Calculates proportion/percent differences between subsequent (or lagged) elements of a numeric vector.

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`x` |
Numeric vector. |

`lag` |
Controls spacing between proportion/percent differences. For example, lag of 1 means you want to calculate differences between elements 1 and 2, 2 and 3, 3 and 4, and so on; a lag of 2 means you want calculate differences between elements 1 and 3, 2 and 4, 3 and 5, and so on. |

`percent` |
TRUE for percent differences, FALSE for proportion differences. Percent differences are just proportion differences multiplied by 100. |

Each proportion/percent difference is based on two numbers, say x1 and x2. The proportion difference is defined as (x1 - x2) / mean(x1, x2), and the percent difference is the same quantity multiplied by 100. Notice that this is NOT proportion/percent change, which divides by the initial value x1 rather than the average of x1 and x2. See `pchanges`

for the proportion/percent change operation.

Numeric vector.

This function uses C++ code to achieve a 3-4 times speed increase compared to the base R code: len <- length(x); p1 <- x[2: len]; p2 <- x[1: (len - 1)]; (p1 - p2) / (0.5 * (p1 + p2)).

Dane R. Van Domelen

Acknowledgment: This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-0940903.

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