Wolfram Language & System 11.0 (2016)|Legacy Documentation

This is documentation for an earlier version of the Wolfram Language.View current documentation (Version 11.2)


evaluates expr once in each Wolfram Language session, always returning the result from the first evaluation.


  • When first used in a session, Once[expr] evaluates expr and returns the result.
  • Subsequent uses of Once[expr] do not evaluate expr again, and return the result from when expr was first evaluated.
  • Once has attribute HoldFirst.
  • Unset[Once[expr]] removes any stored value for Once[expr].
Introduced in 2016