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x <- 10
if (x > 0) {
    cat("positive\n")
} else if (x < 0) {
    cat("negative\n")
} else {
    cat("zero\n")
}

Logical operators: && (and), || (or), ! (not). Use double && and || for SCALAR conditions inside if. Single & and | are vectorized — they return a vector of TRUE/FALSE.

if (x > 0 && x < 100) { ... }    # scalar — what you want for if()
ages > 18 & ages < 65            # vector — for filtering

ifelse(test, yes, no) is the vectorized if — works element-wise:

ages <- c(15, 30, 50, 70)
ifelse(ages >= 18, "adult", "minor")
# "minor" "adult" "adult" "adult"