How do I sort a vector of pairs based on the second element of the pair

Question

If I have a vector of pairs  std  vector lt std  pair lt int  int gt   gt  vec   Is there and easy way to sort the list in increasing order based on the second element of the pair  I know I can write a little function object that will do the work  but is there a way to use existing parts of the STL and std  less to do the work directly  EDIT   I understand that I can write a separate function or class to pass to the third argument to sort   The question is whether or not I can build it out of standard stuff   I d really something that looks like  std  sort vec begin    vec end    std  something magic lt int  int  std  less gt

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For something reusable   template lt template  lt typename gt  class P   std  less  gt  struct compare pair second       template lt class T1  class T2 gt  bool operator   const std  pair lt T1  T2 gt  amp  left  const std  pair lt T1  T2 gt  amp  right            return P lt T2 gt    left second  right second              You can use it as  std  sort foo begin    foo end    compare pair second lt  gt        or  std  sort foo begin    foo end    compare pair second lt std  less gt

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You can use boost like this   std  sort a begin    a end               boost  bind  amp std  pair lt int  int gt   second   1   lt            boost  bind  amp std  pair lt int  int gt   second   2      I don t know a standard way to do this equally short and concise  but you can grab boost  bind it s all consisting of headers

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With C  0x we can use lambda functions   using namespace std  vector lt pair lt int  int gt  gt  v                      sort v begin    v end            const pair lt int  int gt  amp  lhs  const pair lt int  int gt  amp  rhs                 return lhs second  lt  rhs second         In this example the return type bool is implicitly deduced   Lambda return types  When a lambda-function has a single statement  and this is a return-statement  the compiler can deduce the return type  From C  11    5 1 2 4                  If the compound-statement is of the form   return expression     the type of the returned expression after lvalue-to-rvalue conversion  4 1   array-to-pointer conversion  4 2   and function-to-pointer conversion  4 3     otherwise  void       To explicitly specify the return type use the form      - gt  Type      like in   sort v begin    v end            const pair lt int  int gt  amp  lhs  const pair lt int  int gt  amp  rhs  - gt  bool                if  lhs second    0                   return true               return lhs second  lt  rhs second

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For something reusable   template lt template  lt typename gt  class P   std  less  gt  struct compare pair second       template lt class T1  class T2 gt  bool operator   const std  pair lt T1  T2 gt  amp  left  const std  pair lt T1  T2 gt  amp  right            return P lt T2 gt    left second  right second              You can use it as  std  sort foo begin    foo end    compare pair second lt  gt        or  std  sort foo begin    foo end    compare pair second lt std  less gt

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You d have to rely on a non standard select2nd

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With C  0x we can use lambda functions   using namespace std  vector lt pair lt int  int gt  gt  v                      sort v begin    v end            const pair lt int  int gt  amp  lhs  const pair lt int  int gt  amp  rhs                 return lhs second  lt  rhs second         In this example the return type bool is implicitly deduced   Lambda return types  When a lambda-function has a single statement  and this is a return-statement  the compiler can deduce the return type  From C  11    5 1 2 4                  If the compound-statement is of the form   return expression     the type of the returned expression after lvalue-to-rvalue conversion  4 1   array-to-pointer conversion  4 2   and function-to-pointer conversion  4 3     otherwise  void       To explicitly specify the return type use the form      - gt  Type      like in   sort v begin    v end            const pair lt int  int gt  amp  lhs  const pair lt int  int gt  amp  rhs  - gt  bool                if  lhs second    0                   return true               return lhs second  lt  rhs second

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Try swapping the elements of the pairs so you can use std  sort   as normal

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You d have to rely on a non standard select2nd

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EDIT  using c  14  the best solution is very easy to write thanks to lambdas that can now have parameters of type auto  This is my current favorite solution  std  sort v begin    v end       auto  amp left  auto  amp right        return left second  lt  right second          Just use a custom comparator  it s an optional 3rd argument to std  sort   struct sort pred       bool operator   const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right            return left second  lt  right second            std  sort v begin    v end    sort pred       If you re using a C  11 compiler  you can write the same using lambdas   std  sort v begin    v end       const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right        return left second  lt  right second        EDIT  in response to your edits to your question  here s some thoughts     if you really wanna be creative and be able to reuse this concept a lot  just make a template   template  lt class T1  class T2  class Pred   std  less lt T2 gt   gt  struct sort pair second       bool operator   const std  pair lt T1 T2 gt  amp left  const std  pair lt T1 T2 gt  amp right            Pred p          return p left second  right second              then you can do this too   std  sort v begin    v end    sort pair second lt int  int gt        or even  std  sort v begin    v end    sort pair second lt int  int  std  greater lt int gt   gt        Though to be honest  this is all a bit overkill  just write the 3 line function and be done with it  -P

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You can use boost like this   std  sort a begin    a end               boost  bind  amp std  pair lt int  int gt   second   1   lt            boost  bind  amp std  pair lt int  int gt   second   2      I don t know a standard way to do this equally short and concise  but you can grab boost  bind it s all consisting of headers

