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.

You want to write 10000011111101110001010 or 34333234323334? — hjpotter92
– hjpotter92, Commented Jun 30, 2014 at 9:04
I want to write 10000011111101110001010, but not in string format, I want to write 4 bytes, the size of an integer. — Merni
– Merni, Commented Jun 30, 2014 at 9:14
I don't think Lua has built in support for this. Your best bet would be to add some C functions to turn numbers into the appropriate strings. — Mankarse
– Mankarse, Commented Jun 30, 2014 at 10:25
But if I have over 100k values it will take a lot of space. And Im in corona so I cant call any c functions directly. — Merni
– Merni, Commented Jun 30, 2014 at 11:03

legends2k · Accepted Answer · 2014-12-08 06:26:54Z

6

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

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

edited Dec 8, 2014 at 6:26

legends2k

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answered Jul 1, 2014 at 19:39

lipp

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Comments

lhf · Accepted Answer · 2014-06-30 11:53:04Z

3

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.

edited Jun 30, 2014 at 11:53

answered Jun 30, 2014 at 11:48

lhf

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4 Comments

Merni Over a year ago

would you read four bytes at a time or would you read everything and then later split it up in 4 bytes?

Merni Over a year ago

one more question:), if a read one byte at a time it works, if I read 4 bytes, local bytes = f:read(4) local firstByte = string.byte(bytes), how do I get the rest of the bytes, I cant really divide by 256 because its not a pointer to the first position, just a byte

lhf Over a year ago

@Merni, ask a separate question or edit this one with your efforts for reading.

lhf Over a year ago

Since Lua 5.3 you can use string.pack to write values in binary.

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