Clever Geek Handbook

Only fetch data from columns matching character strings

I have a dataset that looks something like this (but much more)

Jul_08 <- c(1,0,2,0,3)
Aug_08 <- c(0,0,1,0,1)
Sep_08 <- c(0,1,0,0,1)
month<-c("Jul_08","Aug_08","Jul_08","Sep_08","Jul_08")
dataset <- data.frame(Jul_08 = Jul_08, Aug_08 = Aug_08, Sep_08=Sep_08,month=month)

For each row, I would like to highlight the value for the selected month only, as specified in the "month" field. In other words, for a given row, if the "month" column is July_08, then for the new "value" column, I would like to include the anchor referring to the "Jul_08" column from that row.

In essence, the output will add this column of values ​​to the dataset

value<-c(1,0,2,0,3)

Creating this final dataset

dataset.value<-cbind(dataset,value)
+3


source to share


2 answers


You can use matrix indexing:

w <- match(month, names(dataset))

dataset$value <- dataset[ cbind(seq_len(nrow(dataset)), w) ]


Here the vector w

specifies R where the column takes on from and is seq_len

used to use the same row, so the column value

is built by taking 1st column in 1st row, then 2nd column and 2nd row, 1st column for 3rd row, etc.

+3


source


You can use lapply

:

value <- unlist(lapply(1:nrow(dataset),
                function(r){ 
                   dataset[r,as.character(dataset[r,'month'])] 
                }))
> value
[1] 1 0 2 0 3

Or alternatively:

value <- diag(as.matrix(dataset[,as.character(dataset$month)]))
> value
[1] 1 0 2 0 3


Then you can cbind

create a new column as in your example.

Some notes:

  • I prefer unlist(lapply(...))

    more sapply

    , as the automatic simplification implemented in the sapply function sometimes surprises me. But I am sure that this time you can use it without any problem.
  • as.character

    only needed if the column month

    is a factor (as in the example), otherwise redundant (but I would leave it to be safe).
+2


source






More articles:


All Articles