Tom - 10 months ago 43

R Question

I have my dataframe set up like this:

`set.seed(50)`

m <- matrix(nrow=4,ncol=9)

m[1,] <- 0

for(i in 2:4){

for(j in 1:9){

m[i,j] <- m[i-1,j] + runif(1,max = .25)

}

}

df <- data.frame(pond=rep(c('A','B','C'),4,each = 3),

variable=(rep(c('most','least','random'),3)),

rank=rep(c(0,.1,.2,.3),each=9),

value= as.vector(t(m)))

I'd like to find the point at which these lines intersect with the line with equation y=0.5-x as shown in this plot:

I've had some success finding the coordinates of intersection using the solve command as outlined here. I'm not sure how to iterate this process over all the variables and ponds in the data frame, especially as the rank at which the line crosses varies between pond/variables.

Answer

```
set.seed(50)
m<-matrix(nrow=4,ncol=9)
m[1,]<-0
for(i in 2:4){
for(j in 1:9){
m[i,j]<-m[i-1,j]+runif(1,max = .25)
}
}
df<-data.frame(pond=rep(c('A','B','C'),4,each = 3),
variable=(rep(c('most','least','random'),3)),
rank=rep(c(0,.1,.2,.3),each=9),
value= as.vector(t(m)))
S <- lapply(split(df,df$pond),function(x){split(x,x$variable)})
Ix <-
lapply( S,
function(L)
{
lapply( L,
function(M)
{
a <- -1
b <- 0.5
intersection <- rep(NA,nrow(M)-1)
for ( n in 1:nrow(M)-1 )
{
x1 <- M$rank[n]
x2 <- M$rank[n+1]
y1 <- M$value[n]
y2 <- M$value[n+1]
det <- a*(x1-x2)+y2-y1
x <- (x1*(y2-y1)+(b-y1)*(x2-x1)) / det
lambda <- (a*x1-y1+b)/det
intersection[n] <-
ifelse( (0<=lambda) && (lambda<=1),
x,
NA )
}
intersection
}
)
}
)
```

Result:

```
> Ix
$A
$A$least
[1] NA NA 0.2653478
$A$most
[1] NA NA 0.2809349
$A$random
[1] NA NA 0.2718672
$B
$B$least
[1] NA NA 0.2548668
$B$most
[1] NA 0.1800216 NA
$B$random
[1] NA NA 0.2706433
$C
$C$least
[1] NA 0.1771962 NA
$C$most
[1] NA 0.1836434 NA
$C$random
[1] NA NA 0.2811595
```

The values in `Ix`

are the "rank" values. `NA`

means that the line segment doesn't intersect.

Source (Stackoverflow)