找位置元件到另一個

Question

比方說，我們有兩個數據幀：

df1 <- data.frame(A = letters[1:3], B = letters[4:6], C = letters[7:9], stringsAsFactors = FALSE) 
    A B C 
1 a d g 
2 b e h 
3 c f i 

df2 <- data.frame(V1 = 1:3, V2 = 4:6, V3 = 7:9) 
    V1 V2 V3 
1 1 4 7 
2 2 5 8 
3 3 6 9 

我需要建立一個函數，它作爲輸入的單個值或從所述一個含元件的載體數據幀並根據其位置索引從另一個數據幀返回元素。

函數應該像這樣工作：

> matchdf(values = c("a", "e", "i"), dfin = df1, dfout = df2) 
[1] 1 5 9 

> matchdf(values = c(1, 5, 9), dfin = df2, dfout = df1) 
[1] "a" "e" "i" 

> matchdf(values = c(1, 1, 1), dfin = df2, dfout = df1) 
[1] "a" "a" "a" 

這是我到目前爲止已經試過：

requiere(dplyr) 
toVec <- function(df) df %>% as.matrix %>% as.vector 
matchdf <- function(values, dfin, dfout) toVec(dfout)[toVec(dfin) %in% values] 

# But sometimes the output values aren't in correct order: 
> matchdf(c("c", "i", "h"), dt1, dt2) 
[1] 3 8 9 
# should output 3 9 8 

> matchdf(values = c("a", "a", "a"), dfin = dt1, dfout = dt2) 
[1] 1 
# Should output 1 1 1 

隨意使用data.table或/和dplyr如果它簡化了任務。我更喜歡沒有for循環的解決方案。

假設：從DF1

• 元素是從不同DF2
• 暗淡（DF1）=暗淡（DF2）

Answer 1

matchdf <- function(values, dfin, dfout){ 
     unlist(sapply(values, 
         function(val) dfout[dfin == val], 
         USE.NAMES = F) 
       ) 
} 

matchdf(c("c", "i", "h"), df1, df2) 
#should output 3 9 8 
[1] 3 9 8 
matchdf(values = c("a", "a", "a"), dfin = df1, dfout = df2) 
#should output 1 1 1 
[1] 1 1 1 
matchdf(values = c("X", "Y", "a"), dfin = df1, dfout = df2) 
#should output vector, not list 
[1] 1