Erasing elements from a vector

Question

I want to clear a element from a vector using the erase method  But the problem here is that the element is not guaranteed to occur only once in the vector  It may be present multiple times and I need to clear all of them  My code is something like this   void erase std  vector lt int gt  amp  myNumbers in  int number in        std  vector lt int gt   iterator iter   myNumbers in begin        std  vector lt int gt   iterator endIter   myNumbers in end        for   iter    endIter    iter                if  iter    number in                        myNumbers in erase iter                      int main int argc  char  argv          std  vector lt int gt  myNmbers      for int i   0  i  lt  2    i                myNmbers push back i           myNmbers push back i              erase myNmbers  1        return 0      This code obviously crashes because I am changing the end of the vector while iterating through it  What is the best way to achieve this  I e  is there any way to do this without iterating through the vector multiple times or creating one more copy of the vector

User · Answer

To erase 1st element you can use   vector lt int gt  mV  1  2  3  4  5     vector lt int gt   iterator it    it   mV begin     mV erase it

User · Answer

You can iterate using the index access  To avoid O n 2  complexity you can use two indices  i - current testing index  j - index to store next item and at the end of the cycle new size of the vector    code   void erase std  vector lt int gt  amp  v  int num      size t j   0    for  size t i   0  i  lt  v size      i        if  v i     num  v j      v i            trim vector to new size   v resize j       In such case you have no invalidating of iterators  complexity is O n   and code is very concise and you don t need to write some helper classes  although in some case using helper classes can benefit in more flexible code   This code does not use erase method  but solves your task   Using pure stl you can do this in the following way  this is similar to the Motti s answer     include  lt algorithm gt   void erase std  vector lt int gt  amp  v  int num        vector lt int gt   iterator it   remove v begin    v end    num       v erase it  v end

User · Answer

Use the remove erase idiom   std  vector lt int gt  amp  vec   myNumbers     use shorter name vec erase std  remove vec begin    vec end    number in   vec end       What happens is that remove compacts the elements that differ from the value to be removed  number in  in the beginning of the vector and returns the iterator to the first element after that range  Then erase removes these elements  whose value is unspecified

User · Answer

Use the remove erase idiom   std  vector lt int gt  amp  vec   myNumbers     use shorter name vec erase std  remove vec begin    vec end    number in   vec end       What happens is that remove compacts the elements that differ from the value to be removed  number in  in the beginning of the vector and returns the iterator to the first element after that range  Then erase removes these elements  whose value is unspecified

User · Answer

Depending on why you are doing this  using a std  set might be a better idea than std  vector   It allows each element to occur only once  If you add it multiple times  there will only be one instance to erase anyway  This will make the erase operation trivial  The erase operation will also have lower time complexity than on the vector  however  adding elements is slower on the set so it might not be much of an advantage   This of course won t work if you are interested in how many times an element has been added to your vector or the order the elements were added

User · Answer

Depending on why you are doing this  using a std  set might be a better idea than std  vector   It allows each element to occur only once  If you add it multiple times  there will only be one instance to erase anyway  This will make the erase operation trivial  The erase operation will also have lower time complexity than on the vector  however  adding elements is slower on the set so it might not be much of an advantage   This of course won t work if you are interested in how many times an element has been added to your vector or the order the elements were added

User · Answer

Calling erase will invalidate iterators  you could use  void erase std  vector lt int gt  amp  myNumbers in  int number in        std  vector lt int gt   iterator iter   myNumbers in begin        while  iter    myNumbers in end                  if   iter    number in                        iter   myNumbers in erase iter                     else                        iter                      Or you could use std  remove if together with a functor and std  vector  erase  struct Eraser       Eraser int number in    number in number in         int number in      bool operator   int i  const               return i    number in            std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end    Eraser number in    myNumbers end      Instead of writing your own functor in this case you could use std  remove  std  vector lt int gt  myNumbers  myNumbers erase std  remove myNumbers begin    myNumbers end    number in   myNumbers end      In C  11 you could use a lambda instead of a functor  std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end     number in  int number   return number    number in      myNumbers end      In C  17 std  experimental  erase and std  experimental  erase if are also available  in C  20 these are  finally  renamed to std  erase and std  erase if  note  in Visual Studio 2019 you ll need to change your C   language version to the latest experimental version for support   std  vector lt int gt  myNumbers  std  erase if myNumbers  Eraser number in       or use lambda  or  std  vector lt int gt  myNumbers  std  erase myNumbers  number in

User · Answer

Use the remove erase idiom   std  vector lt int gt  amp  vec   myNumbers     use shorter name vec erase std  remove vec begin    vec end    number in   vec end       What happens is that remove compacts the elements that differ from the value to be removed  number in  in the beginning of the vector and returns the iterator to the first element after that range  Then erase removes these elements  whose value is unspecified

User · Answer

To erase 1st element you can use   vector lt int gt  mV  1  2  3  4  5     vector lt int gt   iterator it    it   mV begin     mV erase it

User · Answer

Use the remove erase idiom   std  vector lt int gt  amp  vec   myNumbers     use shorter name vec erase std  remove vec begin    vec end    number in   vec end       What happens is that remove compacts the elements that differ from the value to be removed  number in  in the beginning of the vector and returns the iterator to the first element after that range  Then erase removes these elements  whose value is unspecified

User · Answer

Depending on why you are doing this  using a std  set might be a better idea than std  vector   It allows each element to occur only once  If you add it multiple times  there will only be one instance to erase anyway  This will make the erase operation trivial  The erase operation will also have lower time complexity than on the vector  however  adding elements is slower on the set so it might not be much of an advantage   This of course won t work if you are interested in how many times an element has been added to your vector or the order the elements were added

