CODEWITHSUNDEEP
rfor-loopmatrixaddition
khawkes
khawkes

Reputation: 15

How to add a vector to each column in a matrix in R?

I have a matrix called r2, and for each column of that matrix, i need to add another column of data - ie. I want the result to be the first element of the first column + the first element in the second column; second element + second element; and so on. I can get it to work when I separate one column and add it, but I can't loop over the columns of the matrix. See below:

This is the matrix r2:

      [,1]   [,2]   [,3]   [,4]   [,5]   [,6]
[1,] 51.032 45.480 42.638 59.808 40.309 51.135
[2,] 50.591 45.415 47.962 57.020 39.670 52.518
[3,] 55.695 50.099 50.205 63.256 45.174 50.475
[4,] 46.422 44.233 44.322 53.229 34.943 41.707
[5,] 52.357 44.523 44.374 57.014 41.672 53.920
[6,] 53.610 44.448 41.846 60.451 42.163 52.706
[7,] 51.728 44.612 43.305 56.844 39.048 48.710
[8,] 52.790 45.266 43.426 58.365 41.577 54.630

and here is the column I want to add to each column of the matrix, called BDtool["Constant"]:

-26.701
-29.287
-30.964
-24.246
-29.194
-30.444
-26.524
-29.771

this code works to add a column of r2 to the Constant column:

r3 <- r2[,1]+BDtool["Constant"]

and the outcome looks like:

[1,]   24.434
[2,]   23.231
[3,]   19.511
[4,]   17.461
[5,]   24.726
[6,]   22.262
[7,]   22.186
[8,]   24.859

this is exactly what i want. However, I can't get any kind of for-loop to work. I just want to iterate over every column in r2 and, for each column, get an output like the one above. i've fiddled with various versions of:

r3 <- for (i in r2[,1:6]) {
    i +BDtool["Constant"]
}

but r3 always ends up NULL. How do i make this work? Thank you for any advice you can offer!

Upvotes: 1

Views: 1411

Answers (2)

Dirk is no longer here
Dirk is no longer here

Reputation: 368231

One way is to use a helper matrix. Say you have

> M <- matrix(1:9,3,3); M
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
>

and you want to add a vector c(1,2,3) to each column. Well, we can create an matrix of ones and multiply it with that vector:

> idV <- matrix(1,3,3) * c(1,2,3); idV
     [,1] [,2] [,3]
[1,]    1    1    1
[2,]    2    2    2
[3,]    3    3    3
>

After that we just add the two matrices:

> M + idV
     [,1] [,2] [,3]
[1,]    2    5    8
[2,]    4    7   10
[3,]    6    9   12
>

You can set that up easily in a helper function.

Edit: But as Ronan reminds us in his answer above, the recycling rule makes that automagic.

Upvotes: 1

Ronak Shah
Ronak Shah

Reputation: 388982

You can use r3 <- r2 + BDtool$Constant

This works because of R's recycling property. BDtool$Constant is added first to the first column of r2 matrix and the values in BDtool$Constant are recycled and reused for the second column again and so on for all other columns.

This simplified example may help clarify -

r2 <- matrix(1:18, 6, 3)
v <- 1:6
r2 + v
#     [,1] [,2] [,3]
#[1,]    2    8   14
#[2,]    4   10   16
#[3,]    6   12   18
#[4,]    8   14   20
#[5,]   10   16   22
#[6,]   12   18   24

Upvotes: 3

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