How to write a simple for loop that will populate a new column based on values in an old column, using key-value pairs?

user_11370167 0
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I'm working with data that look like this:

label
a
b
c

I have a key-value index I've created (with code like that below) to use in creating a new column based on this existing column.

values <- c("word1", "word2", "word3")
keys <- c("a", "b", "c")
index <- setNames(as.list(values), keys)

With this index made, I was expecting to be able to create a new column with a simple one line for loop, like below.

df$newcol <- for (x in df$label){index$x}

This code doesn't seem to produce any change in my dataframe at all, though. What am I not understanding here?

dataframe r for-loop list
3 Answers

Try check for (x in df$label){print(index$x)}.

It will return

NULL
NULL
NULL

Instead of $ inside function, try using [] to call some values. And, make a new vector first then append it to df$newcol will work.

newcol <- c()
for (x in df$label){
  newcol <- c(newcol, index[x])
  }
df$newcol <- newcol
df

  label newcol
1     a  word1
2     b  word2
3     c  word3

Or, little change to for loop

for (x in 1:nrow(df)){
  df$newcol[x] <- index[df$label[x]]
}
df

  label newcol
1     a  word1
2     b  word2
3     c  word3
user_167291750

I would use dplyr::recode:

df$newcol <- dplyr::recode(df$label, !!!index)

Output:

> df
  label newcol
1     a  word1
2     b  word2
3     c  word3
>
user_87083640

You may want to use a data frame rather than a list index. Then using match().

index <- data.frame(values, keys)
df$newcol <- index[match(df$label, index$keys), 'values']
df
#   label newcol
# 1     a  word1
# 2     b  word2
# 3     c  word3
# 4     a  word1
# 5     b  word2
# 6     c  word3

Data:

df <- data.frame(label=letters[c(1:3, 1:3)])

index <- structure(list(values = c("word1", "word2", "word3"), keys = c("a", 
"b", "c")), class = "data.frame", row.names = c(NA, -3L))
user_65740380
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