# rhdf5 - HDF5 interface for R In rhdf5: R Interface to HDF5

if(file.exists('myhdf5file.h5'))
file.remove('myhdf5file.h5')
if(file.exists('newfile.h5'))
file.remove('newfile.h5')
if(file.exists('newfile2.h5'))
file.remove('newfile2.h5')
if(file.exists('newfile3.h5'))
file.remove('newfile3.h5')


# Introduction

The package is an R interface for HDF5. On the one hand it implements R interfaces to many of the low level functions from the C interface. On the other hand it provides high level convenience functions on R level to make a usage of HDF5 files more easy.

# Installation of the HDF5 package

To install the r Biocpkg("rhdf5") package, you need a current version (>3.5.0) of R (www.r-project.org). After installing R you can run the following commands from the R command shell to install r Biocpkg("rhdf5").

install.packages("BiocManager")
BiocManager::install("rhdf5")


# High level R-HDF5 functions

## Creating an HDF5 file and group hierarchy

An empty HDF5 file is created by

library(rhdf5)
h5createFile("myhdf5file.h5")


The HDF5 file can contain a group hierarchy. We create a number of groups and list the file content afterwards.

h5createGroup("myhdf5file.h5","foo")
h5createGroup("myhdf5file.h5","baa")
h5createGroup("myhdf5file.h5","foo/foobaa")
h5ls("myhdf5file.h5")


Objects can be written to the HDF5 file. Attributes attached to an object are written as well, if write.attributes=TRUE is given as argument to h5write. Note that not all R-attributes can be written as HDF5 attributes.

A = matrix(1:10,nr=5,nc=2)
h5write(A, "myhdf5file.h5","foo/A")
B = array(seq(0.1,2.0,by=0.1),dim=c(5,2,2))
attr(B, "scale") <- "liter"
h5write(B, "myhdf5file.h5","foo/B")
C = matrix(paste(LETTERS[1:10],LETTERS[11:20], collapse=""),
nr=2,nc=5)
h5write(C, "myhdf5file.h5","foo/foobaa/C")
df = data.frame(1L:5L,seq(0,1,length.out=5),
c("ab","cde","fghi","a","s"), stringsAsFactors=FALSE)
h5write(df, "myhdf5file.h5","df")
h5ls("myhdf5file.h5")


If a dataset with the given name does not yet exist, a dataset is created in the HDF5 file and the object obj is written to the HDF5 file. If a dataset with the given name already exists and the datatype and the dimensions are the same as for the object obj, the data in the file is overwritten. If the dataset already exists and either the datatype or the dimensions are different, h5write() fails.

## Writing and reading objects with file, group and dataset handles

File, group and dataset handles are a simpler way to read (and partially to write) HDF5 files. A file is opened by H5Fopen.

h5f = H5Fopen("myhdf5file.h5")
h5f


The $ and & operators can be used to access the next group level. While the $ operator reads the object from disk, the & operator returns a group or dataset handle.

h5f$df h5f&'df'  Both of the following code lines return the matrix C. Note however, that the first version reads the whole tree /foo in memory and then subsets to /foobaa/C, and the second version only reads the matrix C. The first $ in h5f$foo$foobaa$C reads the dataset, the other $ are accessors of a list. Remind that this can have severe consequences for large datasets and datastructures.

h5f$foo$foobaa$C h5f$"/foo/foobaa/C"


One can as well return a dataset handle for a matrix and then read the matrix in chunks for out-of-memory computations. \warning{Please read the next section on subsetting, chunking and compression for more details}.

h5d = h5f&"/foo/B"
h5d[]
h5d[3,,]


The same works as well for writing to datasets. \warning{Remind that it is only guarenteed that the data is written on disk after a call to H5Fflush or after closing of the file.}

h5d[3,,] = 1:4
H5Fflush(h5f)


Remind again that in the following code the first version does not change the data on disk, but the second does.

h5f$foo$B = 101:120
h5f\$"/foo/B" = 101:120


It is important to close all dataset, group, and file handles when not used anymore

H5Dclose(h5d)
H5Fclose(h5f)


or close all open HDF5 handles in the environment by

h5closeAll()


\subsection{Writing and reading with subsetting, chunking and compression} The r Biocpkg("rhdf5") package provides two ways of subsetting. One can specify the submatrix with the R-style index lists or with the HDF5 style hyperslabs. Note, that the two next examples below show two alternative ways for reading and writing the exact same submatrices. Before writing subsetting or hyperslabbing, the dataset with full dimensions has to be created in the HDF5 file. This can be achieved by writing once an array with full dimensions as in Section \ref{sec_writeMatrix} or by creating a dataset. Afterwards the dataset can be written sequentially.

