Populate a new column in a dataframe with a lookup from a double matrix

Question

I have a dataframe df:

colour  shape
\'red\'   circle
\'blue\'  square
\'blue\'  circle
\'green\' sphere

And a double matrix m with named rows/

Answer 1

merge() is your friend here. To use it, we need an appropriate data frame to merge with containing the stacked version of your ID matrix. I create that as newdf with the code below:

df <- data.frame(matrix(1:9, ncol = 3))
colnames(df) <- c("circle","square","sphere")
rownames(df) <- c("red","blue","green")

newdf <- cbind.data.frame(ID = unlist(df), 
                          expand.grid(colour = rownames(df), 
                                      shape = colnames(df)))

Which results in:

> newdf
        ID colour  shape
circle1  1    red circle
circle2  2   blue circle
circle3  3  green circle
square1  4    red square
square2  5   blue square
square3  6  green square
sphere1  7    red sphere
sphere2  8   blue sphere
sphere3  9  green sphere

Then with your original data in object df2, defined using

df2 <- data.frame(colour = c("red","blue","blue","green"),
                  shape = c("circle","square","circle","sphere"))

use merge()

> merge(newdf, df2, sort = FALSE)
  colour  shape ID
1    red circle  1
2   blue circle  2
3   blue square  5
4  green sphere  9

You can store that and rearrange the columns if you need that:

> res <- merge(newdf, df2, sort = FALSE)
> res <- res[,c(3,1,2)]
> res
  ID colour  shape
1  1    red circle
2  2   blue circle
3  5   blue square
4  9  green sphere