Number of head lines to read in (better to be large enough), the lines will be checked if they contain meta information, default is 200
extnum
The column number to be extracted from vcf, default is 10; 0 for not extracting any column; extnum should be between 10 and total column number
keepall
Keep unextracted column in output, default is TRUE
Value
A list containing (1) Content: a vector showing what is contained; (2) Meta: a data frame containing meta-information of the file;
(3) VCF: a data frame, the main part of VCF file; (4) file_sample_name: the file name and sample name,
in case when multiple samples exist in one file, file and sample names might be different