New: Identifying Local Maximas using For & If Statements in R - r

Question

Thanks to those who helped on my last q, I made some modifications and the code worked. However, when I increase the length of r, the max values are not being correctly stored. Can anyone identify why?
Sorry, I'm a noob.
Code:
r <- c(1, 2, 3, 4, 5, 4, 3, 4, 5, 6, 7, 5, 2, 4, 8, 11, 12, 9)
Peaks <- c()
indPeaks <- c()
n <- length(r)-1
for (x in 1:n) {
if ((x-1)<1){
if(r[x+1]<r[x]) {
Peaks <-r[x]
indPeaks <- which(r == r[x])
}
}
else{
if(r[x-1]<r[x]&r[x+1]<r[x]) {
Peaks <-r[x]
indPeaks <- which(r == r[x])
}
}
}
Returns (Environment):
indPeaks
17 # should be: 5 11 17
Peaks
12 #should be: 5 7 12
The code worked for
r <- c(1, 2, 3, 4, 5, 4, 3, 4, 5, 6, 7)

Answer 1

Hi and welcome to StackOverflow.
The reason why your previous code worked is because it only contained 1 peak.
You need to combine the previous value and the current value if you are using for-loops for this purpose. So in your case, when you assign the values to Peaks and indPeaks. So the code will be:
r <- c(1, 2, 3, 4, 5, 4, 3, 4, 5, 6, 7, 5, 2, 4, 8, 11, 12, 9)
Peaks <- c()
indPeaks <- c()
n <- length(r)-1
for (x in 1:n) {
if ((x-1)<1){
if(r[x+1]<r[x]) {
Peaks <-c(Peaks,r[x])
indPeaks <- c(indPeaks,x)
}
}
else{
if(r[x-1]<r[x]&r[x+1]<r[x]) {
Peaks <- c(Peaks,r[x])
indPeaks <- c(indPeaks, x)
}
}
}
With the output:
> Peaks
[1] 5 7 12
> indPeaks
[1] 5 11 17
Note that I changed the last part of your indPeaks <- c(indPeaks, x). Where the previous version was incorrect, since you had several values in r identical to r[x]. Note: I have not tried to put an effort in optimizing your code, I have just fixed so that you get the expected output. :)