How can I add rows to an R data frame every other row?
Brief: how can I add m rows to my m X n data frame, where each new row is inserted after each existing row? I will essentially copy the existing row, but make a change to one variable.
More detail: in reference to another question, I think I can do what I want with rgl's segments3d function. I have a set of x,y,z points, but these are just one end point of a set of line segments. The other end point is so many metres away in the Z dimension, given as a fourth variable: X,Y,Z,Z_Length; in my terminology it's easting,northing,elevation,length.
According to the rgl docs, "Points are taken in pairs by segments3d". So, I think I need to modify my data frame to have extra entries every second line with an altered Z variable (by subtracting Z_Length from Z). Visually, it needs to go from this:
+-------+---------+----------+-----------+---------+
| Label | easting | northing | elevation | length |
+-------+---------+----------+-----------+---------+
| 47063 | 554952 | 5804714 | 32.68 | 619.25 |
| 47311 | 492126 | 5730703 | 10.40 | 1773.00 |
+-------+---------+----------+-----------+---------+
to this:
+-------+---------+----------+-----------+---------+
| Label | easting | northing | elevation | length |
+-------+---------+----------+-----------+---------+
| 47063 | 554952 | 5804714 | 32.68 | 619.25 |
| 47063 | 554952 | 5804714 | -586.57 | 619.25 |
| 47311 | 492126 | 5730703 | 10.40 | 1773.00 |
| 47311 | 492126 | 5730703 | -1762.26 | 1773.00 |
+-------+---------+----------+-----------+---------+
A data sample at the linked question is available.
Answer
Your sample data:
orig.df <- read.table(text = "
Label easting northing elevation length
47063 554952 5804714 32.68 619.25
47311 492126 5730703 10.40 1773.00", header = TRUE)
Create your data to be inserted:
insert.df <- transform(orig.df, elevation = elevation - length)
Append it to your original data:
out.df <- rbind(orig.df, insert.df)
Reorder the rows:
n <- nrow(orig.df)
out.df[kronecker(1:n, c(0, n), "+"), ]
# Label easting northing elevation length
# 1 47063 554952 5804714 32.68 619.25
# 3 47063 554952 5804714 -586.57 619.25
# 2 47311 492126 5730703 10.40 1773.00
# 4 47311 492126 5730703 -1762.60 1773.00