如何更改data.frame中的单个值（值为新级别）？

我想更改data.frame中的单个值，即NA，使用@akrun在注释中指出的代码

df[307，1]：

x <- factor(c("a", "b"))
x[3] <- "c"
Warning message:
In `[<-.factor`(`*tmp*`, 3, value = "c") :
  invalid factor level, NA generated
# one solution:
x <- factor(c("a", "b"))
x <- factor(c(as.character(x), "c"))

# a second solution:
x <- factor(c("a", "b"))
levels(x) <- c("a", "b", "c")
x[3] <- "c"

x该列为
因子
类。将其更改为
字符
或在现有系数列中添加级别，然后替换
x <- factor(c("a", "b"))
x[3] <- "c"
Warning message:
In `[<-.factor`(`*tmp*`, 3, value = "c") :
  invalid factor level, NA generated
# one solution:
x <- factor(c("a", "b"))
x <- factor(c(as.character(x), "c"))

# a second solution:
x <- factor(c("a", "b"))
levels(x) <- c("a", "b", "c")
x[3] <- "c"