v/examples/mini_calculator.v

// 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)
	}
}