R sum by groups for matrices

Question

In the following matrix, I am having 3 columns:

       [,1]       [,2] [,3]

          1 0.11651699    1

          1 0.03850202    1

          0 0.11651699   NA

          0 0.11651699   NA

          1 0.04110752   39

          1 0.03599296   39

          1 0.05440237   41

          1 0.11651699   42

          1 0.06298718   42

          0 0.11651699   NA

          0 0.11651699   NA

          0 0.11651699   NA

I want to create a fourth column in my matrix that stores the sum of column 2 for each group(column 3). The expected out below

      [,1]       [,2]  [,3]   [,4]

          1 0.11651699    1    0.155019 = (0.11651699  + 0.03850202)

          1 0.03850202    1    0.155019 = (0.11651699  + 0.03850202)

          0 0.11651699   NA    1

          0 0.11651699   NA    1

          1 0.04110752   39    0.07710048 = (0.04110752 + 0.03599296) 

          1 0.03599296   39    0.07710048 = (0.04110752 + 0.03599296)

          1 0.05440237   41    0.09290439 = (0.03850202 + 0.05440237)

          1 0.11651699   42    0.1795042  = (0.11651699 + 0.06298718)

          1 0.06298718   42    0.1795042  = (0.11651699 + 0.06298718)

          0 0.11651699   NA    1

          0 0.11651699   NA    1

          1 0.03850202   41    0.09290439 = (0.03850202 + 0.05440237)

I know that I cannot use dplyr and groupby because that only works with dataframes and I am dealing with a matrix object. So, I tried to perform the aggregate(df1[,2] ~ df1[,3], df, sum) and it worked but its not easy taking the results from the aggregate function and creating the fourth column as shown in the expected output.

Answer 1

For achieving that you can simply use ave() inside the cbind() :

mat1 <- cbind(mat, ave(mat[, 2], mat[, 3], FUN = sum))

#Changing 4th column to 1 for NA values in column 3.

mat1[is.na(mat[, 3]), 4] <- 1

mat1

#      [,1]       [,2] [,3]       [,4]

# [1,]    1 0.11651699    1 0.15501901

# [2,]    1 0.03850202    1 0.15501901

# [3,]    0 0.11651699   NA 1.00000000

# [4,]    0 0.11651699   NA 1.00000000

# [5,]    1 0.04110752   39 0.07710048

# [6,]    1 0.03599296   39 0.07710048

# [7,]    1 0.05440237   41 0.09290439

# [8,]    1 0.11651699   42 0.17950417

# [9,]    1 0.06298718   42 0.17950417

#[10,]    0 0.11651699   NA 1.00000000

#[11,]    0 0.11651699   NA 1.00000000

#[12,]    0 0.03850202   41 0.09290439

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