use std::io;
use regex::Regex;
-// describe the operators
-enum Operator {
- NUM, ADD, SUB, MUL, DIV,
+enum Operator {
+ ADD,
+ SUB,
+ MUL,
+ DIV,
}
// structure to hold an expression in the stack
struct Expression {
value : f32,
- operator : Operator,
}
// function to compute a result by popping the stack and
// pushing back the result
-fn get_result (t : &mut Vec::<Expression>) {
- // pop the stack to get the operator
- // if nothing - panic with error
- let op = match t.pop () {
- Some(x) => x,
- None => panic! ("Illegal expression!"),
- };
+fn get_result (t : &mut Vec<Expression>, op : Operator) -> Result<f32,String> {
// pop the stack for last operand
// if nothing - panic with error
- let n1 = match t.pop () {
- Some (x) => x,
- None => panic! ("Illegal expression!"),
- };
+ let n1 = t.pop ();
+ if n1.is_none () {
+ return Err ("Illegal expression".to_string());
+ }
// pop the stack for the first operand
// if nothing - panic with error
- let n2 = match t.pop () {
- Some (x) => x,
- None => panic! ("Illegal expression!"),
- };
-
- let mut res : f32 = 0.0;
+ let n2 = t.pop ();
+ if n2.is_none () {
+ return Err ("Illegal expression!".to_string());
+ }
+
+ let num1 = n1.unwrap().value;
+ let num2 = n2.unwrap().value;
// depending on the operation, set the result
- match op.operator {
- Operator::ADD => res = n1.value + n2.value,
- Operator::SUB => res = n2.value - n1.value,
- Operator::MUL => res = n1.value * n2.value,
- Operator::DIV => res = n2.value / n1.value,
- _ => panic! ("Illegal operator in expression!"),
+ match op {
+ Operator::ADD => {
+ t.push (Expression{value: num2 + num1});
+ Ok (num2 + num1)
+ },
+ Operator::SUB => {
+ t.push (Expression{value: num2 - num1});
+ Ok (num2 + num1)
+ },
+ Operator::MUL => {
+ t.push (Expression{value: num2 * num1});
+ Ok (num2 * num1)
+ },
+ Operator::DIV => {
+ t.push (Expression{value: num2 / num1});
+ Ok (num2 / num1)
+ },
}
- // push the result back to the stack
- t.push (Expression {value : res, operator : Operator::NUM });
}
// evaluation function
-fn evaluate (expr : &str) {
+fn evaluate (expr : &str, match_num : ®ex::Regex) -> Result<f32,String> {
let mut ops = Vec::<Expression>::new ();
// tokenize the individual words by splitting at whitespace
let words = expr.split_whitespace ();
- // regular expression to match a number
- let match_num = Regex::new (r"^\d+?.*?\d*?$").unwrap ();
+
// iterate over the words
for word in words {
match word {
// if the word matches one of +, -, *, / then push it on the stack
// and immediately evaluate the expression by popping the operator
// and the last two operands and push the result back on to the stack
- "+" => {ops.push (Expression{ value: 0.0, operator: Operator::ADD });
- get_result (&mut ops) },
- "-" => {ops.push (Expression{ value: 0.0, operator: Operator::SUB });
- get_result (&mut ops) },
- "*" => {ops.push (Expression{ value: 0.0, operator: Operator::MUL});
- get_result (&mut ops)},
- "/" => {ops.push (Expression{ value: 0.0, operator: Operator::DIV});
- get_result (&mut ops) },
+ "+" => {
+ let m = get_result (&mut ops, Operator::ADD);
+ if ! m.is_ok () {
+ return Err (m.unwrap_err().to_string());
+ }
+ },
+ "-" => {
+ let m = get_result (&mut ops, Operator::SUB);
+ if ! m.is_ok () {
+ return Err (m.unwrap_err().to_string());
+ }
+ },
+ "*" => {
+ let m = get_result (&mut ops, Operator::MUL);
+ if ! m.is_ok () {
+ return Err (m.unwrap_err().to_string());
+ }
+ },
+ "/" => {
+ let m = get_result (&mut ops, Operator::DIV);
+ if ! m.is_ok () {
+ return Err (m.unwrap_err().to_string());
+ }
+ },
// if word matches a number, push it on to the stack
_ => if match_num.is_match (word) {
- let num : f32 = word.parse ().unwrap ();
- ops.push (Expression {value: num, operator: Operator::NUM });
- }
+ let num = word.parse ().unwrap ();
+ ops.push (Expression { value: num });
+ }
// if word doesn't match either operator or number then panic.
else {
- panic! ("Error parsing expression!
- Must be a number or operator (+,-,* or /)");
- }
+ return Err ("Error parsing expression! \
+ Must be a number or operator (+,-,* or /)".to_string());
+ }
}
}
+
+ if ops.len () > 1 {
// if the stack has more than one value, it means that the postfix
// expression is not complete - so display the stack status
- if ops.len () > 1 {
- println! ("Postfix Expression not complete. Current stack: ");
- for exp in ops {
- println! ("{}", exp.value);
- }
- }
+ return Err ("Postfix expression incomplete!".to_string());
+ } else {
// stack has only one item which is the result so display it
- else {
- println! ("Result: {}", ops[0].value);
+ let res = ops[0].value;
+ return Ok (res);
}
- println! ();
}
fn main() {
// get a line from input and evaluate it
let mut expr = String::new ();
- // loop until a blank line is received
+ // regular expression to match a number
+ let match_num = Regex::new (r"^\d+?.*?\d*?$").unwrap ();
+ // loop until a blank line is received
loop {
expr.clear ();
println!("Enter an expression (postfix) (blank to quit): ");
break;
}
- evaluate (&expr);
+ let res = evaluate (&expr, &match_num);
+ if res.is_ok () {
+ println! ("Result is : {}", res.unwrap());
+ }
+ else {
+ eprintln! ("Error: {}", res.unwrap_err());
+ }
}
}
projects / evpf.git / blobdiff