Iterating lines
io.lines and file:lines let you loop over a file one line at a time.
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Iterating lines
file:lines returns an iterator over the lines of an open file, and io.lines(path) opens the file, iterates and closes it automatically at the end. Each line comes without the newline character. This is memory friendly for large files because only one line is held at a time.
Note:
io.lines raises an error if the file cannot be opened, so wrap it in pcall when the file may not exist.
Example: Iterating lines
local path = "/tmp/lua_demo_lines.txt"
local w = assert(io.open(path, "w"))
w:write("alpha\nbeta\n\ngamma\n")
w:close()
local n = 0
for line in io.lines(path) do
n = n + 1
print(n, "[" .. line .. "]")
end
print(pcall(io.lines, "/tmp/definitely_missing_file.txt"))
-- Output:
-- 1 [alpha]
-- 2 [beta]
-- 3 []
-- 4 [gamma]
-- false cannot open file '/tmp/definitely_missing_file.txt' (No such file or directory)Login to try C/C++/Java/PHP code in the editor
Common Mistakes
- Forgetting that lines strips the newline
- Using io.lines on a missing file without pcall
- Reading the whole file into memory when a line loop is enough
Chapter Summary
- io.lines(path) auto-closes the file
- file:lines iterates an open file
- Lines exclude the newline
- Good for big files
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