){
      if(top==-1) return -1;
      return stack[top];
      }

      void display(){
      for(int i=top ; i>-1 ; i–) printf(“%d “,stack[i]);
      printf(“\n\n”);
      }

      int main(){

      n = 10;

      printf(“Initializing the stack with size 10\n\n”);

      stack = (int*)malloc(n*sizeof(int));

      printf(“Pushing elements into the stack\n1\n2\n3\n\n”);

      push(1);
      push(2);
      push(3);

      printf(“Displaying elements of the stack -\n”);

      display();

      printf(“The top of the stack = %d\n\n”,peek());

      printf(“Pop the top of the stack = %d\n\n”,pop());

      printf(“Pop the top of the stack = %d\n\n”,pop());

      printf(“Displaying elements of the stack -\n”);

      display();

      return 0;
      }

    1. #include
      #include
      #include

      #define SIZE 40

      int pop();
      void push(int);

      char postfix[SIZE];
      int stack[SIZE], top = -1;

      int main()
      {
      int i, a, b, result, pEval;
      char ch;

      for(i=0; i<SIZE; i++)
      {
      stack[i] = -1;
      }
      printf("\nEnter a postfix expression: ");
      scanf("%s",postfix);

      for(i=0; postfix[i] != '\0'; i++)
      {
      ch = postfix[i];

      if(isdigit(ch))
      {
      push(ch-'0');
      }
      else if(ch == '+' || ch == '-' || ch == '*' || ch == '/')
      {
      b = pop();
      a = pop();

      switch(ch)
      {
      case '+': result = a+b;
      break;
      case '-': result = a-b;
      break;
      case '*': result = a*b;
      break;
      case '/': result = a/b;
      break;
      }

      push(result);
      }

      }

      pEval = pop();

      printf("\nThe postfix evaluation is: %d\n",pEval);

      return 0;
      }

      void push(int n)
      {
      if (top -1)
      {
      n = stack[top];
      stack[top–] = -1;
      return n;
      }
      else
      {
      printf(“Stack is empty!\n”);
      exit(-1);
      }
      }

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