R gotcha: logical-and operator for combining conditions is & not &&
From the help page for Logical Operators, accessible by ?"&&":
& and && indicate logical AND and | and || indicate logical OR. The shorter form performs elementwise comparisons in much the same way as arithmetic operators. The longer form evaluates left to right examining only the first element of each vector. Evaluation proceeds only until the result is determined. The longer form is appropriate for programming control-flow and typically preferred in if clauses.
(R version 2.13-0)
In other words, when using subset, use the single &.
Here is an illustration of the difference:
c(1,1,0,0) & c(1,0,1,0)[1] TRUE FALSE FALSE FALSEc(1,1,0,0) && c(1,0,1,0)[1] TRUEIf this looks quirky compared to other programming paradigms, remember that R needs to provide a vectorised form of the operator.
In R, you actually want the & operator rather than && to do a pairwise AND operation, the && does a bitwise AND. The same rule applies for OR: if you want to do a logical OR rather than a bitwise OR, you want the | operator.