Solved:Lab 9: Postfix Evaluation

Read a postfix expression from the given input file, and evaluate the value of the postfix expression using stack ADT.

1. Input

Obtain a postfix expression from the given input file (expr_input.txt). The expression ends with #. A detailed specification of operators and operands is provided below.

  • Available operators: +, -, *, /, and %

  • Operands: single-digit numbers (1, 2, 3, 4, 5, 6, 7, 8, and 9)

  • Conditions:

  • The expression should be no more than 100 characters.

  • The delimiter for the end of the expression is ‘#‘.

  • No exception handling is required for checking whether the input file exists.

2. Data structure

struct Stack {

char *key;

int top;

int max_stack_size;

};

3. Evaluation algorithm

  • Iteratively obtain tokens until you meet the end of an expression.

  • There are two rules for popping and pushing the operands from/to the stack:

  • When you meet an operand (number), push it onto the stack.

  • When you meet an operator, pop two operands from the stack and perform the operation, and push the result back to the stack.

4. Program description

  • name: p6.c

  • input: a postfix expression in a file

  • output: evaluation result in the standard output

Submit to the gitlab your code. ( ~2020/5/21 23:59 )