[R] How to quickly find the position of the first non-zero element of a vector

[Bryan Keller]

Anyone familiar with a quicker method to find the position of the first non-zero element of a vector?

#The first way: 
print(min(which(vector != 0)))

#The second way:
for(i in 1:length(vector)) {
	if (vector[i] != 0) {
		print(i)
		break
		}
	}

The first way seems to be faster for larger vectors (or when the first non-zero is deeper in)...

vlarge <- c(numeric(20),1:20)

#Method 1: 
system.time(
for (i in 1:30000) {
	min(which(vlarge != 0))})
#  user  system elapsed 
#  0.49    0.00    0.48

#Method 2:
system.time(
for (i in 1:30000) {
for (i in 1:40) {
  if (vlarge[i] != 0) {
    break
  }
}})
#  user  system elapsed 
#  0.99    0.02    0.99

--------------------------------------------------
The second way seems to be faster for smaller vectors (or when the first non-zero is closer to the front)...

vsmall <- c(numeric(5),1:5)

#Method 1: 
system.time(
for (i in 1:30000) {
	min(which(vsmall != 0))})
#  user  system elapsed 
#  0.41    0.00    0.42

#Method 2:
system.time(
for (i in 1:30000) {
for (i in 1:10) {
  if (vsmall[i] != 0) {
    break
  }
}})
#  user  system elapsed 
#  0.31    0.02    0.31

Or, might the fastest way be to choose between the two methods on-the-fly based on length of the vector, etc.?

Bryan


-------------
Bryan Keller, Doctoral Student/Project Assistant
Educational Psychology - Quantitative Methods
The University of Wisconsin - Madison