R Vectors

Context: FIT1043_MOC · R’s core 1-D structure · columns of a data frame are vectors · built with c() Problem it solves: create a vector, index its elements, do element-wise arithmetic, and test for missing values.

Quick Revision

• 🎯 Trigger: store/operate on many values ➔ build with c(); index with square brackets; arithmetic is element-wise.
• ⚡ Key Constraint: R indexing is 1-based, and negative indices exclude (they don’t count from the end).

🔧 Minimal Working Example

B <- c(5, 6, 3, 0)          # concatenate into a vector
B <- c(B, c(1, 2))          # [1] 5 6 3 0 1 2  (grow it)
x <- c("Jan","Feb","Mar","April")
x[c(1,3,4)]                 # "Jan" "Mar" "April"  (by position)
x[c(-1,-4)]                 # "Feb" "Mar"          (exclude 1 and 4)
x[1:3]                      # "Jan" "Feb" "Mar"    (range)

Expected output: as commented — positive indices select, negative indices exclude, a:b is a range.

• Create/grow ➔ c(...) concatenates values (and vectors) into one vector.
• Index ➔ positive positions select; negative positions exclude; 1:3 selects a range.
• Element-wise arithmetic ➔ same-length vectors combine per element: c(1,2,4)*c(12,4,3) → 12 8 12.
• Missing values ➔ anyNA(x) → TRUE if any NA; is.na(x) → a logical vector flagging each NA.

🔀 Variations

• Element-wise ops ➔ v1 * v2, v1 + v2, etc., require equal length; each position is computed independently.
• NA detection ➔ is.na(c("Java", NA, "R", NA)) → FALSE TRUE FALSE TRUE.

✍️ Practice

Practice 1: Given x <- c("Java", NA, "Python", "R", NA), check whether it has any missing values and flag which elements are missing.

Reference solution

anyNA(x)     # [1] TRUE
is.na(x)     # [1] FALSE TRUE FALSE FALSE TRUE

• Key move: anyNA gives one TRUE/FALSE; is.na gives a per-element logical vector.

⚠️ Common Mistakes

• 💡 Negative index = exclude, not from-the-end ➔ x[-1] drops the first element (unlike Python’s x[-1]).
• 💡 Recycling on unequal lengths ➔ element-wise ops assume equal length; mismatched lengths recycle the shorter one (often a bug).