You need {} for (i in 1:500){ .......commands.......} Like: x<-rep(c(0.8,1.2),250) for (i in 1:500) { if (x[i]>1) x[i]=1 } BR JEsper -- View this message in context: http://r.789695.n4.nabble.com/How-to-perform-a-substitution-in-a-loop-tp2325048p2325073.html Sent from the R help mailing list archive at Nabble.com.