CODEWITHSUNDEEP
fileiolua
matjojo
matjojo

Reputation: 113

How to read specific line using io.read()

This is what I'm trying to do:

io.open("__Equivalent-Exchange__/config/EMCfixed.lua", "r")
var1 = io.read(,*n)

Now I want to be able to set the row of the file that is specified with io.open. What is found was this:

The reference manual state:

When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file. When called with a file handle, it simply sets this file handle as the default input file. When called without parameters, it returns the current default input file.

That didn't help me so I found the io.lines piece.:

Opens the given file name in read mode and returns an iterator function that works like file:lines(···) over the opened file. When the iterator function detects the end of file, it returns no values (to finish the loop) and automatically closes the file.

The call io.lines() (with no file name) is equivalent to io.input():lines("*l") that is, it iterates over the lines of the default input file. In this case it does not close the file when the loop ends."

BUT, How can I specify what line to read?

P.S. for other info on the topic I found this page, I didn't understand it. But it might help your in the process of helping me.

Upvotes: 1

Views: 4926

Answers (1)

Henrik Ilgen
Henrik Ilgen

Reputation: 1967

You cannot jump directly to a specified line, because that would require you to know where that line is. Without reading the whole file up to at least this point, this is only possible if the lines have a fixed length (in which case you could use file:seek). However, if you do not have fixed-length lines, you'll have to iterate over the lines, counting as you go:

function getNthLine(fileName, n)
    local f = io.open(fileName, "r")
    local count = 1

    for line in f:lines() do
        if count == n then
            f:close()
            return line
        end
        count = count + 1
    end

    f:close()
    error("Not enough lines in file!")
end

Edit: Note that you should not use this function if you are searching for multiple lines of the same file (e.g. you need lines 3, 5 and 8). In this case, the function above would open the file three times - that's a waste of system resources. Instead, you could define a function to be called on every number and check for matching line numbers there:

function checkLine(lineNumber, lineContent)
    -- Disregard odd line numbers
    if lineNumber % 2 == 0 then
        -- do something with lineContent
    end
end

local f = io.open(fileName, "r")
local count = 1

for line in f:lines() do
    checkLine(count, line)
end

f:close()

Upvotes: 3

Related Questions