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

[jim holtman]

try .which.mine

Here is what is does on my system:

> vlarge <- c(numeric(20),1:20)
> system.time(
+ for (i in 1:30000) {
+        which.min(vlarge != 0)})
   user  system elapsed
   0.30    0.00    0.35
>
> #Method 2:
> system.time(
+ for (i in 1:30000) {
+ for (i in 1:40) {
+  if (vlarge[i] != 0) {
+    break
+  }
+ }})
   user  system elapsed
   2.25    0.01    2.65
>


On Tue, Sep 15, 2009 at 7:51 PM, Bryan Keller <bskeller at wisc.edu> wrote:
> 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
>
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> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

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