Multiply matrix with a vector row wise

Question

I've got a logical matrix and I need to multiply each column by the sum of this column using apply. For example :

> a
     [,1] [,2] [,3] [,4]
[1,]    1    1    1    1
[2,]    0    0    0    0
[3,]    1    1    0    1
[4,]    1    0    0    1
> b <- colSums(a)
> b
[1] 3 2 1 3

And I want to get the following matrix :

 > a
     [,1] [,2] [,3] [,4]
[1,]    3    2    1    3
[2,]    0    0    0    0
[3,]    3    2    0    3
[4,]    3    0    0    3

I did it with for but since I need to apply my function to a huge dataset I need to code with apply. Thank you.

Answer 1

You can take the transpose ( t ) of the matrix 'a' and then multiply with the vector ('b'), take the transpose ( t ) of the output to get the desired result.

 t(t(a)*b)

Or we can make the lengths of the 'a' and 'b' same by replicating the elements of 'b'. By doing b[col(a)] , we get each element of 'b' replicated by the index provided by the col .

 a*b[col(a)]

For better understanding

  col(a)
  #     [,1] [,2] [,3] [,4]
  #[1,]    1    2    3    4
  #[2,]    1    2    3    4
  #[3,]    1    2    3    4
  #[4,]    1    2    3    4

 b[col(a)] #is a vector
 #[1] 3 3 3 3 2 2 2 2 1 1 1 1 3 3 3 3

 a*b[col(a)]
 #     [,1] [,2] [,3] [,4]
 #[1,]    3    2    1    3
 #[2,]    0    0    0    0
 #[3,]    3    2    0    3
 #[4,]    3    0    0    3

Answer 2

In addition to @akrun's answer, if you really do want to use apply :

apply(a,2,function(x)x*sum(x))
#     [,1] [,2] [,3] [,4]
#[1,]    3    2    1    3
#[2,]    0    0    0    0
#[3,]    3    2    0    3
#[4,]    3    0    0    3

2 means you work on columns (ie the second dimension). So each operation is done on a vector corresponding to a column, hence the use of sum (which works on vector) instead of colSums (which works on a matrix).