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EDIT  using c  14  the best solution is very easy to write thanks to lambdas that can now have parameters of type auto  This is my current favorite solution  std  sort v begin    v end       auto  amp left  auto  amp right        return left second  lt  right second          Just use a custom comparator  it s an optional 3rd argument to std  sort   struct sort pred       bool operator   const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right            return left second  lt  right second            std  sort v begin    v end    sort pred       If you re using a C  11 compiler  you can write the same using lambdas   std  sort v begin    v end       const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right        return left second  lt  right second        EDIT  in response to your edits to your question  here s some thoughts     if you really wanna be creative and be able to reuse this concept a lot  just make a template   template  lt class T1  class T2  class Pred   std  less lt T2 gt   gt  struct sort pair second       bool operator   const std  pair lt T1 T2 gt  amp left  const std  pair lt T1 T2 gt  amp right            Pred p          return p left second  right second              then you can do this too   std  sort v begin    v end    sort pair second lt int  int gt        or even  std  sort v begin    v end    sort pair second lt int  int  std  greater lt int gt   gt        Though to be honest  this is all a bit overkill  just write the 3 line function and be done with it  -P

User · Answer

For something reusable   template lt template  lt typename gt  class P   std  less  gt  struct compare pair second       template lt class T1  class T2 gt  bool operator   const std  pair lt T1  T2 gt  amp  left  const std  pair lt T1  T2 gt  amp  right            return P lt T2 gt    left second  right second              You can use it as  std  sort foo begin    foo end    compare pair second lt  gt        or  std  sort foo begin    foo end    compare pair second lt std  less gt

User · Answer

You d have to rely on a non standard select2nd

User · Answer

EDIT  using c  14  the best solution is very easy to write thanks to lambdas that can now have parameters of type auto  This is my current favorite solution  std  sort v begin    v end       auto  amp left  auto  amp right        return left second  lt  right second          Just use a custom comparator  it s an optional 3rd argument to std  sort   struct sort pred       bool operator   const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right            return left second  lt  right second            std  sort v begin    v end    sort pred       If you re using a C  11 compiler  you can write the same using lambdas   std  sort v begin    v end       const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right        return left second  lt  right second        EDIT  in response to your edits to your question  here s some thoughts     if you really wanna be creative and be able to reuse this concept a lot  just make a template   template  lt class T1  class T2  class Pred   std  less lt T2 gt   gt  struct sort pair second       bool operator   const std  pair lt T1 T2 gt  amp left  const std  pair lt T1 T2 gt  amp right            Pred p          return p left second  right second              then you can do this too   std  sort v begin    v end    sort pair second lt int  int gt        or even  std  sort v begin    v end    sort pair second lt int  int  std  greater lt int gt   gt        Though to be honest  this is all a bit overkill  just write the 3 line function and be done with it  -P

User · Answer

You can use boost like this   std  sort a begin    a end               boost  bind  amp std  pair lt int  int gt   second   1   lt            boost  bind  amp std  pair lt int  int gt   second   2      I don t know a standard way to do this equally short and concise  but you can grab boost  bind it s all consisting of headers

User · Answer

For something reusable   template lt template  lt typename gt  class P   std  less  gt  struct compare pair second       template lt class T1  class T2 gt  bool operator   const std  pair lt T1  T2 gt  amp  left  const std  pair lt T1  T2 gt  amp  right            return P lt T2 gt    left second  right second              You can use it as  std  sort foo begin    foo end    compare pair second lt  gt        or  std  sort foo begin    foo end    compare pair second lt std  less gt

User · Answer

Try swapping the elements of the pairs so you can use std  sort   as normal

User · Answer

Its pretty simple you use the sort function from algorithm and add your own compare function  vector lt  pair lt int int  gt   gt  v  sort v begin   v end   myComparison     Now you have to make the comparison based on the second selection  so declare you  myComparison  as  bool myComparison const pair lt int int gt   amp a const pair lt int int gt   amp b           return a second lt b second

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Its pretty simple you use the sort function from algorithm and add your own compare function  vector lt  pair lt int int  gt   gt  v  sort v begin   v end   myComparison     Now you have to make the comparison based on the second selection  so declare you  myComparison  as  bool myComparison const pair lt int int gt   amp a const pair lt int int gt   amp b           return a second lt b second

User · Answer

EDIT  using c  14  the best solution is very easy to write thanks to lambdas that can now have parameters of type auto  This is my current favorite solution  std  sort v begin    v end       auto  amp left  auto  amp right        return left second  lt  right second          Just use a custom comparator  it s an optional 3rd argument to std  sort   struct sort pred       bool operator   const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right            return left second  lt  right second            std  sort v begin    v end    sort pred       If you re using a C  11 compiler  you can write the same using lambdas   std  sort v begin    v end       const std  pair lt int int gt   amp left  const std  pair lt int int gt   amp right        return left second  lt  right second        EDIT  in response to your edits to your question  here s some thoughts     if you really wanna be creative and be able to reuse this concept a lot  just make a template   template  lt class T1  class T2  class Pred   std  less lt T2 gt   gt  struct sort pair second       bool operator   const std  pair lt T1 T2 gt  amp left  const std  pair lt T1 T2 gt  amp right            Pred p          return p left second  right second              then you can do this too   std  sort v begin    v end    sort pair second lt int  int gt        or even  std  sort v begin    v end    sort pair second lt int  int  std  greater lt int gt   gt        Though to be honest  this is all a bit overkill  just write the 3 line function and be done with it  -P

User · Answer

You d have to rely on a non standard select2nd

User · Answer

You can use boost like this   std  sort a begin    a end               boost  bind  amp std  pair lt int  int gt   second   1   lt            boost  bind  amp std  pair lt int  int gt   second   2      I don t know a standard way to do this equally short and concise  but you can grab boost  bind it s all consisting of headers

[c++] How do I sort a vector of pairs based on the second element of the pair?

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