Julia File I/O Guide — Reading, Writing, and File Operations

In this tutorial, you will learn about Julia File I/O Guide. We cover key concepts, practical examples, and best practices to help you master this topic.

Julia file I/O uses open() with do-block for automatic resource management, read() for entire content, eachline() for line iteration, and CSV.jl/DataFrames.jl for tabular data -- with write(), append(), and FileIO.jl for various formats.

Basic File Reading

# Read entire file as string
content = read("file.txt", String)

# Read lines into array
lines = readlines("file.txt")

# Iterate line by line (lazy)
for line in eachline("file.txt")
    println(line)
end

File Writing

# Write string
write("output.txt", "Hello, Julia!\n")

# Write array
lines = ["line 1", "line 2", "line 3"]
write("output.txt", join(lines, "\n"))

# Append
open("log.txt", "a") do io
    write(io, "New log entry\n")
end

# Write with formatting
using Printf
open("data.txt", "w") do io
    @printf(io, "Value: %.2f\n", 3.14159)
end

File Handle Operations

# Open with do-block (auto-close)
open("file.txt", "r") do io
    for line in eachline(io)
        println(line)
    end
end

# Manual open/close
io = open("file.txt", "r")
try
    data = read(io, String)
finally
    close(io)
end

# File position
io = open("data.bin", "r")
seek(io, 0)     # beginning
seekend(io)     # end
position(io)    # current position
skip(io, 100)   # skip bytes

CSV Files

using CSV
using DataFrames

# Read CSV
df = CSV.File("data.csv") |> DataFrame

# Read with options
df = CSV.read("data.csv", DataFrame;
    delim = ',',
    header = 1,
    types = Dict(:age => Int, :name => String)
)

# Write CSV
CSV.write("output.csv", df)

# Write with options
CSV.write("output.csv", df; delim = '|')

Common Mistakes

1. Forgetting to close files

Use do-blocks (open(f -> ..., "r")) for automatic closing. Manual open requires explicit close in a try/finally block.

2. Path issues

Use joinpath("dir", "file.txt") for platform-independent paths. Use @__FILE__ and dirname for relative paths.

3. Encoding errors

read("file.txt", String) assumes UTF-8. For other encodings, use read("file.txt", String, enc"ISO-8859-1").

Practice Questions

1. How do you read a file line by line? for line in eachline("file.txt") iterates without loading the entire file into memory.

2. How do you write to a file? write("output.txt", content) or use open("f", "w") do io; write(io, data); end.

3. How do you read a CSV file? using CSV; df = CSV.read("file.csv", DataFrame) loads into a DataFrame.

FAQ

{{< faq question="What file modes does open support?" >}} "r" (read), "w" (write/truncate), "a" (append), "r+" (read/write), "w+" (read/write/truncate). {{< /faq >}}

{{< faq question="How do I handle binary files?" >}} Use read("file.bin") which returns Vector{UInt8}. Write with write("file.bin", data::Vector{UInt8}). {{< /faq >}}

{{< faq question="What is the difference between read and readlines?" >}} read reads the entire file as a single string. readlines reads into an array of strings (one per line). {{< /faq >}}

What's Next

Now learn about plotting in Julia.