examples: calculator

examples/mini_calculator.v

@@ -0,0 +1,134 @@
+// Q: What's this?
+// A: This is a mini "home-made" calculator. You may also regard it as a very elementary version of "interpreter".
+
+import os
+
+const (
+	NUMERIC_CHAR = [`0`,`1`,`2`,`3`,`4`,`5`,`6`,`7`,`8`,`9`,`.`,`e`,`E`]
+)
+
+// Convert expression to Reverse Polish Notation. 
+fn expr_to_rev_pol(expr string) ?[]string {
+	if expr == '' {
+		return error('err: empty expression')
+	}
+	mut stack := []string
+	mut rev_pol := []string
+	mut pos := 0
+	for pos<expr.len {
+		mut end_pos := pos
+		for end_pos<expr.len && expr[end_pos] in NUMERIC_CHAR {
+			end_pos++
+		}
+		if end_pos>pos {
+			stack << expr[pos..end_pos]
+			pos = end_pos
+		}
+		else if end_pos==pos {
+			op := expr[pos].str()
+			match op {
+				'(' { 
+					stack << op
+				}
+				'*', '/' {
+					for stack.len>0 && !(stack.last() in ['(', '+', '-']) {
+						rev_pol << stack.last()
+						stack.delete(stack.len-1)
+					}
+					stack << op
+				}
+				'+', '-' {
+					for stack.len>0 && stack.last() != '(' {
+						rev_pol << stack.last()
+						stack.delete(stack.len-1)
+					}
+					stack << op
+				}
+				')' {
+					for stack.len>0 && stack.last() != '(' {
+						rev_pol << stack.last()
+						stack.delete(stack.len-1)
+					}
+					stack.delete(stack.len-1)
+				}
+				else {
+					return error('err: invalid character `${op}`')
+				}
+			}
+			pos++
+		}
+	}
+	for stack.len>0 {
+		top := stack.last()
+		rev_pol << top 
+		stack.delete(stack.len-1)
+	}
+	return rev_pol
+}
+
+// Evaluate the result of Reverse Polish Notation.
+fn eval_rev_pol(rev_pol []string) ?f64 {
+	mut stack := []f64
+	for item in rev_pol {
+		if is_num_string(item) {
+			stack << item.f64()
+		}
+		else {
+			if stack.len>=2 {
+				oprand_r := stack.last()
+				stack.delete(stack.len-1)
+				oprand_l := stack.last()
+				stack.delete(stack.len-1)
+				match item {
+					'+' { stack << oprand_l+oprand_r }
+					'-' { stack << oprand_l-oprand_r }
+					'*' { stack << oprand_l*oprand_r }
+					'/' { 
+						if oprand_r == 0 {
+							return error('err: divide by zero')
+						}
+						stack << oprand_l/oprand_r
+					}
+					else {}
+				}
+			}
+			else {
+				return error('err: invalid expression')
+			}
+		}
+	}
+	return stack[0]
+}
+
+fn is_num_string(str string) bool {
+	for c in str {
+		if !(c in NUMERIC_CHAR) {
+			return false
+		}
+	}
+	return true
+}
+
+
+fn main() {
+	println('Please enter the expression you want to calculate, e.g. 1e2+(3-2.5)*6/1.5 .')
+	println('Enter \'exit\' or \'EXIT\' to quit.')
+	mut expr_count := 0
+	for {
+		expr_count++
+		print('[$expr_count] ')
+		expr := os.get_line().trim_space()
+		if expr in ['exit', 'EXIT'] {
+			break
+		}
+		rev_pol := expr_to_rev_pol(expr) or {
+			eprintln(err)
+			continue
+		}
+		res := eval_rev_pol(rev_pol) or { 
+			eprintln(err)
+			continue
+		}
+		println(res)
+	}
+}