Stephen Milborrow milbo at sonic.net
Mon Apr 14 22:22:51 CEST 2008

> Le sam. 12 avr. à 12:47, carlos martinez a écrit :
> Looking for a simple, effective a minimum execution time solution.
> For a vector as:
> c(0,0,1,0,1,1,1,0,0,1,1,0,1,0,1,1,1,1,1,1)
> To transform it to the following vector without using any loops:
> (0,0,1,0,1,2,3,0,0,1,2,0,1,0,1,2,3,4,5,6)

Here is a fast solution using the Ra just-in-time compiler

if (length(x) > 1)
    for (i in 2:length(x))
        if (x[i])
            x[i] <- x[i-1] + 1

The times in seconds for various solutions mailed to r-devel are listed
below. There is some variation between runs and with the contents of x. The
times shown are for

set.seed(1066);  x <- as.double(runif(1e6) > .5)

This was tested on a WinXP 3 GHz Pentium D with Ra 1.0.7 (based on R 2.6.2).
The code to generate these results is attached.

vin     24
greg   11
had    3.9
dan    1.4
dan2  1.4
jit       0.25    # code is shown above, 7 secs with standard R 2.6.2>

Stephen Milborrow
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