Read/Write highscore from a file (Lua - Corona SDK)

Question

Here's my issue: I have a file with the highscore written in it (just the first line, no nicknames, just the highscore), I need to read that line and compare it with the actual score obtained in the game session, if the score is higher, overwrite the file with the new value, but if I try to read it I get a null value... Seems like i'm not reading it the right way. What's wrong with my code?

Thanks for the help!

local path = system.pathForFile( "data.sav", system.DocumentsDirectory )

local file = io.open( path, "w+" )

highscore_letta = file:read("*n")
print(highscore_letta)

if (_G.player_score > tonumber(highscore_letta)) then
   file:write(_G.player_score)
end

io.close( file )

Answer 1

I had this problem myself. I found out that if you open a file in "w+" mode, the current contents are deleted, so that you can write new contents. So to read and write you have to open the file twice. First, you open the file in "rb" mode and get the file contents, then close it. Then you reopen it in "wb" mode, write the new number, and close it.

In Windows, you need "b" in the file mode. Otherwise, the strings that you are reading and writing may be modified in unexpected ways: for instance, a newline ( "\\n" ) may be replaced with carriage return–newline ( "\\r\\n" ).

The file modes that Lua supports are borrowed from the C language. (I found a description on page 305 of what I guess is a draft of the C specification .) I think the Lua manual sort of assumes that you will know what these modes mean, as an experienced C programmer would, but to me it wasn't at all obvious.

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Thus to read a number and then write a new one:

local filepath = "path/to/file"
-- Create a file handle that will allow you to read the current contents.
local read_file = io.open(filepath, "rb")
number = read_file:read "*n" -- Read one number. In Lua 5.3, use "n"; the asterisk is not needed.
read_file:close() -- Close the file handle.

local new_number = 0 -- Replace this with the number you actually want to write.
-- Create a file handle that allows you to write new contents to the file,
-- while deleting the current contents.
write_file = io.open(filepath, "wb")
write_file:write(new_number) -- Overwrite the entire contents of the file.
write_file:flush() -- Make sure the new contents are actually saved.
write_file:close() -- Close the file handle.

I created a script to do these operations automatically, as they're somewhat annoying to type every time.

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The mode "r+" or "r+b" is supposed to allow you to read and write, but I couldn't get it to work when the original contents are longer than the new contents. If the original contents are "abcd" , four bytes, and the new contents are "efg" , three bytes, and you write at offset 0 in the file, the file will now have "efgd" : the last byte of the original contents is not deleted.