I am trying to interpolate a row of a data frame at a single point. I can use approx to get a single value, but need to end up with a vector or single line with one value for each column of the data frame.
df <- data.frame(
x = c(1.1, 1.2, 1.3, 1.4),
y1 = c(-.6,-.4, .4, 1.2),
y2 = c(-.7, -.4, .4, .46),
y3 = c(-.8, -.3, .5, 1.2)
)
df
x y1 y2 y3
1 1.1 -0.6 -0.70 -0.8
2 1.2 -0.4 -0.40 -0.3
3 1.3 0.4 0.40 0.5
4 1.4 1.2 0.46 1.2
target_x <- 1.24
## Works
approx(df$x, df$y1, target_x)$y
>[1] -0.08
### looking for something like approx(df$x, df[,2:4], target_x)
## should return:
> [1] -0.08 -0.08 0.02
Linear interpolation is fine, I do not need to extrapolate beyond the existing table. Either base R or dplyr approaches are preferred.
You're looking for
sapply(df[, 2:4], \(y) approx(x=df$x, y=y, xout=target_x)$y)
# y1 y2 y3
# -0.08 -0.08 0.02
Data:
df <- structure(list(x = c(1.1, 1.2, 1.3, 1.4), y1 = c(-0.6, -0.4,
0.4, 1.2), y2 = c(-0.7, -0.4, 0.4, 0.46), y3 = c(-0.8, -0.3,
0.5, 1.2)), class = "data.frame", row.names = c(NA, -4L))
target_x <- 1.24