R將轉換應用於data.frame的多列

Question

我想將data.frame中的幾列從chr轉換為數值，我想在一行中完成。 這是我正在嘗試做的事情：

items[,2:4] <- as.numeric(sub("\\$","",items[,2:4]))

但是我收到一個錯誤消息：

Warning message:
NAs introduced by coercion

如果我按列做它，盡管它起作用：

items[,2:2] <- as.numeric(sub("\\$","",items[,2:2]))
items[,3:3] <- as.numeric(sub("\\$","",items[,3:3]))
items[,4:4] <- as.numeric(sub("\\$","",items[,4:4]))

我在這里想念什么？ 為什么要為多列指定此命令？ 這是我不知道的奇怪的R特質嗎？

示例數據：

Name, Cost1,  Cost2,  Cost3,  Cost4
A,    $10.00, $15.50, $13.20, $45.45
B,    $45.23, $34.23, $34.24, $23.34
C,    $23.43, $45.23, $65.23, $34.23
D,    $76.34, $98.34, $90.34, $45.09

Answer 1

您的問題是， gsub將其x參數轉換為character 。 如果一個list （ data.frame實際上是一個list ）被轉換為character則會發生有線連接：

as.character(list(a=c("1", "1"), b="1"))
# "c(\"1\", \"1\")" "1"

# and "c(\"1\", \"1\")" can not convert into a numeric
as.numeric("c(\"1\", \"1\")")
# NA

一種解決方案是unlist x參數：

items[, 2:5] <- as.numeric(gsub("\\$", "", unlist(items[, 2:5])))

Answer 2

是的，這里有： apply是您要查找的命令：

items<-read.table(text="Name, Cost1,  Cost2,  Cost3,  Cost4
A,    $10.00, $15.50, $13.20, $45.45
B,    $45.23, $34.23, $34.24, $23.34
C,    $23.43, $45.23, $65.23, $34.23
D,    $76.34, $98.34, $90.34, $45.09", header=TRUE,sep=",")

items[,2:4]<-apply(items[,2:4],2,function(x){as.numeric(gsub("\\$","",x))})
items
  Name Cost1 Cost2 Cost3   Cost4
1    A 10.00 15.50 13.20  $45.45
2    B 45.23 34.23 34.24  $23.34
3    C 23.43 45.23 65.23  $34.23
4    D 76.34 98.34 90.34  $45.09

Answer 3

一種更有效的方法是：

items[-1] <- lapply(items[-1], function(x) as.numeric(gsub("$", "", x, fixed = TRUE)))
items
#   Name Cost1 Cost2 Cost3 Cost4
# 1    A 10.00 15.50 13.20 45.45
# 2    B 45.23 34.23 34.24 23.34
# 3    C 23.43 45.23 65.23 34.23
# 4    D 76.34 98.34 90.34 45.09

---

到目前為止答案的一些基准

fun1 <- function() {
  A[-1] <- lapply(A[-1], function(x) as.numeric(gsub("$", "", x, fixed=TRUE)))
  A
}
fun2 <- function() {
  A[, 2:ncol(A)] <- as.numeric(gsub("\\$", "", unlist(A[, 2:ncol(A)])))
  A
}
fun3 <- function() {
  A[, 2:ncol(A)] <- apply(A[,2:ncol(A)], 2, function(x) { as.numeric(gsub("\\$","",x)) })
  A
}

這是一些示例數據和處理時間

set.seed(1)
A <- data.frame(Name = sample(LETTERS, 10000, TRUE),
                matrix(paste0("$", sample(99, 10000*100, TRUE)), 
                       ncol = 100))
system.time(fun1())
#    user  system elapsed 
#    0.72    0.00    0.72 
system.time(fun2())
#    user  system elapsed 
#    5.84    0.00    5.85 
system.time(fun3())
#    user  system elapsed 
#    4.14    0.00    4.14