<br />
> tmp=list()<br />
> system.time(replicate(1e5,"as"%in%names(tmp)))<br />
user system elapsed <br />
2.01 0.00 2.20 <br />
> system.time(replicate(1e5,is.null(tmp[["as"]])))<br />
user system elapsed <br />
0.35 0.00 0.37 <br />
> tmp=as.list(1:1000)<br />
> names(tmp)=paste("n",1:1000)<br />
> system.time(replicate(1e5,is.null(tmp[["as"]])))<br />
user system elapsed <br />
4.51 0.01 4.76 <br />
> system.time(replicate(1e5,"as"%in%names(tmp)))<br />
user system elapsed <br />
8.22 0.00 8.86 <br />