Logical condition while subsetting not giving correct values

Question

I wanted to subset data frame project I was working with, using a logical. I am getting a paradoxical result. The part of the logical preceding the ROLL.NO.

Answer 1

From ?base::Logic, help('&'), help('|'), etc

See Syntax for the precedence of these operators: unlike many other languages (including S) the AND and OR operators do not have the same precedence (the AND operators have higher precedence than the OR operators).

which explains why

TRUE | TRUE & FALSE
# [1] TRUE

which is essentially

TRUE | (TRUE & FALSE)

which is also true, and a simplification of what you are doing here:

(project$DC31==1&project$D14==2) |
  (project$DC31==2&project$D14==1) &
  !is.na(project$DC31) &
  !is.na(project$D14) &
  project$ROLL.NO. == 3131

since you expect the result only to contain some project$ROLL.NO. == 3131 I assume, so even if some of these are false, if one or more OR is true, you may get some that are not ROLL.NO. which are not 3131

Also note that ! has a higher precedence than logicals