CODEWITHSUNDEEP
rdata.table
Nick Allen
Nick Allen

Reputation: 1467

Data Table - Select Value of Column by Name From Another Column

I have a data table with a number of columns containing values. I have another column which defines which one of those columns whose value I need to select. I am having trouble finding a way to do this.

Here is a simple example. 

> d <- data.table(
     value.1 = c("one", "uno", "1"),
     value.2 = c("two", "dos", "2"),
     name.of.col = c("value.1","value.2","value.1"))

> d
   value.1 value.2 name.of.col
1:     one     two     value.1
2:     uno     dos     value.2
3:       1       2     value.1

I would like to add a column 'value.of.col' which contains the value of the column specified by 'name.of.col'.

> d
   value.1 value.2 name.of.col  value.of.col
1:     one     two     value.1  one
2:     uno     dos     value.2  dos
3:       1       2     value.1  1

Upvotes: 5

Views: 1929

Answers (3)

agstudy
agstudy

Reputation: 121608

Another option: 

d[ , value.of.col := diag(as.matrix(.SD)), .SDcols = d[ , name.of.col]]
> d
   value.1 value.2 name.of.col value.of.col
1:     one     two     value.1          one
2:     uno     dos     value.2          dos
3:       1       2     value.1            1

EDIT add a faster solution:

d[ , value.of.col :=
      melt(d,id.vars='name.of.col')[name.of.col==variable, value]]

Upvotes: 6

BrodieG
BrodieG

Reputation: 52687

You can use matrix indexing to pull values from the first and second columns:

mx.idx <- d[, cbind(1:nrow(d), match(name.of.col, names(d)))]
d[, 
  value.of.col:=
    as.matrix(d[, 1:2])[mx.idx]
 ]
d
#    value.1 value.2 name.of.col value.of.col
# 1:     one     two     value.1          one
# 2:     uno     dos     value.2          dos
# 3:       1       2     value.1            1

Upvotes: 2

Matt Dowle
Matt Dowle

Reputation: 59612

The following should be memory efficient and a little easier to read/follow. 

for (i in unique(d[["name.of.col"]]))
    d[ name.of.col==i, value.of.col:=get(i) ]

d
   value.1 value.2 name.of.col value.of.col
1:     one     two     value.1          one
2:     uno     dos     value.2          dos
3:       1       2     value.1            1

Upvotes: 2

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