# sparseMatrixStats

# Installation

You can install the release version of sparseMatrixStats from BioConductor:

    if (!requireNamespace("BiocManager", quietly = TRUE))
        install.packages("BiocManager")

    BiocManager::install("sparseMatrixStats")

# Introduction

The sparseMatrixStats package implements a number of summary functions for sparse matrices from the Matrix package.

Let us load the package and create a synthetic sparse matrix.

    library(sparseMatrixStats)
    #> Loading required package: MatrixGenerics
    #> Loading required package: matrixStats
    #>
    #> Attaching package: 'MatrixGenerics'
    #> The following objects are masked from 'package:matrixStats':
    #>
    #>     colAlls, colAnyNAs, colAnys, colAvgsPerRowSet, colCollapse,
    #>     colCounts, colCummaxs, colCummins, colCumprods, colCumsums,
    #>     colDiffs, colIQRDiffs, colIQRs, colLogSumExps, colMadDiffs,
    #>     colMads, colMaxs, colMeans2, colMedians, colMins, colOrderStats,
    #>     colProds, colQuantiles, colRanges, colRanks, colSdDiffs, colSds,
    #>     colSums2, colTabulates, colVarDiffs, colVars, colWeightedMads,
    #>     colWeightedMeans, colWeightedMedians, colWeightedSds,
    #>     colWeightedVars, rowAlls, rowAnyNAs, rowAnys, rowAvgsPerColSet,
    #>     rowCollapse, rowCounts, rowCummaxs, rowCummins, rowCumprods,
    #>     rowCumsums, rowDiffs, rowIQRDiffs, rowIQRs, rowLogSumExps,
    #>     rowMadDiffs, rowMads, rowMaxs, rowMeans2, rowMedians, rowMins,
    #>     rowOrderStats, rowProds, rowQuantiles, rowRanges, rowRanks,
    #>     rowSdDiffs, rowSds, rowSums2, rowTabulates, rowVarDiffs, rowVars,
    #>     rowWeightedMads, rowWeightedMeans, rowWeightedMedians,
    #>     rowWeightedSds, rowWeightedVars
    # Matrix defines the sparse Matrix class
    # dgCMatrix that we will use
    library(Matrix)
    # For reproducibility
    set.seed(1)

Create a synthetic table with customers, items, and how often they bought that item.

    customer_ids <- seq_len(100)
    item_ids <-  seq_len(30)
    n_transactions <- 1000
    customer <- sample(customer_ids, size = n_transactions, replace = TRUE,
                        prob = runif(100))
    item <- sample(item_ids, size = n_transactions, replace = TRUE,
                   prob = runif(30))

    tmp <- table(paste0(customer, "-", item))
    tmp2 <- strsplit(names(tmp), "-")
    purchase_table <- data.frame(
      customer = as.numeric(sapply(tmp2, function(x) x[1])),
      item = as.numeric(sapply(tmp2, function(x) x[2])),
      n = as.numeric(tmp)
    )

    head(purchase_table, n = 10)
    #>    customer item n
    #> 1         1   15 2
    #> 2         1   20 1
    #> 3         1   22 1
    #> 4         1   23 1
    #> 5         1   30 1
    #> 6       100   11 1
    #> 7       100   15 3
    #> 8       100   17 1
    #> 9       100   19 1
    #> 10      100   20 2

Let us turn the table into a matrix to simplify the analysis:

    purchase_matrix <- sparseMatrix(purchase_table$customer, purchase_table$item,
                    x = purchase_table$n,
                    dims = c(100, 30),
                    dimnames = list(customer = paste0("Customer_", customer_ids),
                                    item = paste0("Item_", item_ids)))
    purchase_matrix[1:10, 1:15]
    #> 10 x 15 sparse Matrix of class "dgCMatrix"
    #>   [[ suppressing 15 column names 'Item_1', 'Item_2', 'Item_3' ... ]]
    #>              item
    #> customer
    #>   Customer_1  . . . . . . . . . . . . . . 2
    #>   Customer_2  . . 2 . . . . . . . . . . . 1
    #>   Customer_3  . . 1 . . . . . 1 . . 1 . . 1
    #>   Customer_4  . . . . . 1 . . 1 1 . 1 . 1 3
    #>   Customer_5  . . . . . . . . . . . 1 . . .
    #>   Customer_6  . . . 1 . 1 1 . . 1 1 . . . 1
    #>   Customer_7  2 . 1 . . 1 . 1 1 . 1 2 . . 1
    #>   Customer_8  . . . . . . . . 1 . . . . . .
    #>   Customer_9  . . . . . 2 . . 1 . . . . 1 .
    #>   Customer_10 . . . . . . . . . . . . . . .

We can see that some customers did not buy anything, where as some bought a lot.

`sparseMatrixStats` can help us to identify interesting patterns in this data:

    # How often was each item bough in total?
    colSums2(purchase_matrix)
    #>  Item_1  Item_2  Item_3  Item_4  Item_5  Item_6  Item_7  Item_8  Item_9 Item_10
    #>      34      13      30      10      12      34      29      31      71      28
    #> Item_11 Item_12 Item_13 Item_14 Item_15 Item_16 Item_17 Item_18 Item_19 Item_20
    #>      20      40       0      48      52      27      30      55      14      46
    #> Item_21 Item_22 Item_23 Item_24 Item_25 Item_26 Item_27 Item_28 Item_29 Item_30
    #>       8      41      40      73      56      83      20      18      13      24

    # What is the range of number of items each
    # customer bought?
    head(rowRanges(purchase_matrix))
    #>            [,1] [,2]
    #> Customer_1    0    2
    #> Customer_2    0    2
    #> Customer_3    0    3
    #> Customer_4    0    3
    #> Customer_5    0    1
    #> Customer_6    0    4