\paragraph{Influence of chunk size and compression level} The chosen chunk size and compression level have a strong impact on the reading and writing time as well as on the resulting file size. In an example an integer vector of size 10e7 is written to an HDF5 file. The file is written in subvectors of size 10'000. The definition of the chunk size influences the reading as well as the writing time. If the chunk size is much smaller or much larger than actually used, the runtime performance decreases dramatically. Furthermore the file size is larger for smaller chunk sizes, because of an overhead. The compression can be much more efficient when the chunk size is very large. The following figure illustrates the runtime and file size behaviour as a function of the chunk size for a small toy dataset.

After the creation of the dataset, the data can be written sequentially to the HDF5 file. Subsetting in R-style needs the specification of the argument index to h5read() and h5write().

h5createDataset("myhdf5file.h5", "foo/S", c(5,8),
storage.mode = "integer", chunk=c(5,1), level=7)
h5write(matrix(1:5,nr=5,nc=1), file="myhdf5file.h5",
name="foo/S", index=list(NULL,1))
h5write(6:10, file="myhdf5file.h5",
name="foo/S", index=list(1,2:6))
h5write(matrix(11:40,nr=5,nc=6), file="myhdf5file.h5",
name="foo/S", index=list(1:5,3:8))
h5write(matrix(141:144,nr=2,nc=2), file="myhdf5file.h5",
name="foo/S", index=list(3:4,1:2))
h5write(matrix(151:154,nr=2,nc=2), file="myhdf5file.h5",
name="foo/S", index=list(2:3,c(3,6)))


The HDF5 hyperslabs are defined by some of the arguments start, stride, count, and block. These arguments are not effective, if the argument index is specified.

h5createDataset("myhdf5file.h5", "foo/H", c(5,8), storage.mode = "integer",
chunk=c(5,1), level=7)
h5write(matrix(1:5,nr=5,nc=1), file="myhdf5file.h5", name="foo/H",
start=c(1,1))
h5write(6:10, file="myhdf5file.h5", name="foo/H",
start=c(1,2), count=c(1,5))
h5write(matrix(11:40,nr=5,nc=6), file="myhdf5file.h5", name="foo/H",
start=c(1,3))
h5write(matrix(141:144,nr=2,nc=2), file="myhdf5file.h5", name="foo/H",
start=c(3,1))
h5write(matrix(151:154,nr=2,nc=2), file="myhdf5file.h5", name="foo/H",
start=c(2,3), stride=c(1,3))
start=c(2,2), count=c(2,2))
start=c(2,2), stride=c(1,2),count=c(2,2))
start=c(2,1), stride=c(1,3),count=c(2,2), block=c(1,2))


## Saving multiple objects to an HDF5 file (h5save)

A number of objects can be written to the top level group of an HDF5 file with the function h5save() (as analogous to the base R function save()).

A = 1:7;  B = 1:18; D = seq(0,1,by=0.1)
h5save(A, B, D, file="newfile2.h5")
h5dump("newfile2.h5")


## List the content of an HDF5 file

The function h5ls() provides some ways of viewing the content of an HDF5 file.

h5ls("myhdf5file.h5")
h5ls("myhdf5file.h5", all=TRUE)
h5ls("myhdf5file.h5", recursive=2)


## Dump the content of an HDF5 file

The function h5dump() is similar to the function h5ls(). If used with the argument load=FALSE, it produces the same result as h5ls(), but with the group structure resolved as a hierarchy of lists. If the default argument load=TRUE is used all datasets from the HDF5 file are read.

h5dump("myhdf5file.h5",load=FALSE)
D <- h5dump("myhdf5file.h5")


## Reading HDF5 files with external software

The content of the HDF5 file can be checked with the command line tool h5dump (available on linux-like systems with the HDF5 tools package installed) or with the graphical user interface HDFView (http://www.hdfgroup.org/hdf-java-html/hdfview/) available for all major platforms.

system2("h5dump", "myhdf5file.h5")


Please note, that arrays appear as transposed matrices when opening it with a C-program (h5dump or HDFView). This is due to the fact the fastest changing dimension on C is the last one, but on R it is the first one (as in Fortran).

