snp_readBGEN: Read BGEN files into a "bigSNP"
In bigsnpr: Analysis of Massive SNP Arrays

snp_readBGEN

R Documentation

Read BGEN files into a "bigSNP"

Description

Function to read the UK Biobank BGEN files into a bigSNP.

Usage

snp_readBGEN(
  bgenfiles,
  backingfile,
  list_snp_id,
  ind_row = NULL,
  bgi_dir = dirname(bgenfiles),
  read_as = c("dosage", "random"),
  ncores = 1
)

Arguments

`bgenfiles`	Character vector of paths to files with extension ".bgen". The corresponding ".bgen.bgi" index files must exist.
`backingfile`	The path (without extension) for the backing files (".bk" and ".rds") that are created by this function for storing the bigSNP object.
`list_snp_id`	List of character vectors of SNP IDs to read, with one vector per BGEN file. Each SNP ID should be in the form `"<chr>_<pos>_<a1>_<a2>"` (e.g. `"1_88169_C_T"` or `"01_88169_C_T"`). If you have one BGEN file only, just wrap your vector of IDs with `list()`. This function assumes that these IDs are uniquely identifying variants.
`ind_row`	An optional vector of the row indices (individuals) that are used. If not specified, all rows are used. Don't use negative indices. You can access the sample IDs corresponding to the genotypes from the .sample file, and use e.g. `match()` to get indices corresponding to the ones you want.
`bgi_dir`	Directory of index files. Default is the same as `bgenfiles`.
`read_as`	How to read BGEN probabilities? Currently implemented: as dosages (rounded to two decimal places), the default, as hard calls, randomly sampled based on those probabilities (similar to PLINK option '`⁠--hard-call-threshold random⁠`').
`ncores`	Number of cores used. Default doesn't use parallelism. You may use `bigstatsr::nb_cores()`.

Details

For more information on this format, please visit BGEN webpage.

This function is designed to read UK Biobank imputation files. This assumes that variants have been compressed with zlib, that there are only 2 possible alleles, and that each probability is stored on 8 bits. For example, if you use qctool to generate your own BGEN files, please make sure you are using options '⁠-ofiletype bgen_v1.2 -bgen-bits 8 -assume-chromosome⁠'.

If the format is not the expected one, this will result in an error or even a crash of your R session. Another common source of error is due to corrupted files; e.g. if using UK Biobank files, compare the result of tools::md5sum() with the ones at https://biobank.ndph.ox.ac.uk/ukb/refer.cgi?id=998.

You can look at some example code from my papers on how to use this function:

Value

The path to the RDS file ⁠<backingfile>.rds⁠ that stores the bigSNP object created by this function.
Note that this function creates another file (.bk) which stores the values of the FBM (⁠$genotypes⁠). The rows corresponds to the order of ind_row; the columns to the order of list_snp_id. The ⁠$map⁠ component of the bigSNP object stores some information on the variants (including allele frequencies and INFO scores computed from the imputation probabilities). However, it does not have a ⁠$fam⁠ component; you should use the individual IDs in the .sample file (filtered with ind_row) to add external information on the individuals.
You shouldn't read from BGEN files more than once. Instead, use snp_attach to load the "bigSNP" object in any R session from backing files.