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Discard

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

Discard[expr, crit]

Drops the elements e of expr for which crit[e] is True (the complement of Select). On an association crit tests the values.

Discard[expr, crit, n]

Drops only the first n such elements (n may be Infinity).

Examples (7)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (3)

In[1]:= Discard[{1, 2, 3, 4, 5, 6}, EvenQ]
Out[1]= {1, 3, 5}

In[2]:= Discard[<|"a" -> 1, "b" -> 2, "c" -> 4|>, EvenQ]
Out[2]= <|"a" -> 1|>

In[3]:= Discard[{1, 2, 3, 4, 5, 6}, EvenQ, 2]
Out[3]= {1, 3, 5, 6}

Applications (4)

The complement of Select

In[4]:= Discard[{1, 2, 3, 4, 5}, EvenQ]
Out[4]= {1, 3, 5}

In[5]:= Discard[Range[10], # > 5 &]
Out[5]= {1, 2, 3, 4, 5}

Drop at most the first match

In[6]:= Discard[{1, 2, 3, 4, 5}, OddQ, 1]
Out[6]= {2, 3, 4, 5}

Over an association, tests the values

In[7]:= Discard[<|a -> 1, b -> 2, c -> 3|>, OddQ]
Out[7]= <|b -> 2|>

Implementation notes

Algorithm. builtin_discard is the complement of Select: it keeps the elements of a non-atomic expression for which the criterion does not give True. Discard[expr, crit] tests every element; Discard[expr, crit, n] discards at most the first n matches and keeps the rest regardless. The result reuses the original head, so it works on any non-atomic expression, and on an association it tests the values while keeping the keys.

Data structures. A single pass builds a keep array of element copies; the criterion is applied with eval_call1 (evaluate crit[elem]) and the verdict read with is_true_sym. A visible NDArray is de-listed first via ops_delist_visible.

Complexity / limits. O(n) evaluations of the criterion. A non-atomic first argument is required (Discard::normal), and the optional limit must be a non-negative integer or Infinity (Discard::innf); both diagnostics route through the message funnel so Quiet[]/Check[] behave. crit is arbitrary, so there is no packed fast path.

Attributes: Protected.

References

See also: Select

Notes & additional examples

Notes

Discard[expr, crit] keeps the elements for which crit does not return True ā€” exactly the complement of Select[expr, crit]. The optional third argument Discard[expr, crit, n] discards at most the first n matching elements and keeps everything after. It works on any non-atomic expression and, over an association, tests the values while preserving keys.