基于不同数据框中的另一列，使用 for 循环填充空列

Question

I have this data frame df and I would like to use this df to loop through my df2;

Station_ID    Station_Name
1             New York
2             London
3             Madrid
4             Rome
....

I have another data frame df2;

Station        x1          x2   
1              10           5
1               8           6
2              21           9
4              12           7

I would like to achieve;

Station       Station_Name     x1          x2   
    1         New York         10           5
    1         New York          8           6
    2         London           21           9
    4         Rome             12           7

What I have done so far;

df2 <- df2 %>%
  add_column(Station_Name = NA)

for (i in 1:nrow(df2$Station_Name)) {
  if (df$Station_ID == df2$Station) {
    df2$Sitation_Name <- df$Station_Name
    
  }
  
}

Error in 1:nrow(df2$Station_Name): argument of length 0

also just being curious, how would you I suggest I do if I had 5 different data frames instead and I had to write a loop which would go through all those different data frames to add their corresponding name?

Answer 1

Instead of a loop the natural way would be to use a left_join :

library(dplyr)

df2 <- left_join(df2, df, by = c("Station" = "Station_ID"))

df2
#>   Station x1 x2 Station_Name
#> 1       1 10  5     New York
#> 2       1  8  6     New York
#> 3       2 21  9       London
#> 4       4 12  7         Rome

Or using base R:

df2 <- merge(df2, df, by.x = "Station", by.y = "Station_ID", all.x)

df2
#>   Station x1 x2 Station_Name
#> 1       1 10  5     New York
#> 2       1  8  6     New York
#> 3       2 21  9       London
#> 4       4 12  7         Rome

DATA

df <- structure(list(Station_ID = 1:4, Station_Name = c(
  "New York",
  "London", "Madrid", "Rome"
)), class = "data.frame", row.names = c(
  NA,
  -4L
))

df2 <- structure(list(Station = c(1L, 1L, 2L, 4L), x1 = c(
  10L, 8L, 21L,
  12L
), x2 = c(5L, 6L, 9L, 7L)), class = "data.frame", row.names = c(
  NA,
  -4L
))

Answer 2

As Stefan, I would also advise to use dplyr here. If you still wanted to do the loop, this would be my solution:

#Loop
df <- data.frame(Station_ID= c(1:4),
                 Station_Name= c("NY", "Lon", "Mad", "Rome"))

df2 <- data.frame(Station= c(1:4),
                  X1= c(10,8,21,12),
                  X2= c(5,6,9,7))



for (i in 1:nrow(df2)) {
 
  df2$Station_Name[i] <- df$Station_Name[i]
  
}

df2
#>   Station X1 X2 Station_Name
#> 1       1 10  5           NY
#> 2       2  8  6          Lon
#> 3       3 21  9          Mad
#> 4       4 12  7         Rome

^{Created on 2022-12-25 with reprex v2.0.2}

The issue you had, was that df$Station_Name is actually just a vector and thus nrow() cannot be applied.

Answer 3

Using data.table

library(data.table)
setDT(df2)[df, on = .(Station = Station_ID), nomatch = FALSE]
   Station x1 x2 Station_Name
1:       1 10  5     New York
2:       1  8  6     New York
3:       2 21  9       London
4:       4 12  7         Rome