    # What is the variance in the number of items
    # each customer bought?
    head(rowVars(purchase_matrix))
    #> Customer_1 Customer_2 Customer_3 Customer_4 Customer_5 Customer_6
    #> 0.23448276 0.40919540 0.39195402 0.74022989 0.06436782 0.81034483

    # How many items did a customer not buy at all, one time, 2 times,
    # or exactly 4 times?
    head(rowTabulates(purchase_matrix, values = c(0, 1, 2, 4)))
    #>             0 1 2 4
    #> Customer_1 25 4 1 0
    #> Customer_2 25 2 3 0
    #> Customer_3 25 4 0 0
    #> Customer_4 19 8 1 0
    #> Customer_5 28 2 0 0
    #> Customer_6 20 7 2 1

## Alternative Matrix Creation

In the previous section, I demonstrated how to create a sparse matrix from scratch using the `sparseMatrix()` function. However, often you already have an existing matrix and want to convert it to a sparse representation.

    mat <- matrix(0, nrow=10, ncol=6)
    mat[sample(seq_len(60), 4)] <- 1:4
    # Convert dense matrix to sparse matrix
    sparse_mat <- as(mat, "dgCMatrix")
    sparse_mat
    #> 10 x 6 sparse Matrix of class "dgCMatrix"
    #>
    #>  [1,] . . . . . .
    #>  [2,] . . . . 4 .
    #>  [3,] . . . . . .
    #>  [4,] . . . . . .
    #>  [5,] . . . . . .
    #>  [6,] 1 . . 2 . .
    #>  [7,] . . 3 . . .
    #>  [8,] . . . . . .
    #>  [9,] . . . . . .
    #> [10,] . . . . . .

The sparseMatrixStats package is a derivative of the matrixStats package and implements it’s API for sparse matrices. For example, to calculate the variance for each column of `mat` you can do

    apply(mat, 2, var)
    #> [1] 0.1 0.0 0.9 0.4 1.6 0.0

However, this is quite inefficient and matrixStats provides the direct function

    matrixStats::colVars(mat)
    #> [1] 0.1 0.0 0.9 0.4 1.6 0.0

Now for sparse matrices, you can also just call

    sparseMatrixStats::colVars(sparse_mat)
    #> [1] 0.1 0.0 0.9 0.4 1.6 0.0

# Benchmark

If you have a large matrix with many exact zeros, working on the sparse representation can considerably speed up the computations.

I generate a dataset with 10,000 rows and 50 columns that is 99% empty

    big_mat <- matrix(0, nrow=1e4, ncol=50)
    big_mat[sample(seq_len(1e4 * 50), 5000)] <- rnorm(5000)
    # Convert dense matrix to sparse matrix
    big_sparse_mat <- as(big_mat, "dgCMatrix")

I use the bench package to benchmark the performance difference:

    bench::mark(
      sparseMatrixStats=sparseMatrixStats::colVars(big_sparse_mat),
      matrixStats=matrixStats::colVars(big_mat),
      apply=apply(big_mat, 2, var)
    )
    #> # A tibble: 3 × 6
    #>   expression             min   median `itr/sec` mem_alloc `gc/sec`
    #>   <bch:expr>        <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
    #> 1 sparseMatrixStats   59.3µs  62.37µs    15720.      448B     6.15
    #> 2 matrixStats         2.14ms   2.15ms      464.   78.62KB     0
    #> 3 apply               4.11ms   4.25ms      215.    9.54MB   125.

As you can see `sparseMatrixStats` is ca. 50 times fast than `matrixStats`, which in turn is 7 times faster than the `apply()` version.

# Session Info

    sessionInfo()
    #> R version 4.5.2 (2025-10-31)
    #> Platform: x86_64-pc-linux-gnu
    #> Running under: Ubuntu 24.04.3 LTS
    #>
    #> Matrix products: default
    #> BLAS:   /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3
    #> LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so;  LAPACK version 3.12.0
    #>
    #> locale:
    #>  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
    #>  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=C
    #>  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
    #>  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
    #>  [9] LC_ADDRESS=C               LC_TELEPHONE=C
    #> [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
    #>
    #> time zone: Etc/UTC
    #> tzcode source: system (glibc)
    #>
    #> attached base packages:
    #> [1] stats     graphics  grDevices utils     datasets  methods   base
    #>
    #> other attached packages:
    #> [1] Matrix_1.7-4             sparseMatrixStats_1.23.0 MatrixGenerics_1.23.0
    #> [4] matrixStats_1.5.0        BiocStyle_2.39.0
    #>
    #> loaded via a namespace (and not attached):
    #>  [1] vctrs_0.7.1         cli_3.6.5           knitr_1.51
    #>  [4] rlang_1.1.7         xfun_0.56           bench_1.1.4
    #>  [7] jsonlite_2.0.0      glue_1.8.0          buildtools_1.0.0
    #> [10] htmltools_0.5.9     maketools_1.3.2     sys_3.4.3
    #> [13] sass_0.4.10         rmarkdown_2.30      grid_4.5.2
    #> [16] tibble_3.3.1        evaluate_1.0.5      jquerylib_0.1.4
    #> [19] fastmap_1.2.0       profmem_0.7.0       yaml_2.3.12
    #> [22] lifecycle_1.0.5     BiocManager_1.30.27 compiler_4.5.2
    #> [25] pkgconfig_2.0.3     Rcpp_1.1.1          lattice_0.22-9
    #> [28] digest_0.6.39       R6_2.6.1            utf8_1.2.6
    #> [31] pillar_1.11.1       magrittr_2.0.4      bslib_0.10.0
    #> [34] tools_4.5.2         cachem_1.1.0