在数据框中，使用R中向量的名称创建列

Question

I am trying to create a bunch of columns in a data frame with their names dependent on values in two vectors. I will do my best to explain and provide sample data but do apologize in advance for any unclarity.

# my data frame
df <- data.frame(id = c(1, 2, 3), var1 = c("a", "c", "e"), var2 = c("b", "d", "f"))

# the two vectors
v1 <- c("x", "y", "z")
v2 <- c(10, 20, 30)

I would then like to do something like this:

for (i in 1:length(v1)) {
  for (j in 1:length(v2)) {
    dataset$v1[i]_v2[j] <- "some value"
  }
  rm(j)
}
rm(i)

Of course the tricky part in which this approach won't work is dataset$v1[i]_v2[j] . But is there maybe some way to work around this? I would be grateful for any help!

Answer 1

You can do the assignment like this:

for (i in 1:length(v1)) {
  for (j in 1:length(v2)) {
    # use [[]] to access the column
    df[[paste(v1[i], v2[j], sep='_')]] <- "some value"
    # this would also work:
    # df[, paste(v1[i], v2[j], sep='_')] <- "some value"
  }
  rm(j)
}
rm(i)

In case you literally want to fill all columns with "some_value" this is even easier:

column.names <- as.vector(sapply(v1, function(x) paste(x, v2, sep='_')))
df[,column.names] <- 'some_value'