+use std::io;
+use regex::Regex;
+
+// describe the operators
+enum Operator {
+ NUM, 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!"),
+ };
+ // pop the stack for last operand
+ // if nothing - panic with error
+ let n1 = match t.pop () {
+ Some (x) => x,
+ None => panic! ("Illegal expression!"),
+ };
+ // 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;
+
+ // 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!"),
+ }
+ // push the result back to the stack
+ t.push (Expression {value : res, operator : Operator::NUM });
+}
+
+// evaluation function
+fn evaluate (expr : &str) {
+ 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) },
+ // 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 });
+ }
+ // if word doesn't match either operator or number then panic.
+ else {
+ panic! ("Error parsing expression!
+ Must be a number or operator (+,-,* or /)");
+ }
+ }
+ }
+ // 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);
+ }
+ }
+ // stack has only one item which is the result so display it
+ else {
+ println! ("Result: {}", ops[0].value);
+ }
+ println! ();
+}
+
+fn main() {
+ // get a line from input and evaluate it
+ let mut expr = String::new ();
+ // loop until a blank line is received
+ loop {
+ expr.clear ();
+ println!("Enter an expression (postfix) (blank to quit): ");
+ // read a line of text
+ io::stdin().read_line (&mut expr).expect ("Error reading line!");
+ // trim the text
+ let expr = expr.trim ();
+ // quit if the expression is blank
+ if expr == "" {
+ break;
+ }
+
+ evaluate (&expr);
+ }
+}
projects / evpf.git / commitdiff