How to sort an STL vector

Question

I would like to sort a vector   vector lt myClass gt  object    Where myclass contains many int variables  How can I sort my vector on any specific data variable of myClass

User · Answer

Like explained in other answers you need to provide a comparison function. If you would like to keep the definition of that function close to the sort call (e.g. if it only makes sense for this sort) you can define it right there with boost::lambda. Use boost::lambda::bind to call the member function.

To e.g. sort by member variable or function data1:

#include <algorithm>
#include <vector>
#include <boost/lambda/bind.hpp>
#include <boost/lambda/lambda.hpp>
using boost::lambda::bind;
using boost::lambda::_1;
using boost::lambda::_2;

std::vector<myclass> object(10000);
std::sort(object.begin(), object.end(),
    bind(&myclass::data1, _1) < bind(&myclass::data1, _2));

User · Answer

std  sort object begin    object end    pred       where  pred   is a function object defining the order on objects of myclass  Alternatively  you can define myclass  operator lt    For example  you can pass a lambda   std  sort object begin    object end                  myclass const amp  a  myclass const amp  b    return a v  lt  b v        Or if you re stuck with C  03  the function object approach  v is the member on which you want to sort    struct pred       bool operator   myclass const  amp  a  myclass const  amp  b  const           return a v  lt  b v

User · Answer

Overload less than operator  then sort  This is an example I found off the web     class MyData   public    int m iData    string m strSomeOtherData    bool operator lt  const MyData  amp rhs  const   return m iData  lt  rhs m iData        std  sort myvector begin    myvector end       Source  here

User · Answer

this is my approach to solve this generally  It extends the answer from Steve Jessop by removing the requirement to set template arguments explicitly and adding the option to also use functoins and pointers to methods  getters    include  lt vector gt   include  lt iostream gt   include  lt algorithm gt   include  lt string gt   include  lt functional gt   using namespace std   template  lt typename T  typename U gt  struct CompareByGetter       U  T   getter    const      CompareByGetter U  T   getter    const    getter getter          bool operator   const T  amp lhs  const T  amp rhs             lhs  getter     lt   rhs  getter               template  lt typename T  typename U gt  CompareByGetter lt T U gt  by U  T   getter    const        return CompareByGetter lt T U gt  getter           sort by template  lt typename T  typename U gt  struct CompareByMember       U T   field      CompareByMember U T   f    field f         bool operator   const T  amp lhs  const T  amp rhs            return lhs  field  lt  rhs  field            template  lt typename T  typename U gt  CompareByMember lt T U gt  by U T   f        return CompareByMember lt T U gt  f      template  lt typename T  typename U gt  struct CompareByFunction       function lt U T  gt  f      CompareByFunction function lt U T  gt  f    f f         bool operator   const T amp  a  const T amp  b  const           return f a   lt  f b             template  lt typename T  typename U gt  CompareByFunction lt T U gt  by function lt U T  gt  f        CompareByFunction lt T U gt  cmp f       return cmp     struct mystruct       double x y z      string name      double length   const           return sqrt  x x   y y   z z              ostream amp  operator lt  lt   ostream amp  os  const mystruct amp  ms        return os  lt  lt        lt  lt  ms x  lt  lt        lt  lt  ms y  lt  lt        lt  lt  ms z  lt  lt        lt  lt  ms name  lt  lt    len     lt  lt  ms length    lt  lt          template  lt class T gt  ostream amp  operator lt  lt   ostream amp  os  std  vector lt T gt  v        os  lt  lt           for  auto it   begin v   it    end v     it            if   it    begin v                  os  lt  lt                         os  lt  lt   it            os  lt  lt           return os     void sorting         vector lt mystruct gt  vec1      1 1 0  a     0 1 2  b     -1 -5 0  c     0 0 0  d           function lt string const mystruct amp   gt  f      const mystruct amp  v  return v name         cout  lt  lt   unsorted     lt  lt  vec1  lt  lt  endl      sort begin vec1   end vec1   by  amp mystruct  x         cout  lt  lt   sort by x    lt  lt  vec1  lt  lt  endl      sort begin vec1   end vec1   by  amp mystruct  length        cout  lt  lt   sort by len    lt  lt  vec1  lt  lt  endl      sort begin vec1   end vec1   by f         cout  lt  lt   sort by name    lt  lt  vec1  lt  lt  endl

User · Answer

A pointer-to-member allows you to write a single comparator  which can work with any data member of your class    include  lt algorithm gt   include  lt vector gt   include  lt string gt   include  lt iostream gt   template  lt typename T  typename U gt  struct CompareByMember          This is a pointer-to-member  it represents a member of class T        The data member has type U     U T   field      CompareByMember U T   f    field f         bool operator   const T  amp lhs  const T  amp rhs            return lhs  field  lt  rhs  field            struct Test       int a      int b      std  string c      Test int a  int b  std  string c    a a   b b   c c            for convenience  this just lets us print out a Test object std  ostream  amp operator lt  lt  std  ostream  amp o  const Test  amp t        return o  lt  lt  t c     int main         std  vector lt Test gt  vec      vec push back Test 1  10   y         vec push back Test 2  9   x             sort on the string field     std  sort vec begin    vec end             CompareByMember lt Test std  string gt   amp Test  c        std  cout  lt  lt   sorted by string field  c         std  cout  lt  lt  vec 0   lt  lt       lt  lt  vec 1   lt  lt    n           sort on the first integer field     std  sort vec begin    vec end             CompareByMember lt Test int gt   amp Test  a        std  cout  lt  lt   sorted by integer field  a         std  cout  lt  lt  vec 0   lt  lt       lt  lt  vec 1   lt  lt    n           sort on the second integer field     std  sort vec begin    vec end             CompareByMember lt Test int gt   amp Test  b        std  cout  lt  lt   sorted by integer field  b         std  cout  lt  lt  vec 0   lt  lt       lt  lt  vec 1   lt  lt    n       Output   sorted by string field  c  x y sorted by integer field  a  y x sorted by integer field  b  x y

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