# Difference between revisions of "Extracting Unique Elements From a List"

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(buh!? This certainly isn't the best solution for preserving any order, as it clearly doesn't preserve the order.) |
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There are several possible solutions for this problem. Here are some of these: | There are several possible solutions for this problem. Here are some of these: | ||

− | Using lists:usort | + | ===Using lists:usort=== |

The lists module contains a wealth of list processing functionality. One possible solution to this problem is to use the lists:usort function, which takes a list and returns a sorted copy of the original list, with all duplicates removed: | The lists module contains a wealth of list processing functionality. One possible solution to this problem is to use the lists:usort function, which takes a list and returns a sorted copy of the original list, with all duplicates removed: | ||

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</code> | </code> | ||

− | Using the sets Module | + | ===Using the sets Module=== |

Erlang standard libraries includes a module, sets, with a variety of functions related to generating, creating, and manipulating mathematical sets. | Erlang standard libraries includes a module, sets, with a variety of functions related to generating, creating, and manipulating mathematical sets. | ||

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Note that sets:to_list(sets:from_list(L)) produces an unreliably arranged list. | Note that sets:to_list(sets:from_list(L)) produces an unreliably arranged list. | ||

− | Using a General Balanced Tree Set (gb_set) | + | ===Using a General Balanced Tree Set (gb_set)=== |

Erlang's standard libraries includes an implementation of Professor Arne Andersson's General Balanced Trees. These structures are more costly than sorting lists for small sets, but this is a much more efficient implementation when working with large sets of data. | Erlang's standard libraries includes an implementation of Professor Arne Andersson's General Balanced Trees. These structures are more costly than sorting lists for small sets, but this is a much more efficient implementation when working with large sets of data. |

## Revision as of 06:17, 9 March 2007

## Contents |

## Problem

You want to eliminate duplicate values from a list.

## Solution

There are several possible solutions for this problem. Here are some of these:

### Using lists:usort

The lists module contains a wealth of list processing functionality. One possible solution to this problem is to use the lists:usort function, which takes a list and returns a sorted copy of the original list, with all duplicates removed:

1> UL = [1,2,8,7,8,10,3,12,3,99,188,3,2,1,3,5,15,72]. [1,2,8,7,8,10,3,12,3,99,188,3,2,1,3,5,15,72] 2> lists:usort(UL). [1,2,3,5,7,8,10,12,15,72,99,188]

### Using the sets Module

Erlang standard libraries includes a module, sets, with a variety of functions related to generating, creating, and manipulating mathematical sets.

10> Set = sets:from_list(UL). {sets,12, 16, 16, 8, 80, 48, {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]}, {{[],[99,3],[],[],"\274\f",[15],[2],[5],"H\b",[],[],[1],[],[7],"\n",[]}}} 11> sets:to_list(Set). [3,99,12,188,15,2,5,8,72,1,7,10]

Note that sets:to_list(sets:from_list(L)) produces an unreliably arranged list.

### Using a General Balanced Tree Set (gb_set)

Erlang's standard libraries includes an implementation of Professor Arne Andersson's General Balanced Trees. These structures are more costly than sorting lists for small sets, but this is a much more efficient implementation when working with large sets of data.

The gb_set:from_list function will produce an ordered set of elements (dropping duplicates). The set can then be extracted back to a list for other use:

3> GBSet = gb_sets:from_list(UL). {12, {10, {5,{2,{1,nil,nil},{3,nil,nil}},{8,{7,nil,nil},nil}}, {72,{15,{12,nil,nil},nil},{188,{99,nil,nil},nil}}}} 4> gb_sets:to_list(GBSet). [1,2,3,5,7,8,10,12,15,72,99,188]