Basic usages: reading and writing data

JLD2 provides a few different options to save and load data:

• FileIO interface
• Single object storage
• File handles
• UnPack Extension

FileIO interface

The save and load functions, provided by FileIO, are one of the simplest ways to use JLD2. The save function accepts an AbstractDict yielding the key/value pairs, where the key is a string representing the name of the dataset and the value represents its contents:

julia> using JLD2
julia> save("example.jld2", Dict("hello" => "world", "foo" => :bar))

The save function can also accept the dataset names and contents as arguments:

julia> using JLD2
julia> save("example.jld2", "hello", "world", "foo", :bar)

For save and load to automatically detect that you want to save a JLD2 file use the file suffix ".jld2".

If called with a filename argument only, the load function loads all datasets from the given file into a Dict:

julia> using JLD2
julia> load("example.jld2")Dict{String, Any} with 2 entries:
  "hello" => "world"
  "foo"   => :bar

When called with a single dataset name, load returns the contents of that dataset from the file:

julia> using JLD2
julia> load("example.jld2", "hello")"world"

When called with multiple dataset names, load returns the contents of the given datasets as a tuple:

julia> using JLD2
julia> load("example.jld2", "foo", "hello")(:bar, "world")

jldsave

jldsave makes use of julia's keyword argument syntax to store files. This can be useful, when your data variables already have the correct name, e.g. use jldsave(file; variablename) instead of `save(file, "variablename", variablename)

jldsave(filename; kwargs...)
jldsave(filename, compress; kwargs...)
jldsave(filename, compress, iotype; kwargs...)

jldsave("example.jld2"; a=1, b=2, c)

jldopen("example.jld2", "w") do f
    f["a"] = 1
    f["b"] = 2
    f["c"] = c
end

Single object storage

If only a single object needs to stored and loaded from a file, one can use save_object and load_object functions.

save_object(filename, x)

hello = "world"
save_object("example.jld2", hello)

load_object(filename)

hello = "world"
save_object("example.jld2", hello)
hello_loaded = load_object("example.jld2")

File handles

It is also possible to interact with JLD2 files using a file-like interface. The jldopen function accepts a file name and an argument specifying how the file should be opened:

jldopen(file, mode::AbstractString; iotype=MmapIO, compress=false, typemap=JLD2.default_typemap)

Data can be written to the file using write(f, "name", data) or f["name"] = data, or read from the file using read(f, "name") or f["name"]. When you are done with the file, remember to call close(f).

Like open, jldopen also accepts a function as the first argument, permitting do-block syntax:

using JLD2
jldopen("example.jld2", "w") do f
    write(f, "variant1", 1.0)
    f["variant2"] = (rand(5), rand(Bool, 3))
end

f = jldopen("example.jld2")
v1 = read(f, "variant1")
v2 = f["variant2"]
close(f)
v1, v2

(1.0, ([0.1866124705718487, 0.48635242918709354, 0.06575952041788313, 0.47406168498982415, 0.2593027604508331], Bool[0, 1, 0]))

Groups

JLD2 files allow for nesting datasets into groups which may be useful for organizing your data. You may construct groups explicitly:

using JLD2
jldopen("example.jld2", "w") do file
    mygroup = JLD2.Group(file, "mygroup")
    mygroup["mystuff"] = 42
    display(file)
end

or implicitly, by saving a variable with a name containing slashes as path delimiters:

using JLD2
save("example.jld2", "mygroup/mystuff", 42)

Similarly, you can access groups directly:

using JLD2
jldopen("example.jld2") do file
    file["mygroup"]["mystuff"]
end

42

or using slashes as path delimiters:

using JLD2
load("example.jld2", "mygroup/mystuff")

42

When loading files with nested groups these will be unrolled into paths by default but yield nested dictionaries but with the nested keyword argument.

julia> load("example.jld2")ERROR: UndefVarError: `load` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: a global variable of this name may be made accessible by importing FileIO in the current active module Main
Hint: a global variable of this name also exists in JLD2.
julia> load("example.jld2"; nested=true)ERROR: UndefVarError: `load` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: a global variable of this name may be made accessible by importing FileIO in the current active module Main
Hint: a global variable of this name also exists in JLD2.

UnPack Extension

When additionally loading the UnPack.jl package, its @unpack and @pack! macros can be used to quickly save and load data from the file-like interface. Example:

using JLD2, UnPack
file = jldopen("example.jld2", "w")
x, y = rand(2)

@pack! file = x, y # equivalent to file["x"] = x; file["y"] = y
@unpack x, y = file # equivalent to x = file["x"]; y = file["y"]
close(file)