Description Usage Arguments Details Value Author(s) See Also Examples

A hack to speed up tapply(x, group, sum) for the special case where x is sorted by group.

1 | ```
tapplysum.fast(x, groups)
``` |

`x` |
A numeric vector. |

`groups` |
A grouping factor. |

Works by computing the cumulative sum of x and taking the difference
at the indices where the groups change. Standard `tapply`

can
be slow when there are a large number of groups, due to the overhead
of factor manipulation.

Vector containing the group sums of `x`

.

C. H. Jackson <chris.jackson@mrc-bsu.cam.ac.uk>

1 2 3 4 | ```
x <- factor(rep(1:1000, each=100))
y <- rnorm(1000*100)
system.time(tapply(y, x, sum))
system.time(tapplysum.fast(y, x))
``` |

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