Evaluate an expression inside Once:

Any subsequent evaluation of Once[expr] simply returns the previous result:

Evaluate an expression only once and remember the result persistently in a local file:

Evaluate an expression only once per front end session:

The value is remembered even after a kernel restart:

Ensure that an initialization file is read only once:

Perform a time-consuming computation only once, even if the cell containing it is evaluated again:

Once[Get[package]] is similar to Needs[package] and causes a package to be read only once:

Once has attribute HoldFirst and compares unevaluated expressions:

Find all persistently cached results of evaluations of Once:

Delete them:

Once[expr] is never evaluated again, even if variables in expr may have changed:

Use Unset[Once[expr]] to clear a remembered value:

Use Once for a form of dynamic programming to turn recursion into iteration:

Find the number of cached values generated:

Remove the cached values before redoing the timing experiment: