Sum a list of matrices

Question

I have a list where each element is a 5*5 matrix. Eg

[[1]]   
           V1          V2          V3          V4          V5      
      [1,]   0.000000   46.973700   21.453500  338.547000   10.401600 
      [2,]  43.020500    0.000000  130.652000  840.526000   56.363700 
      [3,]  12.605600  173.238000    0.000000  642.075000   19.628100 
      [4,] 217.946000  626.368000  481.329000    0.000000  642.341000 
      [5,] 217.946000  626.368000  481.329000    0.000000  642.341000 
[[2]]   
           V1          V2          V3          V4          V5      
      [1,]   0.000000   47.973700   21.453500  338.547000   10.401600 
      [2,]  143.020500    0.000000  130.652000  840.526000   56.363700 
      [3,]  312.605600  17.238000    0.000000  642.075000   19.628100 
      [4,]  17.946000  126.368000  481.329000    0.000000  642.341000
      [5,] 217.946000  626.368000  481.329000    0.000000  642.341000  
...

How can I use an apply-like function to sum matrix [1] to [n], and return a 5*5 matrix as a result (each element is a sum of the corresponding elements in each of the matrix in the list) ?

I suggest you edit your post to give a reproducible example and make clearer what you're after. An example of what your final result would like like would also be helpful. In it's current state it will likely be closed. — Tyler Rinker, Jul 25 '12 at 02:08
Are you summing a list of 5 * 5 matrices? to give a 5 *5 matrix? — mnel, Jul 25 '12 at 02:25

mnel · Accepted Answer · 2012-07-26T01:31:44.733

120

Use Reduce.

## dummy data

.list <- list(matrix(1:25, ncol = 5), matrix(1:25, ncol = 5))

Reduce('+', .list)
##       [,1] [,2] [,3] [,4] [,5]
## [1,]    2   12   22   32   42
## [2,]    4   14   24   34   44
## [3,]    6   16   26   36   46
## [4,]    8   18   28   38   48
## [5,]   10   20   30   40   50

edited Jul 26 '12 at 01:31

answered Jul 25 '12 at 02:20

mnel

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That is nice! Thank you very much! – Seen Jul 25 '12 at 02:25
@GSee I tried to used the do.call, but it returns an error as "operator needs one or two arguments" – Seen Jul 25 '12 at 02:32
1

`do.call` will only work with a list of length 2. I've removed this from the answer. – mnel Jul 25 '12 at 02:36
Another function I've never seen! Thanks! – Chris Beeley Jun 15 '13 at 15:18
What would be the equivalent of '+' to get the max? – GabrielMontenegro Feb 23 '18 at 10:57
@GabrielMontenegro `pmax` (`pmin` for minimum) – mnel Mar 06 '18 at 01:43

score 15 · Answer 2 · answered Jul 25 '12 at 16:11

15

I think @mnel's answer is the more efficient but this is another approach:

apply(simplify2array(.list), c(1,2), sum)

    [,1] [,2] [,3] [,4] [,5]
[1,]    2   12   22   32   42
[2,]    4   14   24   34   44
[3,]    6   16   26   36   46
[4,]    8   18   28   38   48
[5,]   10   20   30   40   50

answered Jul 25 '12 at 16:11

Jilber Urbina

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Nice trick though in this case Reduce is faster (Reduce is the fastest answer here that extends to multiple correlation matrices in a list). +1 – Tyler Rinker Jul 25 '12 at 21:36
1

And to the contrary of Reduce, it also works with mean instead of +. – nassimhddd Mar 04 '13 at 16:30
2

this also gives options like `na.rm`. +1 – mts Jul 22 '15 at 13:39

score 5 · Answer 3 · answered Jul 25 '12 at 02:56

5

You could you do.call with some monkeying around but it loses its eloquence:

.list <- list(matrix(1:25, ncol=5), matrix(1:25,ncol=5), matrix(1:25,ncol=5))

x <- .list[[1]]
lapply(seq_along(.list)[-1], function(i){
    x <<- do.call("+", list(x, .list[[i]]))
})
x

answered Jul 25 '12 at 02:56

Tyler Rinker

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But, then you don't need `do.call`; you could have `x <<- x + .list[[i]]` inside your `lapply`. – GSee Jul 25 '12 at 03:06
1

very true. `Reduce` is much more eloquent though on a bunch of matrices the `lapply` approach may be faster (sometimes the Higher Order Functions suffer speed issues). – Tyler Rinker Jul 25 '12 at 03:13

Sum a list of matrices

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