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Diffstat (limited to 'system/doc/efficiency_guide/commoncaveats.xml')
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diff --git a/system/doc/efficiency_guide/commoncaveats.xml b/system/doc/efficiency_guide/commoncaveats.xml index c1d71e745a..854dd54660 100644 --- a/system/doc/efficiency_guide/commoncaveats.xml +++ b/system/doc/efficiency_guide/commoncaveats.xml @@ -169,53 +169,5 @@ multiple_setelement(T0) -> {Bin1,Bin2} = split_binary(Bin, Num)</code> </section> - <section> - <title>Operator "--"</title> - <p>The "<c>--</c>" operator has a complexity - proportional to the product of the length of its operands. - This means that the operator is very slow if both of its operands - are long lists:</p> - - <p><em>DO NOT</em></p> - <code type="none"><![CDATA[ - HugeList1 -- HugeList2]]></code> - - <p>Instead use the <seealso marker="stdlib:ordsets">ordsets</seealso> - module in STDLIB:</p> - - <p><em>DO</em></p> - <code type="none"> - HugeSet1 = ordsets:from_list(HugeList1), - HugeSet2 = ordsets:from_list(HugeList2), - ordsets:subtract(HugeSet1, HugeSet2)</code> - - <p>Obviously, that code does not work if the original order - of the list is important. If the order of the list must be - preserved, do as follows:</p> - - <p><em>DO</em></p> - <code type="none"><![CDATA[ - Set = gb_sets:from_list(HugeList2), - [E || E <- HugeList1, not gb_sets:is_element(E, Set)]]]></code> - - <note><p>This code behaves differently from "<c>--</c>" - if the lists contain duplicate elements (one occurrence - of an element in HugeList2 removes <em>all</em> - occurrences in HugeList1.)</p> - <p>Also, this code compares lists elements using the - "<c>==</c>" operator, while "<c>--</c>" uses the "<c>=:=</c>" operator. - If that difference is important, <c>sets</c> can be used instead of - <c>gb_sets</c>, but <c>sets:from_list/1</c> is much - slower than <c>gb_sets:from_list/1</c> for long lists.</p></note> - - <p>Using the "<c>--</c>" operator to delete an element - from a list is not a performance problem:</p> - - <p><em>OK</em></p> - <code type="none"> - HugeList1 -- [Element]</code> - - </section> - </chapter> |