WolframLang: Delete Element in List

By Xah Lee. Date: . Last updated: .

Delete by Index Range

Drop[ expr, n]
Drop 1st to nth Drop 
Drop[ {1, 2, 3, 4, 5}, 3 ] === {4, 5}
Drop[ expr, -n]
Drop nth to last. 
Drop[ {1, 2, 3, 4, 5}, -3 ] === {1, 2}
Drop[ expr, {n}]
Drop just nth. 
Drop[ {1, 2, 3, 4, 5}, {3} ] === {1, 2, 4, 5}
Drop[ expr, {m,n}]
Drop mth to nth. 
Drop[ {1, 2, 3, 4, 5}, {2,4} ] === {1, 5}

Delete by Position

Delete[expr, n]
if n is a integer, consider it as position spec {n}. Result is same as Delete[expr, {n}] and Delete[expr, {{n}}] 
x = {a, {{b, c}, d}, e};

Delete[ x, 1 ] === {{{b, c}, d}, e}
Delete[ x, {1} ] === {{{b, c}, d}, e}
Delete[ x, {{1}} ] === {{{b, c}, d}, e}
Delete[expr, {pos1, pos2, pos3 etc}]
Delete a list of elements by Position Delete 
x = {a, {{b, c}, d}, e};

Delete[ x, {{1}} ] === {{{b, c}, d}, e}

Delete[ x, {{2}} ] === {a, e}

Delete[ x, {{1}, {2}} ] === {e}

Delete[ x, {{2, 1}} ] === {a, {d}, e}

Delete[ x, {{2, 1, 1}} ] === {a, {{c}, d}, e}

Delete by Pattern

DeleteCases
Delete elements by Pattern DeleteCases 
DeleteCases[ {1, 3, 2, 4}, 2 ] === {1, 3, 4}

DeleteCases[ {a, b, c}, c ] === {a, b}

DeleteCases[ {a, f[3], c}, f[3] ] === {a, c}

DeleteCases[ {a, f[3], g[4], f[5]}, f[_] ] === {a, g[4]}

DeleteCases[ {{2}, {4, 8}, {c}}, {_ , _} ] === {{2}, {c}}

Delete Duplicates

DeleteDuplicates[expr]
Delete duplicates DeleteDuplicates 
DeleteDuplicates[ {3, x, 3, x, 4 } ] === {3, x, 4}

the Nothing element

Nothing
when a list element is changed to Nothing, it is removed in a list. Nothing 
{3, Nothing, 4} === {3, 4}

The Nothing is not removed when Head is not List. 

f[3, Nothing, 4]
(* f[3, Nothing, 4] *)

This is sometimes very convenient way to delete element, especially when you are mapping a function over a list. [see Map Function to List] 

(* remove odd by replace it with Nothing *)
Map[
 If[ OddQ[ # ] , Nothing , # ]&,
 {1, 2, 3, 4, 5, 6}
 ] ===
{2, 4, 6}

List

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