Question

My code is designed to read digits and turn them into Chinese pinyin:

function digitconverter (digit)
    if digit == "0" then
        cnumber = "ying2 "
    elseif digit == "1" then
        cnumber = "yi1 "
    elseif digit == "2" then
        cnumber = "er2 "
    elseif digit == "3" then
        cnumber = "san1 "
    elseif digit == "4" then
        cnumber = "si4 "
    elseif digit == "5" then
        cnumber = "wu3 "
    elseif digit == "6" then
        cnumber = "liu4 "
    elseif digit == "7" then
        cnumber = "qi1 "
    elseif digit == "8" then
        cnumber = "ba1 "
    elseif digit == "9" then
        cnumber = "jiu3 "
    end
    return cnumber
end

print("Enter a number to be converted:")

repeat
    strnumber = io.read("*line")
    number = tonumber(strnumber)
    if number ~= nil then
        continue = true
    else
        print("Invalid input. Please try again:")
        continue = false
    end
until continue == true
nlength = #strnumber

digits = {}
for d in string.gmatch(number, "%d") do
    digits[#digits + 1] = d
end

convnumber = ""
for d=1,nlength do
    convnumber = convnumber .. digitconverter(digits[d])
end
print(convnumber)

    io.read()

If I enter over 15 digits, it gets stuck (for lack of a better term). It WILL convert every digit, but the 16th will be random and the 17th and on will repeat another random one. I’ve been over it and I can’t figure out where it’s getting hung up. Thoughts?

Answer 1

You’re iterating through the digits of number, not strnumber. The problem is when you get to too many digits, the string representation is going to be in scientific notation:

strnumber = '1234567890123456789'
number = tonumber(strnumber)
print(number) --> 1.2345678901235e+018

---

Side note: Lua is based on hashtables, which gives you (barring hash collisions) constant time lookup. So your digit converter can be simply written as a map:

local digitmap = {
   ["0"] = "ying2 ",
   ["1"] = "yi1 ",
   ["2"] = "er2 ",
   ["3"] = "san1 ",
   ["4"] = "si4 ",
   ["5"] = "wu3 ",
   ["6"] = "liu4 ",
   ["7"] = "qi1 ",
   ["8"] = "ba1 ",
   ["9"] = "jiu3 ",
}

Also, building strings like this is very inefficient:

for d=1,nlength do
   convnumber = convnumber .. digitconverter(digits[d])
end

You’re generating tons of intermediate strings, which requires a lot of allocations and produces a lot of garbage. It’s much faster to put all the values you need to concatenate into a table, then call table.concat. Another advantage is that you can specify a delimiter (right now, you’re hard coding the delimiter into your string table).

Using those techniques, we can rewrite your code like this:

local digitmap = {
   ['0'] = 'ying2',
   ['1'] = 'yi1',
   ['2'] = 'er2',
   ['3'] = 'san1',
   ['4'] = 'si4',
   ['5'] = 'wu3',
   ['6'] = 'liu4',
   ['7'] = 'qi1',
   ['8'] = 'ba1',
   ['9'] = 'jiu3',
}

print('Enter a number to be converted:')
while true do
   strnumber = io.read('*line')
   if not strnumber:match('%D') then
      break
   end
   print('Invalid input. Please try again:')
end

local digits = {}
for digit in string.gmatch(strnumber, '%d') do
   digits[#digits + 1] = digitmap[digit]
end

print(table.concat(digits, ' '))