Midpoints of a Numeric Vector

Compute the midpoints between consecutive elements of a numeric vector. This is useful, for example, when positioning labels in stacked bar plots.

Usage

midx(x, inclZero = FALSE, cumulate = FALSE)

Arguments

x

a numeric vector.

inclZero

logical. If TRUE, a zero is prepended to x before computing midpoints. In this case, the first midpoint equals x[1] / 2. Default is FALSE.

cumulate

logical. If TRUE, returns the cumulative sum of the midpoints. Default is FALSE.

Value

a numeric vector of length length(x) - 1 (or length(x) if inclZero = TRUE) containing the midpoints. Returns an empty numeric vector if fewer than two values are available.

Details

The midpoints are defined as: $$m_i = \frac{x_i + x_{i+1}}{2}$$

When inclZero = TRUE, the computation is performed on c(0, x).

See also

Other vector.window: moveAvg(), quot()

Examples

x <- c(1, 3, 6, 7)

midx(x)
#> [1] 2.0 4.5 6.5
midx(x, inclZero = TRUE)
#> [1] 0.5 2.0 4.5 6.5
midx(x, inclZero = TRUE, cumulate = TRUE)
#> [1]  0.5  2.5  7.0 13.5

# Example: label positions in a stacked bar plot
tab <- matrix(c(401,216,221,254,259,169), nrow = 2, byrow = TRUE)
b <- barplot(tab, beside = FALSE, horiz = TRUE)

xpos <- t(apply(tab, 2, midx, inclZero = TRUE, cumulate = TRUE))
text(x = xpos, y = b, labels = t(tab), col = "red")