該教材是如何制作固定每行70堿基的fasta文件，主要設計如何將特別長的堿基序列均勻切割成每行70個堿基的文件。代碼如下：
如何構建fasta文件及其index索引文件
fasta文件更改header

rm(list = ls())
suppressMessages(library(Biostrings))
suppressMessages(library(tidyverse))

trim_fa<-function(fa_dir,prefix,out_dir){
  trimSeq <- function(Seq, binwidth = 70){
    N_line = nchar(Seq) / binwidth 
    if(N_line > as.integer(N_line)){
      N =  as.integer(N_line) + 1
    } else{
      N =  as.integer(N_line)
    }
    
    out_seq<-vector()
    bases = strsplit(Seq, "")[[1]]
    for(i in 0:(N-1)){
      xseq = paste0(bases[(1 + binwidth*i):(binwidth + i*binwidth)], collapse = "") 
      if(i != (N-1)){
        xseq = paste0(bases[(1 + binwidth*i):(binwidth + i*binwidth)], collapse = "") 
      } else{
        xseq = paste0(bases[(1 + binwidth*i):length(bases)], collapse = "") 
      }
      out_seq<-paste0(out_seq,xseq, "\n")
    }
    return(out_seq)
  }
  fa<-readDNAStringSet(fa_dir)
  seq_name = names(fa)
  sequence = paste(fa)
  df <- data.frame(seq_name, sequence)
  df$sequence<-trimSeq(Seq = as.character(df$sequence))
  df$seq_name<-paste0(">",df$seq_name)
  D <- do.call(rbind, lapply(seq(nrow(df)), function(i) t(df[i, ])))
  write.table(D,paste0(out_dir,prefix,".fa"), row.names = FALSE, col.names = FALSE, quote = FALSE)
}

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