Julia File Handling

Julia provides some basic functions to handle files:

• open() - Open a file
• read() - Read file content
• close() - Close a file

Reading from or writing to a file requires the use of a file handle.

A file handle is essentially a pointer, which points to a specific location in the file.

To read data from a file, the application first calls an operating system function, passing the file name and selecting a path to the file to open it. The open function returns a sequence number, known as the file handle, which is a unique identifier for the opened file.

open() Function

Julia can use the open() function to open a file, which returns a file handle:

Syntax:

open(filename, mode)

filename is the file name, and mode is the read/write mode, which can be one of the following values:

Example:

# Windows open file
foo = open("D://tutorialpro-test//julia//tutorialpro-file.txt")

# Linux or Mac open file
foo = open("./tutorialpro-test/julia/tutorialpro-file.txt")

close() Function

After completing file operations, we need to close the file to avoid resource leaks. Close the file using the close() function:

# Close the file handle foo returned from the previous code
close(foo)

In Julia, it is recommended to wrap any file processing functions in a do block. The advantage of wrapping file processing functions in a do block is that the opened file will be automatically closed when the block completes, invoking the close() function automatically, as shown below:

Example

open("./tutorialpro-test/julia/tutorialpro-file.txt") do file
   # Perform file operations
end

The above code does not require a call to the close() function as it will automatically close the file.

Next, we create a test.jl file with the following code:

test.jl File Code:

# Open file, create tutorialpro-file.txt if it does not exist
io = open("tutorialpro-file.txt", "w");

# Write to file
write(io, "Hello world!\ntutorialpro Julia Test");

# Close file
close(io);

Execute the above code:

$ julia test.jl

Upon successful execution, a tutorialpro-file.txt file is created in the directory of the current code file with the following content:

Hello world!
tutorialpro Julia Test

We can use the do statement, which eliminates the need for the close() function:

test.jl File Code:

open("tutorialpro-file.txt", "w") do io
    write(io, "Hello world!\ntutorialpro Julia Test\nUsing do statement")
end;

Execute the above code:

$ julia test.jl

Upon successful execution, a tutorialpro-file.txt file is created in the directory of the current code file with the following content:

Hello world!
tutorialpro Julia Test
Using do statement

Example

# Read file content
txt = open("tutorialpro-file.txt") do file
    read(file, String)
end;
# Output
println(txt)

read() Function

Using the read() function, we can read the entire content of a file, for example:

txt = read(foo, String)

Example

# Open file
file = open("tutorialpro-file.txt")

# Read file content
txt = read(file, String)

# Close file
close(file)

# Output
println(txt)

Executing the above code, the output is as follows:

$ Julia test.jl
Hello world!
tutorialpro Julia Test

Using the read() function within a do statement:

open(filename) do file
    read(file, String)
end

Example

# Read file content
txt = open("tutorialpro-file.txt") do file
    read(file, String)
end;
# Output
println(txt)

Executing the above code, the output is as follows:

$ julia test.jl
Hello world!
tutorialpro Julia Test

We can use the readlines() function to place file content into an array line by line, with the syntax as follows:

readlines(io::IO=stdin; keep::Bool=false)
readlines(filename::AbstractString; keep::Bool=false)

Example

# Open file
file = open("tutorialpro-file.txt")

# Read file content
txt = readlines(file, keep=true)

# Close file
close(file)

# Output
println(txt)

Executing the above code, the output is as follows:

$ julia test.jl
["Hello world!\n", "tutorialpro Julia Test\n", "Using do statement"]

We can also process files line by line using the eachline() function:

Example

# Open file
open("tutorialpro-file.txt") do file
# Read file content line by line
    for ln in eachline(file)
        # Output string length and string
        println("$(length(ln)), $(ln)")
    end
end

Executing the above code, the output is as follows:

$ julia test.jl
12, Hello world!
17, tutorialpro Julia Test
8, Using do statement

We can also get the current line number:

Example

open("tutorialpro-file.txt") do f
    # Set line number
    line = 1
    while !eof(f)
       x = readline(f)
       println("$line $x")
       # Increment line number
       line += 1
    end
end

Executing the above code, the output is as follows:

$ julia test.jl
1 Hello world!
2 tutorialpro Julia Test
3 Using do statement

stat() Function

The stat() function is used to get file information, with the format as follows:

stat(pathname)

Example

for n in fieldnames(typeof(stat("tutorialpro-file.txt")))
    println(n, ": ", getfield(stat("tutorialpro-file.txt"),n))
end

Executing the above code, the output is as follows:

$ julia test.jl
desc: tutorialpro-file.txt
device: 16777222
inode: 35345094
mode: 33188
nlink: 1
uid: 501
gid: 20
rdev: 0
size: 47
blksize: 4096
blocks: 8
mtime: 1.6524042534674733e9
ctime: 1.6524042534674733e9

abspath() Function

The abspath() function is used to get the absolute path of a file, which can be mapped in a list:

stat(pathname)

Example

println(map(abspath, readdir()))

Executing the above code, the output is as follows:

["/Users/tutorialpro/tutorialpro-test/.Rhistory", "/Users/tutorialpro/tutorialpro-test/.vscode", 
...

More Functions

The following table lists other relevant functions for file processing: