Applying a function for each value in a row to all values in another row, append results

Question

I have read many related posts, but can't seem to find an solution to what seems like it should be a very simple problem. I'm using R and I want to apply the following equation x = R+2B to my data frame. I have a row R which is a vector of values 1:6, and a row B which is the same. I want to create a new vector that is the values of R[i]+2B[1:6]. So for each R value there will be 6 resulting values. In total I will have 36 values in the vector.

> data
  R B
1 1 1
2 2 2
3 3 3
4 4 4
5 5 5
6 6 6

> x <- matrix(ncol=6,nrow=6)
> for (i in nrow(data)){ 
+   x[i] = data$R[i]+(2*data[1:6,2])
 + }
Warning message:
In x[i] = data$R[i] + (2 * data[1:6, 2]) :
number of items to replace is not a multiple of replacement length

Here is what I'm trying, which isn't right. Any thoughts on how to do this simply and efficiently?

Thank you in advance.

Answer 1

You can use sapply:

sapply(data$R, function(x)x + 2*data[1:6,"B"])

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    3    4    5    6    7    8
[2,]    5    6    7    8    9   10
[3,]    7    8    9   10   11   12
[4,]    9   10   11   12   13   14
[5,]   11   12   13   14   15   16
[6,]   13   14   15   16   17   18

If you want a vector instead of a matrix, you can use as.vector

Answer 2

You're looking for expand.grid:

data <- data.frame(R=1:6,B=1:6)
out <- expand.grid(data$R,2*data$B)
apply(out,1,sum)
 [1]  3  4  5  6  7  8  5  6  7  8  9 10  7  8  9 10 11 12  9 10 11 12 13 14 11
[26] 12 13 14 15 16 13 14 15 16 17 18

Answer 3

sapply(data[, 'R'], function(x)x + 2*data[, 'B'])

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    3    4    5    6    7    8
[2,]    5    6    7    8    9   10
[3,]    7    8    9   10   11   12
[4,]    9   10   11   12   13   14
[5,]   11   12   13   14   15   16
[6,]   13   14   15   16   17   18

Each column corresponds to a row of 'R'.