Calculate triangle areas in a loop - R - r

Question

I have a 20 points (X1, Y1,…. Xn, Yn) on a pyramid and a random base point (Xbase, Ybase). I wish to calculate the triangle area between (Xi, Yi; Xi+1, Yi+1; Xbase, Ybase). Therefore, I did a loop that calculate the area but I can not store the area result area in a the data.frame (myDF). Furthermore, is there another elegant way to calculate the area?
Script:
library(ggplot2)
myDF <- data.frame(area=double())
nElem <- 100
xData <- as.data.frame(seq(1,nElem,5))
yData1 <- seq(5,nElem/2,5)
yData2 <- rev(yData1-4)
yData<- as.data.frame((c(yData1, yData2)))
xyDATA<- cbind(xData,yData)
colnames(xyDATA) <- c("xCoord","yCoord")
Xbase <-runif(1, 90, 91)
Ybase <-runif(1, 1.0, 1.5)
for(i in 1:19)
{
x1 <- Xbase
y1 <- Ybase
x2 <- xyDATA[i,1]
y2 <- xyDATA[i,2]
x3 <- xyDATA[i+1,1]
y3 <- xyDATA[i+1,2]
s <- 0.5*sqrt((x2*x3-x3*y2)^2+(x3*y1-x1*y3)^2+(x1*y2-x2*y1)^2)
myDF[i] <-s
}
P1 <- ggplot(xyDATA) + geom_point(aes(x = xCoord, y = yCoord))
P2 <- P1 + geom_point(aes(x = x1, y = y1),colour="red",size=4)
P2
Thanks a lot.

Answer 1

As written you are assigning the value of s to an entire column in the dataframe. You probably want to specify an area column and then assign into a row of that col.
# before the loop, create the column:
DF['area'] <- NA
# Inside the loop
....
myDF[i, "area"] <-s

Answer 2

Here is a solution using the dplyr package:
nElem <- 100
xData <- as.data.frame(seq(1,nElem,5))
yData1 <- seq(5,nElem/2,5)
yData2 <- rev(yData1-4)
yData<- as.data.frame((c(yData1, yData2)))
xyDATA<- cbind(xData,yData)
colnames(xyDATA) <- c("xCoord","yCoord")
Xbase <-runif(1, 90, 91)
Ybase <-runif(1, 1.0, 1.5)
library(dplyr)
myDF <- xyDATA %>%
mutate("s" = 0.5*sqrt(
(xCoord*lead(xCoord)-lead(xCoord)*yCoord)^2+
(lead(xCoord)*Ybase-Xbase*lead(yCoord))^2+
(Xbase*yCoord-xCoord*Ybase)^2
))
head(myDF)
xCoord yCoord s
1 1 5 502.2731
2 6 10 807.6995
3 11 15 1118.5987
4 16 20 1431.4092
5 21 25 1745.1034
6 26 30 2059.2776