R Vector Indexing

How it works

R vector positions start at one. Positive integers select elements; negative integers omit positions. A logical mask selects matching positions and may include missing values unless handled explicitly. Avoid mixing positive and negative indices. Out-of-range indexing with [ can return NA, which is different from a checked successful lookup.

Run this R example

These examples run locally with R, not PHP, Python or JavaScript. The reviewed baseline is R 4.5.2 on Windows x64/UCRT, using only packages shipped with R. This is a tested version, not a newest-release claim. Install your own R from the official R project; an IDE is optional. Save the complete source as indexing.R in a separate learning folder and run Rscript --vanilla indexing.R. Keep code and fictional fixtures separate from a real project.

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Example

local({
  previous_options <- options(warn = 2, digits = 8, stringsAsFactors = FALSE)
  on.exit(options(previous_options), add = TRUE)
  visits <- c(8, 12, 10, 14)
  cat("selected:", paste(visits[c(1, 3)], collapse = ", "), "\n")
  cat("without second:", paste(visits[-2], collapse = ", "), "\n")
  cat("large:", paste(visits[visits >= 12], collapse = ", "), "\n")
  invisible(NULL)
})

visits <- c(8, 12, 10, 14)
cat("selected:", paste(visits[c(1, 3)], collapse = ", "), "\n")
cat("without second:", paste(visits[-2], collapse = ", "), "\n")
cat("large:", paste(visits[visits >= 12], collapse = ", "), "\n")

Expected result

selected: 8, 10
without second: 8, 10, 14
large: 12, 14

c(1, 3) selects the first and third entries. The negative index omits the second. The comparison creates a logical vector the same length as visits, so its positions have an unambiguous relationship to the data.