R: Vector to matrix - convert one-dimensional vector index to two-dimensional matrix index?-CodePudding

Let's say I have a vector:

myVector <- c(1,2,3,4,1,2,3,4,1,2,3,4)

And for some reason index, say, 9 in that vector (having value 1) is important to me:

> myVector[9]
[1] 1

For some another reason, I want to make this vector a matrix with dimensions 6x2 (six rows, two columns).

> myMatrix <- matrix(myVector, nrow = 6, ncol = 2)  # Only either nrow or ncol argument is actually required.
> myMatrix
     [,1] [,2]
[1,]    1    3
[2,]    2    4
[3,]    3    1
[4,]    4    2
[5,]    1    3
[6,]    2    4

Now I would like to know where my vector index 9 is located at in this new matrix. How do I get that information?

Of course, I can see in this case that it is row number 3 and column number 2, but how do I know in general where do the parameters of the transformation (number of rows and columns in the matrix) take my original index?

CodePudding user response：

The matrix you are creating still incorporates the vector it was build on, so indexing should still work.

myVector <- c(1,2,3,4,1,2,3,4,1,2,3,4)
myMatrix <- matrix(myVector, nrow = 6, ncol = 2)
# check pos 9
myMatrix[9] # 1
myVector[9] # 1
# check all positions:
myMatrix==myVector # all true

# or manually:
for (i in seq_along(myVector)) {
  print(myMatrix[i]==myVector[i])
}

CodePudding user response：

After giving myself some time to think this, I figured out that this can be solved with some algebra:

originalIndex <- 9 # this is the index in the original vector
nrows <- 6 # number of rows in the matrix to be created

rowIndex = originalIndex - (ceiling(originalIndex / nrows) - 1) * nrows
columnIndex = ceiling(originalIndex / nrows)

And the print out is as it should:

> rowIndex
[1] 3
> columnIndex
[1] 2