h5_read: Read an HDF5 Object or Attribute

In h5lite: Simplified 'HDF5' Interface

Read an HDF5 Object or Attribute

Description

Reads a dataset, a group, or a specific attribute from an HDF5 file into an R object. Supports partial reading (hyperslabs) to load specific subsets of data without loading the entire object into memory.

Usage

h5_read(file, name = "/", attr = NULL, as = "auto", start = NULL, count = NULL)

Arguments

file	The path to the HDF5 file.
name	The full path of the dataset or group to read (e.g., "/data/matrix").
attr	The name of an attribute to read. If NULL (default), the function reads the object specified by name (and attaches its attributes to the result). If provided (string), the function reads only the specified attribute from name.
as	The target R data type. Global: "auto" (default), "integer", "double", "logical", "bit64", "null". Specific: A named vector mapping names or type classes to R types (see Section "Type Conversion").
start	A numeric vector specifying the 1-based coordinate(s) for a partial read. Most often, this is a single value targeting the most logical structural unit (e.g., the row of a matrix, or the 2D matrix of a 3D array). If NULL (default), the entire dataset is read.
count	A single numeric value specifying the number of elements or units to read. If NULL (default) and start is provided, h5lite reads exactly 1 unit and simplifies the resulting dimensions (see Section "Dimension Simplification").

Value

An R object corresponding to the HDF5 object or attribute. Returns NULL if the object is skipped via as = "null".

Partial Reading (Hyperslabs)

You can read specific subsets of an n-dimensional dataset by utilizing the start and count arguments.

The "Smart" start Parameter

start is designed to be intuitive. Most of the time, you only need to provide a single value. This single value automatically targets the most meaningful dimension of the dataset:

• 1D Vector: start specifies the element.

• 2D Matrix / Data Frame: start specifies the row.

• 3D Array: start specifies the 2D matrix.

The count parameter is a single value that determines how many of those units to read sequentially. For example, start = 5 and count = 3 on a matrix will read 3 complete rows starting at row 5 (automatically spanning all columns).

Multi-Value start and N-Dimensional Arrays

If you need to extract a specific block inside a structural unit, you can provide a vector of values to start. To make indexing intuitive across higher-order arrays, start maps its values to dimensions in the following priority order, targeting the outermost blocks first and specific rows/columns last:

• ⁠N, N-1, ..., 3, 1 (Rows), 2 (Cols)⁠

For example, on a 3D array, start = c(2, 5) targets the 2nd matrix, and the 5th row. The count argument always applies to the last dimension specified in start.

Dimension Simplification (Dropping)

h5lite mimics R's native subsetting behavior regarding dimension preservation:

• Exact Indexing (count = NULL): If you provide start but omit count, h5lite assumes you are targeting an exact point index. It will read 1 unit and drop the targeted dimension. (e.g., reading a specific row of a matrix will return a 1D vector).

• Range Indexing (count provided): If you explicitly provide count (even count = 1), h5lite assumes you are reading a range. The dataset's original structural geometry is preserved. (e.g., reading ⁠start = 5, count = 1⁠ on a matrix will return a 1xN matrix).

Type Conversion (as)

You can control how HDF5 data is converted to R types using the as argument.

1. Mapping by Name:

• as = c("data_col" = "integer"): Reads the dataset/column named "data_col" as an integer.

• as = c("@validated" = "logical"): When reading a dataset, this forces the attached attribute "validated" to be read as logical.

2. Mapping by HDF5 Type Class: You can target specific HDF5 data types using keys prefixed with a dot (.). Supported classes include:

• Integer: .int, .int8, .int16, .int32, .int64

• Unsigned: .uint, .uint8, .uint16, .uint32, .uint64

• Floating Point: .float, .float16, .float32, .float64

Example: as = c(.uint8 = "logical", .int = "bit64")

3. Precedence & Attribute Config:

• Attributes vs Datasets: Attribute type mappings take precedence over dataset mappings. If you specify as = c(.uint = "logical", "@.uint" = "integer"), unsigned integer datasets will be read as logical, but unsigned integer attributes will be read as integer.

• Specific vs Generic: Specific keys (e.g., .uint32) take precedence over generic keys (e.g., .uint), which take precedence over the global default (.).

Note

The @ prefix is only used to configure attached attributes when reading a dataset (attr = NULL). If you are reading a specific attribute directly (e.g., h5_read(..., attr = "id")), do not use the @ prefix in the as argument.

Partial reading (start/count) is currently only supported for datasets, not attributes.

See Also

h5_write()

Examples

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

# --- Setup: Write Test Data ---
h5_write(c(10L, 20L, 30L, 40L, 50L), file, "ints")

m <- matrix(1:50, nrow = 10, ncol = 5, dimnames = list(paste0("r", 1:10), paste0("c", 1:5)))
h5_write(m, file, "matrix_data")

arr <- array(1:24, dim = c(2, 3, 4))
h5_write(arr, file, "array_data")

# --- Standard Reading ---
# Read the entire dataset
x <- h5_read(file, "ints")

# --- Type Conversion ---
# Force integer dataset to be read as numeric (double)
x_dbl <- h5_read(file, "ints", as = "double")
class(x_dbl)

# --- Partial Reading: Single-Value 'start' ---
# Vector: Start at 2nd element, read 3 elements
h5_read(file, "ints", start = 2, count = 3)

# Matrix: Start at row 5, read 3 complete rows (returns 3x5 matrix)
h5_read(file, "matrix_data", start = 5, count = 3)

# 3D Array: Start at 2nd matrix, read 2 complete matrices (returns 2x3x2 array)
h5_read(file, "array_data", start = 2, count = 2)

# --- Partial Reading: Dimension Simplification ---
# Omit 'count' to extract an exact point index and drop the targeted dimension

# Matrix: Extract exactly row 5 (drops row dimension, returns a 1D vector)
h5_read(file, "matrix_data", start = 5)

# Matrix: Extract row 5, but preserve matrix structure (returns 1x5 matrix)
h5_read(file, "matrix_data", start = 5, count = 1)

# --- Partial Reading: Multi-Value 'start' ---
# Matrix: Extract exactly row 5, column 2 (drops both dims, returns a scalar)
h5_read(file, "matrix_data", start = c(5, 2))

# 3D Array: Target matrix 2, row 1. (drops matrix and row dims, returns 1D vector of cols)
h5_read(file, "array_data", start = c(2, 1))

unlink(file)

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