How to match and replace elements between two dataframes

df1<-data.frame(id=c(1,2,3,4), value=c(10,20,30,40))

df2<-data.frame(col1=c(1,2,3,4), col2=c(4,3,2,1))

df2[] <- lapply(df2, function(x) {
   inds <- match(x, df1$id)
   ifelse(is.na(inds),x, df1$value[inds]) 
})


df2

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df <- data.frame(fruits = c("apple", "orange", "pineapple", "banana", "grape"), stringsAsFactors = FALSE)

df_rep <- data.frame(eng = c("apple", "orange", "grape"), esp = c("manzana", "naranja", "uva"), stringsAsFactors = FALSE)

df2 <- merge(df, df_rep, by.x = 'fruits', by.y = 'eng', all.x = TRUE)

df2$fruits <- ifelse(is.na(df2$esp), df2$fruits, df2$esp)

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a <- data.frame(id=letters[1:4], age=c(18,NA,9,NA), sex=c("M","F","F","M"))
a

b <- data.frame(id=c("a","b","d"), age=c(18,32,20))
b

library(data.table)
setDT(a)[b, agei := i.age, on='id'][is.na(age), age := agei][,agei:= NULL][]
a

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library(dplyr)

dt1 <- read.table(text = "
SAMPLE_ID CLASS_ID CLASS VALUE
1         0        0      5
2         0        0      5
3         0        0      3
4         0        0      6
5         0        0      6
6         0        0      3
", header = TRUE, stringsAsFactors = FALSE)

dt2 <- read.table(text = "
SAMPLE_ID REF_VAL CLASS_ID CLASS
1        33        2      cloud
2        45        3      water
3        NA        3      water
4        NA        4      forest
", header = TRUE, stringsAsFactors = FALSE)

dt3 <- read.table(text = "
SAMPLE_ID CLASS_ID CLASS STRATA
5         3       NA      20
6         3      water    19
", header = TRUE, stringsAsFactors = FALSE)

dt <- dt1[,c("SAMPLE_ID", "VALUE")]
dt <- left_join(dt, dplyr::bind_rows(dt2, dt3))
dt <- select(dt, SAMPLE_ID, CLASS_ID, CLASS, VALUE)