1 files changed, 135 insertions, 0 deletions
diff --git a/v_windows/v/examples/mini_calculator.v b/v_windows/v/examples/mini_calculator.v
new file mode 100644
index 0000000..0c22aa3
--- /dev/null
+++ b/v_windows/v/examples/mini_calculator.v
@@ -0,0 +1,135 @@
+// 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].ascii_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 `$op`')
+				}
+			}
+			pos++
+		}
+	}
+	for stack.len > 0 {
+		top := stack.last()
+		rev_pol << top
+		stack.delete(stack.len - 1)
+	}
+	return rev_pol
+}
+
+// Evaluate the result of Reverse Polish Notation.
+fn eval_rev_pol(rev_pol []string) ?f64 {
+	mut stack := []f64{}
+	for item in rev_pol {
+		if is_num_string(item) {
+			stack << item.f64()
+		} else {
+			if stack.len >= 2 {
+				oprand_r := stack.last()
+				stack.delete(stack.len - 1)
+				oprand_l := stack.last()
+				stack.delete(stack.len - 1)
+				match item {
+					'+' {
+						stack << oprand_l + oprand_r
+					}
+					'-' {
+						stack << oprand_l - oprand_r
+					}
+					'*' {
+						stack << oprand_l * oprand_r
+					}
+					'/' {
+						if oprand_r == 0 {
+							return error('err: divide by zero')
+						}
+						stack << oprand_l / oprand_r
+					}
+					else {}
+				}
+			} else {
+				return error('err: invalid expression')
+			}
+		}
+	}
+	return stack[0]
+}
+
+fn is_num_string(str string) bool {
+	for c in str {
+		if c !in numeric_char {
+			return false
+		}
+	}
+	return true
+}
+
+fn main() {
+	println('Please enter the expression you want to calculate, e.g. 1e2+(3-2.5)*6/1.5 .')
+	println("Enter 'exit' or 'EXIT' to quit.")
+	mut expr_count := 0
+	for {
+		expr_count++
+		expr := os.input_opt('[$expr_count] ') or {
+			println('')
+			break
+		}.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)
+	}
+}