Compute maxima and minima for all rows or columns of sparse matrices. Optionally also return which elements are the maxima/minima per row/column.

1 2 3 4 5 |

`X` |
a sparse matrix in a format of the |

`which` |
optionally return a sparse matrix of the same dimensions as |

`ignore.zero` |
By default, only the non-zero elements are included in the computations. However, when |

The basic workhorse of these functions is the function `rollup`

from the package `slam`

.

By default, these functions returns a `sparseVector`

with the non-zero maxima or minima. Use additionally `as.vector`

to turn this into a regular vector.

When `which = T`

, the result is a list of two items:

`max/min` |
the same sparse vector as described above. |

`which` |
a sparse pattern matrix of the kind |

Michael Cysouw

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | ```
# rowMax(X, ignore.zero = FALSE) is the same as apply(X, 1, max)
# however, with large sparse matrices, the 'apply' approach will start eating away at memory
# and things become slower.
X <- rSparseMatrix(1e3, 1e3, 1e2)
system.time(m1 <- rowMax(X, ignore.zero = FALSE))
system.time(m2 <- apply(X, 1, max)) # slower
all.equal(as.vector(m1), m2) # but same result
# to see the effect even stronger, try something larger
# depending on the amount of available memory, the 'apply' approach will give an error
# "problem too large"
## Not run:
X <- rSparseMatrix(1e6, 1e6, 1e6)
system.time(m1 <- rowMax(X, ignore.zero = FALSE))
system.time(m2 <- apply(X, 1, max))
## End(Not run)
# speed depends most strongly on the number of entries in the matrix
# also some performance loss with size of matrix
# up to 1e5 entries is still reasonably fast
X <- rSparseMatrix(1e7, 1e7, 1e5)
system.time(m <- rowMax(X))
## Not run:
X <- rSparseMatrix(1e7, 1e7, 1e7)
system.time(M <- rowMax(X)) # about ten times as slow
## End(Not run)
# apply is not feasably on such large matrices
# Error: problem too large...
## Not run:
m <- apply(X, 1, max)
## End(Not run)
``` |

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