更改R中所有值中最小值的3行

Question

I have a data that looks like this

x	y
3	f
14	f
1	f
7	f

What I want is to change the y value of 3 individuals with the lowest x value from f to t so my data would look like this:

|  x  |  y  |
|:---:|:---:|
|  3  |  t  |
|  14 |  f  |
|  1  |  t  |
|  7  |  t  |

Kind regards

Answer 1

You could do this as a one-liner in base R. Use rank to get the order each entry takes, find which rank is below 4, then use ifelse to select 't' or 'f' based on this.

within(df, y <- ifelse(rank(x) < 4, "t", "f"))
#>    x y
#> 1  3 t
#> 2 14 f
#> 3  1 t
#> 4  7 t

---

Data taken from question

df <- data.frame(x = c(3, 14, 1, 7), y = "f")

Answer 2

library(dplyr)

df <- data.frame(
    x = c(3L, 14L, 1L, 7L),
    y = c("f", "f", "f", "f")
)


df %>% 
  mutate(
    y = case_when(
      x == sort(x)[1] ~ "t",
      x == sort(x)[2] ~ "t",
      x == sort(x)[3] ~ "t",
      TRUE ~ "f"
    )
  )
#>    x y
#> 1  3 t
#> 2 14 f
#> 3  1 t
#> 4  7 t

^{Created on 2022-07-12 by the reprex package (v2.0.1)}

Answer 3

Just arrange the dataframe in ascending order of x, then edit the y value for the first three rows.

data <- data[order(x),]

for (i in 1:3) {

data$x[i] <- 't'

}