shift rows

library(data.table)
library(plyr)
library(dplyr)
library(magrittr)

(dt <- data.table(x=1:6,b=c(2,'',4,5,6,7),c(1,'','',4,5,6),c(1,'','',4,5,''),c('','','',4,5,'')))
old_names = names(dt)
dt %<>% setnames(paste0("V", 1:ncol(dt)))
txt = dt %>% mutate_(Vcobime = paste0("paste(", paste0("V", 1:ncol(dt),
  collapse = ","), ", sep = \",\")")) %>% .$Vcobime
loc = which(substr(txt, sapply(txt, nchar), sapply(txt, nchar)) == ",")
txt[loc] = gsub("(([^,]+,)*)(,*)", "\\3\\1", txt[loc]) %>%
  substr(1, sapply(.,nchar)-1) %>% paste0(",", .)
dt = fread(paste(txt, collapse = "\n")) %>% setnames(old_names)

## test
set.seed(100)
n_cols = 100
n_rows = 2e5
rightShift = sample(0:39, n_rows, TRUE)
m = matrix(rbinom(n_cols*n_rows, 10, 0.5), n_rows)
dt = t(sapply(1:n_rows, function(i){
  c(as.character(m[i,1:(n_cols-rightShift[i])]), rep('', rightShift[i]))
})) %>% data.table

st = proc.time()
txt = dt %>% mutate_(Vcobime = paste0("paste(", paste0("V", 1:ncol(dt),
  collapse = ","), ", sep = \",\")")) %>% .$Vcobime
loc = which(substr(txt, sapply(txt, nchar), sapply(txt, nchar)) == ",")
txt[loc] = gsub("(([^,]+,)*)(,*)", "\\3\\1", txt[loc]) %>%
  substr(1, sapply(.,nchar)-1) %>% paste0(",", .)
res = fread(paste(txt, collapse = "\n"))
proc.time() - st
#   user  system elapsed
#   7.97    0.16    8.14

st = proc.time()
dtT = data.table(t(dt))
len = nrow(dtT)
fun <-function(x){
  idx <- len-length(which(x!=''))
  if (idx !=0) {
    res <- rbind(array(rep('',idx),c(idx,1)), as.matrix(x[which(x!='')],ncol=1))
  } else {
    res <- as.matrix(x[which(x!='')],ncol=1)
  }
  res
}
res2 = dtT[,lapply(.SD,fun)]
res2 = data.table(t(res2))
proc.time() - st
#   user  system elapsed
# 218.23    0.64  220.40

all.equal(res, colwise(as.integer)(res2) %>% data.table) # TRUE