h5_write: Write an R Object to HDF5

In h5lite: Simplified 'HDF5' Interface

Write an R Object to HDF5

Description

Writes an R object to an HDF5 file, creating the file if it does not exist. This function acts as a unified writer for datasets, groups (lists), and attributes.

Usage

h5_write(data, file, name, attr = NULL, as = "auto", compress = "gzip")

Arguments

data	The R object to write. Supported: numeric, complex, logical, character, factor, raw, matrix, data.frame, integer64, POSIXt, NULL, and nested lists.
file	The path to the HDF5 file.
name	The name of the dataset or group to write (e.g., "/data/matrix").
attr	The name of an attribute to write. If NULL (default), data is written as a dataset or group at the path name. If provided (string), data is written as an attribute named attr attached to the object name.
as	The target HDF5 data type. Defaults to "auto". See the Data Type Selection section for a full list of valid options (including "int64", "bfloat16", "utf8[n]", etc.) and how to map sub-components of data.
compress	Compression configuration. Default is "gzip". Pass a basic string to specify the algorithm and level (e.g., "none", "gzip", "zstd-7", "lz4", "blosc1-lz4-9", "blosc2-gzip-3", "blosc2-zstd"), or pass a compress object created by h5_compression() for advanced pipeline control (including scale-offset algorithms, Fletcher32 checksums, or Blosc2 pre-filters). See h5_compression() for the complete list of available codecs and options.

Value

Invisibly returns file. This function is called for its side effects.

Writing Scalars

By default, h5_write saves single-element vectors as 1-dimensional arrays. To write a true HDF5 scalar, wrap the value in I() to treat it "as-is."

Examples

h5_write(I(5), file, "x") # Creates a scalar dataset
h5_write(5, file, "x")    # Creates a 1D array of length 1

Data Type Selection (as Argument)

By default, as = "auto" will automatically select the most appropriate data type for the given object. For numeric types, this will be the smallest type that can represent all values in the vector. For character types, h5lite will use a ragged vs rectangular heuristic, favoring small file size over fast I/O. For R data types not mentioned below, see vignette("data-types") for information on their fixed mappings to HDF5 data types.

Numeric and Logical Vectors

When writing a numeric or logical vector, you can specify one of the following storage types for it:

• Floating Point: "float16", "float32", "float64", "bfloat16"

• Signed Integer: "int8", "int16", "int32", "int64"

• Unsigned Integer: "uint8", "uint16", "uint32", "uint64"

NOTE: NA values must be stored as float64. NaN, Inf, and -Inf must be stored as a floating point type.

Examples

h5_write(1:100, file, "big_ints", as = "int64")
h5_write(TRUE,  file, "my_bool",  as = "float32")

Character Vectors

You can control whether character vectors are stored as variable or fixed length strings, and whether to use UTF-8 or ASCII encoding.

• Variable Length Strings: "utf8", "ascii"

• Fixed Length Strings:

  • "utf8[]" or "ascii[]" (length is set to the longest string)

  • "utf8[n]" or "ascii[n]" (where n is the length in bytes)

NOTE: Variable-length strings allow for NA values but cannot be compressed on disk. Fixed-length strings allow for compression but do not support NA.

Examples

h5_write(letters[1:5],    file, "len10_strs", as = "utf8[10]")
h5_write(c('X', 'Y', NA), file, "var_chars",  as = "ascii")

Lists, Data Frames, and Attributes

Provide a named vector to apply type mappings to sub-components of data. Set "skip" as the type to skip a specific component.

• Specific Name: "col_name" = "type" (e.g., c(score = "float32"))

• Specific Attribute: "@attr_name" = "type"

• Class-based: ".integer" = "type", ".numeric" = "type"

• Class-based Attribute: "@.character" = "type", "@.logical" = "type"

• Global Fallback: "." = "type"

• Global Attribute Fallback: "@." = "type"

Examples

# To strip attributes when writing:
h5_write(data, file, 'no_attrs_obj', as = c('@.' = "skip"))

# To only save the `hp` and `wt` columns:
h5_write(mtcars, file, 'my_df', as = c('hp' = "auto", 'wt' = "float32", '.' = "skip"))

Dimension Scales

h5lite automatically writes names, row.names, and dimnames as HDF5 dimension scales. Named vectors will generate an ⁠<name>_names⁠ dataset. A data.frame with row names will generate an ⁠<name>_rownames⁠ dataset (column names are saved internally in the original dataset). Matrices will generate ⁠<name>_rownames⁠ and ⁠<name>_colnames⁠ datasets. Arrays will generate ⁠<name>_dimscale_1⁠, ⁠<name>_dimscale_2⁠, etc. Special HDF5 metadata attributes link the dimension scales to the dataset. The dimension scales can be relocated with h5_move() without breaking the link.

See Also

h5_read(), h5_compression(), vignette('compression')

Examples

file <- tempfile(fileext = ".h5")

# 1. Writing Basic Datasets
h5_write(1:10, file, "data/integers")
h5_write(rnorm(10), file, "data/floats")
h5_write(letters[1:5], file, "data/chars")

# 2. Writing Attributes
# Write an object first
h5_write(1:10, file, "data/vector")
# Attach an attribute to it using the 'attr' parameter
h5_write(I("My Description"), file, "data/vector", attr = "description")
h5_write(I(100), file, "data/vector", attr = "scale_factor")

# 3. Controlling Data Types
# Store values as 32-bit signed integers
h5_write(1:5, file, "small_ints", as = "int32")

# 4. Writing Complex Structures (Lists/Groups)
my_list <- list(
  meta    = list(id = 1, name = "Experiment A"),
  results = matrix(runif(9), 3, 3),
  valid   = I(TRUE)
)
h5_write(my_list, file, "experiment_1", as = c(id = "uint16"))

# 5. Writing Data Frames (Compound Datasets)
df <- data.frame(
  id    = 1:5,
  score = c(10.5, 9.2, 8.4, 7.1, 6.0),
  grade = factor(c("A", "A", "B", "C", "D"))
)
h5_write(df, file, "records/scores", as = c(grade = "ascii[1]"))

# 6. Fixed-Length Strings
h5_write(c("A", "B"), file, "fixed_str", as = "ascii[10]")

# 7. Review the file structure
h5_str(file)

# 8. Clean up
unlink(file)

h5lite documentation built on May 19, 2026, 1:07 a.m.