需要在字符串上矢量化功能

Question

我有一个300k行数据框，具有这样的列：

   db$performance[1:10]
   [1] "1C1CCCCCCCCCCCCCCCCCCCCCC" "CCCCCCCCCCCCC"             
    "4321CCCCCCCCCCCCCCCCCCCCC" "321CCCCCCCCCCCCCCCCCCCCCC"
    [5] "CCCCCCCCCCCCCC"            "4321CCCCCCCCCCCCC0"  "211CCCCCCCCCCCCCCCCCCCCCC" "BCCCCCCCCC"     [9] "BCCCCCCCCC"                "8"       

我想搜索该列的每一行，并计算在最后(右至左(18个字符元素中发生的" 4"的数量。我拥有的循环解决方案显然很糟糕，因为它非常慢(6分钟或更长时间(。见下文。如何对求解解决方案进行矢量化(使用和/或dplyr？(

谢谢！

substrRight <- function(x, n){
 substr(x, nchar(x)-n, nchar(x))
}
db$NewVar = NA
for (N in 1:nrow(db)){
db$NewVar[N] = str_count( substrRight(db$performance[N],18), "4")
}

Answer 1

str_count和 substr已被矢量化。因此，将功能直接应用于整个列

library(stringr)
str_count(substrRight(db$performance, 18), "4") 
#[1] 0 0 0 0 0 1 0 0 0 0

应该足够快。检查较大数据集上的时间

基准

db1 <- db[rep(seq_len(nrow(db)), 1e5),, drop = FALSE]
system.time({
out <- numeric(nrow(db1))
for (i in seq_len(nrow(db1))){
 out[i]= str_count( substrRight(db1$performance[i],18), "4")
}
})
# user  system elapsed 
# 14.699   0.104  14.755 
system.time({
sapply(db1$performance, function(x) str_count( substrRight(x,18), "4") )
})
# user  system elapsed 
# 14.267   0.075  14.299 
system.time({
str_count(substrRight(db1$performance, 18), "4") 
})
# user  system elapsed 
#  0.437   0.016   0.452 

数据

db <- structure(list(performance = c("1C1CCCCCCCCCCCCCCCCCCCCCC", "CCCCCCCCCCCCC", 
"4321CCCCCCCCCCCCCCCCCCCCC", "321CCCCCCCCCCCCCCCCCCCCCC", "CCCCCCCCCCCCCC", 
"4321CCCCCCCCCCCCC0", "211CCCCCCCCCCCCCCCCCCCCCC", "BCCCCCCCCC", 
"BCCCCCCCCC", "8")), class = "data.frame", row.names = c(NA, 
-10L))