v/examples/mini_calculator.v

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2020-04-01 14:19:57 +02:00
// 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)
}
}