Write numbers to a file in lua in binary format

Question

I have a big array with numbers I would like to write to a file.

But if I do this:

local out = io.open("file.bin", "wb")
local i = 4324234
out:write(i)

I am just writing the number as a string to the file. How do I write the correct bytes for the number to file. And how can I later read from it.

Answer 1

You could use lua struct for more fine-grained control over binary conversion.

local struct = require('struct')
out:write(struct.pack('i4',0x123432))

Answer 2

Try this

function writebytes(f,x)
    local b4=string.char(x%256) x=(x-x%256)/256
    local b3=string.char(x%256) x=(x-x%256)/256
    local b2=string.char(x%256) x=(x-x%256)/256
    local b1=string.char(x%256) x=(x-x%256)/256
    f:write(b1,b2,b3,b4)
end

writebytes(out,i)

and also this

function bytes(x)
    local b4=x%256  x=(x-x%256)/256
    local b3=x%256  x=(x-x%256)/256
    local b2=x%256  x=(x-x%256)/256
    local b1=x%256  x=(x-x%256)/256
    return string.char(b1,b2,b3,b4)
end

out:write(bytes(0x10203040))

These work for 32-bit integers and output the most significant byte first. Adapt as needed.