Sum a List of Matrices

Sum a list of matrices

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

Sum of a list of matrices in R

Since you want to keep top-level list use lapply :

lapply(x, function(l) if(is.list(l)) Reduce(`+`, l) else l)

#[[1]]
#     [,1] [,2] [,3]
#[1,]    3    9   15
#[2,]    5   11   17
#[3,]    7   13   19

#[[2]]
#     [,1] [,2] [,3]
#[1,]    3    6    9
#[2,]    4    7   10
#[3,]    5    8   11

#[[3]]
#     [,1] [,2] [,3]
#[1,]    4    7   10
#[2,]    5    8   11
#[3,]    6    9   12

How to sum all the arrays inside a list of arrays?

Just sum the list:

>>> sum([np.array([[5,0,0],[0,5,0],[0,0,5]]), np.array([[1,1,0],[2,4,0],[2,0,5]])])
array([[ 6,  1,  0],
       [ 2,  9,  0],
       [ 2,  0, 10]])

R: how do apply the sum function in a list?

You need to find someway to group your columns by threes, for example:

grp = (1:ncol(Matrix) -1) %/% 3

or if you know the dimensions:

grp  = rep(0:2,each=3)

To do rowSums in columns of threes, we can do this with a function:

SumCols = function(M,col_grp){
sapply(unique(col_grp),function(i)rowSums(M[,col_grp==i]))
}
SumCols(Matrix,grp)

     [,1] [,2] [,3]
[1,]   18   63  108
[2,]   21   66  111
[3,]   24   69  114
[4,]   27   72  117
[5,]   30   75  120

So put this inside your List of matrices,

Reduce("+",lapply(List[[1]],SumCols,grp))

     [,1] [,2] [,3]
[1,]  540 1890 3240
[2,]  630 1980 3330
[3,]  720 2070 3420
[4,]  810 2160 3510
[5,]  900 2250 3600

Sum list of matrices with NAs

We can write a custom function and use it in Reduce. We replace the NAs with 0's and then we add them.

modifiedSum <- function(x, y) {
  replace(x, is.na(x), 0) + replace(y, is.na(y), 0)
}

Reduce(modifiedSum, lx)

#     [,1] [,2]
#[1,]    1    5
#[2,]    5    6

Rolling sum of matrices in list

library(purrr)
map(split(test_list, ceiling(seq_along(test_list)/3)), ~reduce(.x , `+`))

$`1`
     [,1] [,2] [,3]
[1,]   11   14   17
[2,]   12   15   18
[3,]   13   16   19

$`2`
     [,1] [,2] [,3]
[1,]    3   12   21
[2,]    6   15   24
[3,]    9   18   27

Credit to this answer for the neat splitting code.

Make numpy.sum() return a sum of matrices instead of a single number

You must call np.sum with the optional axis parameter set to 0 (summation over the axis 0, i.e the one created by your list comprehension)

totalsum = np.sum([i * matrix for i in arr], 0)

Alternatively, you can omit the brackets so np.sum evaluate a generator.

totalsum = np.sum(i * matrix for i in arr)

Sum of elements of a matrix (list), but only at indexes given in another matrix (list)

Use a generator expression:

res = sum(v for ii, val in zip(index, matrix) for i, v in zip(ii, val) if i == -1)
print(res)

Output

935

As an alternative:

res = 0
for ii, val in zip(index, matrix):
    for i, v in zip(ii, val):
        if i == -1:
            res += v

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