Write specific bytes inside a file using lua

Question

I want to write a specific byte inside a .txt file, for example: Text File

Something

Code

Som4thing

I want to do like that, but without overwriting the whole file using io.write().

I hope it was easy to understand my question.

Joseph Sible-Reinstate Monica · Accepted Answer · 2020-05-12 22:51:51Z

2

First, you need to figure out what mode to open the file in. r won't let you write, so it's out. a and a+ will only ever let you write to the end, so they're out. w and w+ erase the whole file, so they're out. That leaves r+.

Next, you need to get to the right place in the file. The seek function does that. In your case, you want to go to 3 bytes past the beginning.

Finally, simply write your data and close the file.

local file = io.open('filename.txt', 'r+')
file:seek('set', 3)
file:write('4')
file:close()

answered May 12, 2020 at 22:51

Joseph Sible-Reinstate Monica

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3 Comments

ProgrammingIsLearning Over a year ago

Thank you! I actually didn't know you could do f:write(), I always did io.input(f) io.write('4'). And I actually appreciate that you answer pretty fast.

ProgrammingIsLearning Over a year ago

I just noticed, the byte I want to open will be always the letter position minus 1?

Joseph Sible-Reinstate Monica Over a year ago

@ProgrammingIsLearning Yeah, it's zero-based, unlike pretty much everything else in Lua.

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