Aggregate expression values across cells

Overview

This repository contains a function to aggregate statistics for groups of cells from a gene-by-cell matrix of expression values. It was primarily developed for computing pseudo-bulk expression profiles for clusters of cells, which can then be used for differential expression analysis. The code itself was originally derived from the scran R package, factored out into a separate C++ library for easier re-use.

Quick start

Given a tatami::Matrix and an array of group assignments, the aggregate_across_cells() function will compute the aggregate statistics across all genes for each group.

#include "scran_aggregate/scran_aggregate.hpp"
 
const tatami::Matrix<double, int>& mat = some_data_source();
std::vector<int> groupings = some_groupings();
 
scran_aggregate::AggregateAcrossCellsOptions opt;
auto res = scran_aggregate::aggregate_across_cells(mat, groupings.data(), opt);
 
res.sums; // vector of vectors of per-group sums across genes.
res.sums[0]; // vector of sums for the first group across genes.
res.detected; // vector of vectors of the number of detected cells per gene.

The array of groupings should contain integer assignments to groups 0, 1, 2, etc. For more complex groupings defined from combinations of multiple factors, the combine_factors() utility will create group assignments from unique combinations of those factors:

std::vector<int> grouping1 { 0, 0, 1, 1, 2, 2 };
std::vector<int> grouping2 { 0, 1, 0, 1, 0, 1 };
 
std::vector<int> combined(grouping1.size()); 
auto res = scran_aggregate::combine_factors(
    grouping1.size(), 
    std::vector<int*>{ grouping1.data(), grouping2.data() },
    combined.data()
);
 
combined; // defines unique combinations of (grouping1, grouping2).
res.factors[0]; // values of grouping1 for each unique combination.
res.factors[1]; // values of grouping2 for each unique combination.

We can also use the aggregate_across_genes() function to sum expression values across gene sets, e.g., to compute the activity of a gene signature. This can be done with any number of gene sets, possibly with a different weight for each gene in each set.

std::vector<std::tuple<size_t, const int*, const double*> > gene_sets;
 
std::vector<int> set1 { 0, 5, 10, 20 };
gene_sets.emplace_back(set1.size(), set1.data(), static_cast<double*>(NULL)); // no weight
 
std::vector<int> set2 { 0, 2, 4, 6, 8, 10 };
std::vector<double> weight2 { 0.1, 0.3, 0.3, 0.2, 0.1, 0.05 };
gene_sets.emplace_back(set2.size(), set2.data(), weight2.data()); // weighted
 
scran_aggregate::AggregateAcrossGenesOptions g_opt;
auto g_res = scran_aggregate::aggregate_across_genes(
    mat,
    gene_sets,
    g_opt
);
g_res.sum[0]; // vector of sums for set 1 in each cell.

Check out the reference documentation for more details.

Building projects

CMake with FetchContent

If you're using CMake, you just need to add something like this to your CMakeLists.txt:

include(FetchContent)
 
FetchContent_Declare(
  scran_aggregate
  GIT_REPOSITORY https://github.com/libscran/scran_aggregate
  GIT_TAG master # or any version of interest
)
 
FetchContent_MakeAvailable(scran_aggregate)

Then you can link to scran_aggregate to make the headers available during compilation:

# For executables:
target_link_libraries(myexe libscran::scran_aggregate)
 
# For libaries
target_link_libraries(mylib INTERFACE libscran::scran_aggregate)

CMake with find_package()

find_package(libscran_scran_aggregate CONFIG REQUIRED)
target_link_libraries(mylib INTERFACE libscran::scran_aggregate)

To install the library, use:

mkdir build && cd build
cmake .. -DSCRAN_AGGREGATE_TESTS=OFF
cmake --build . --target install

By default, this will use FetchContent to fetch all external dependencies. If you want to install them manually, use -DSCRAN_AGGREGATE_FETCH_EXTERN=OFF. See the tags in extern/CMakeLists.txt to find compatible versions of each dependency.

Manual

If you're not using CMake, the simple approach is to just copy the files in include/ - either directly or with Git submodules - and include their path during compilation with, e.g., GCC's -I. This requires the external dependencies listed in extern/CMakeLists.txt, which also need to be made available during compilation.