## Removing content from an HDF5 file

As well as adding content to an HDF5 file, it is possible to remove entries using the function h5delete(). To demonstrate it's use, we'll first list the contents of a file and examine the size of the file in bytes.

h5ls("myhdf5file.h5", recursive=2)
file.size("myhdf5file.h5")


We then use h5delete() to remove the df dataset by providing the file name and the name of the dataset, e.g.

h5delete(file = "myhdf5file.h5", name = "df")
h5ls("myhdf5file.h5", recursive=2)


We can see that the df entry has now disappeared from the listing. In most cases, if you have a heirachy within the file, h5delete() will remove children of the deleted entry too. In this example we remove foo and the datasets below it are deleted too. Notice too that the size of the file as decreased.

h5delete(file = "myhdf5file.h5", name = "foo")
h5ls("myhdf5file.h5", recursive=2)
file.size("myhdf5file.h5")


N.B. h5delete() does not explicitly traverse the tree to remove child nodes. It only removes the named entry, and HDF5 will then remove child nodes if they are now orphaned. Hence it won't delete child nodes if you have a more complex structure where a child node has multiple parents and only one of these is removed.

# 64-bit integers

R does not support a native datatype for 64-bit integers. All integers in R are 32-bit integers. When reading 64-bit integers from a HDF5-file, you may run into troubles. r Biocpkg("rhdf5") is able to deal with 64-bit integers, but you still should pay attention.

As an example, we create an HDF5 file that contains 64-bit integers.

x = h5createFile("newfile3.h5")

D = array(1L:30L,dim=c(3,5,2))
d = h5createDataset(file="newfile3.h5", dataset="D64", dims=c(3,5,2),H5type="H5T_NATIVE_INT64")
h5write(D,file="newfile3.h5",name="D64")


There are three different ways of reading 64-bit integers in R. H5Dread() and h5read() have the argument bit64conversion the specify the conversion method.

By setting bit64conversion='int', a coercing to 32-bit integers is enforced, with the risk of data loss, but with the insurance that numbers are represented as native integers.

D64a = h5read(file="newfile3.h5",name="D64",bit64conversion="int")
D64a
storage.mode(D64a)


bit64conversion='double' coerces the 64-bit integers to floating point numbers. doubles can represent integers with up to 54-bits, but they are not represented as integer values anymore. For larger numbers there is still a data loss.

D64b = h5read(file="newfile3.h5",name="D64",bit64conversion="double")
D64b
storage.mode(D64b)


bit64conversion='bit64' is the recommended way of coercing. It represents the 64-bit integers as objects of class integer64 as defined in the package r CRANpkg("bit64"). Make sure that you have installed r CRANpkg("bit64"). The datatype integer64 is not part of base R, but defined in an external package. This can produce unexpected behaviour when working with the data. When choosing this option the package r CRANpkg("bit64") will be loaded.

D64c = h5read(file="newfile3.h5",name="D64",bit64conversion="bit64")
D64c
class(D64c)


# Low level HDF5 functions

## Creating an HDF5 file and a group hierarchy

Create a file.

library(rhdf5)
h5file = H5Fcreate("newfile.h5")
h5file


and a group hierarchy

h5group1 <- H5Gcreate(h5file, "foo")
h5group2 <- H5Gcreate(h5file, "baa")
h5group3 <- H5Gcreate(h5group1, "foobaa")
h5group3


## Writing data to an HDF5 file

Create 4 different simple and scalar data spaces. The data space sets the dimensions for the datasets.

d = c(5,7)
h5space1 = H5Screate_simple(d,d)
h5space2 = H5Screate_simple(d,NULL)
h5space3 = H5Scopy(h5space1)
h5space4 = H5Screate("H5S_SCALAR")
h5space1
H5Sis_simple(h5space1)


Create two datasets, one with integer and one with floating point numbers.

h5dataset1 = H5Dcreate( h5file, "dataset1", "H5T_IEEE_F32LE", h5space1 )
h5dataset2 = H5Dcreate( h5group2, "dataset2", "H5T_STD_I32LE", h5space1 )
h5dataset1


Now lets write data to the datasets.

A = seq(0.1,3.5,length.out=5*7)
H5Dwrite(h5dataset1, A)
B = 1:35
H5Dwrite(h5dataset2, B)


To release resources and to ensure that the data is written on disk, we have to close datasets, dataspaces, and the file. There are different functions to close datasets, dataspaces, groups, and files.

H5Dclose(h5dataset1)
H5Dclose(h5dataset2)

H5Sclose(h5space1)
H5Sclose(h5space2)
H5Sclose(h5space3)
H5Sclose(h5space4)

H5Gclose(h5group1)
H5Gclose(h5group2)
H5Gclose(h5group3)

H5Fclose(h5file)


# Session Info

sessionInfo()

for(file in c('myhdf5file.h5', 'newfile.h5', 'newfile2.h5', 'newfile3.h5')) {
if(file.exists(file)) {
file.remove( file )
}
}


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rhdf5 documentation built on Nov. 8, 2020, 6:56 p.m.