User · Answer

Calling erase will invalidate iterators  you could use  void erase std  vector lt int gt  amp  myNumbers in  int number in        std  vector lt int gt   iterator iter   myNumbers in begin        while  iter    myNumbers in end                  if   iter    number in                        iter   myNumbers in erase iter                     else                        iter                      Or you could use std  remove if together with a functor and std  vector  erase  struct Eraser       Eraser int number in    number in number in         int number in      bool operator   int i  const               return i    number in            std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end    Eraser number in    myNumbers end      Instead of writing your own functor in this case you could use std  remove  std  vector lt int gt  myNumbers  myNumbers erase std  remove myNumbers begin    myNumbers end    number in   myNumbers end      In C  11 you could use a lambda instead of a functor  std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end     number in  int number   return number    number in      myNumbers end      In C  17 std  experimental  erase and std  experimental  erase if are also available  in C  20 these are  finally  renamed to std  erase and std  erase if  note  in Visual Studio 2019 you ll need to change your C   language version to the latest experimental version for support   std  vector lt int gt  myNumbers  std  erase if myNumbers  Eraser number in       or use lambda  or  std  vector lt int gt  myNumbers  std  erase myNumbers  number in

User · Answer

Depending on why you are doing this  using a std  set might be a better idea than std  vector   It allows each element to occur only once  If you add it multiple times  there will only be one instance to erase anyway  This will make the erase operation trivial  The erase operation will also have lower time complexity than on the vector  however  adding elements is slower on the set so it might not be much of an advantage   This of course won t work if you are interested in how many times an element has been added to your vector or the order the elements were added

User · Answer

You can iterate using the index access  To avoid O n 2  complexity you can use two indices  i - current testing index  j - index to store next item and at the end of the cycle new size of the vector    code   void erase std  vector lt int gt  amp  v  int num      size t j   0    for  size t i   0  i  lt  v size      i        if  v i     num  v j      v i            trim vector to new size   v resize j       In such case you have no invalidating of iterators  complexity is O n   and code is very concise and you don t need to write some helper classes  although in some case using helper classes can benefit in more flexible code   This code does not use erase method  but solves your task   Using pure stl you can do this in the following way  this is similar to the Motti s answer     include  lt algorithm gt   void erase std  vector lt int gt  amp  v  int num        vector lt int gt   iterator it   remove v begin    v end    num       v erase it  v end

User · Answer

You can iterate using the index access  To avoid O n 2  complexity you can use two indices  i - current testing index  j - index to store next item and at the end of the cycle new size of the vector    code   void erase std  vector lt int gt  amp  v  int num      size t j   0    for  size t i   0  i  lt  v size      i        if  v i     num  v j      v i            trim vector to new size   v resize j       In such case you have no invalidating of iterators  complexity is O n   and code is very concise and you don t need to write some helper classes  although in some case using helper classes can benefit in more flexible code   This code does not use erase method  but solves your task   Using pure stl you can do this in the following way  this is similar to the Motti s answer     include  lt algorithm gt   void erase std  vector lt int gt  amp  v  int num        vector lt int gt   iterator it   remove v begin    v end    num       v erase it  v end

User · Answer

Calling erase will invalidate iterators  you could use  void erase std  vector lt int gt  amp  myNumbers in  int number in        std  vector lt int gt   iterator iter   myNumbers in begin        while  iter    myNumbers in end                  if   iter    number in                        iter   myNumbers in erase iter                     else                        iter                      Or you could use std  remove if together with a functor and std  vector  erase  struct Eraser       Eraser int number in    number in number in         int number in      bool operator   int i  const               return i    number in            std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end    Eraser number in    myNumbers end      Instead of writing your own functor in this case you could use std  remove  std  vector lt int gt  myNumbers  myNumbers erase std  remove myNumbers begin    myNumbers end    number in   myNumbers end      In C  11 you could use a lambda instead of a functor  std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end     number in  int number   return number    number in      myNumbers end      In C  17 std  experimental  erase and std  experimental  erase if are also available  in C  20 these are  finally  renamed to std  erase and std  erase if  note  in Visual Studio 2019 you ll need to change your C   language version to the latest experimental version for support   std  vector lt int gt  myNumbers  std  erase if myNumbers  Eraser number in       or use lambda  or  std  vector lt int gt  myNumbers  std  erase myNumbers  number in

User · Answer

You can iterate using the index access  To avoid O n 2  complexity you can use two indices  i - current testing index  j - index to store next item and at the end of the cycle new size of the vector    code   void erase std  vector lt int gt  amp  v  int num      size t j   0    for  size t i   0  i  lt  v size      i        if  v i     num  v j      v i            trim vector to new size   v resize j       In such case you have no invalidating of iterators  complexity is O n   and code is very concise and you don t need to write some helper classes  although in some case using helper classes can benefit in more flexible code   This code does not use erase method  but solves your task   Using pure stl you can do this in the following way  this is similar to the Motti s answer     include  lt algorithm gt   void erase std  vector lt int gt  amp  v  int num        vector lt int gt   iterator it   remove v begin    v end    num       v erase it  v end

User · Answer

Calling erase will invalidate iterators  you could use  void erase std  vector lt int gt  amp  myNumbers in  int number in        std  vector lt int gt   iterator iter   myNumbers in begin        while  iter    myNumbers in end                  if   iter    number in                        iter   myNumbers in erase iter                     else                        iter                      Or you could use std  remove if together with a functor and std  vector  erase  struct Eraser       Eraser int number in    number in number in         int number in      bool operator   int i  const               return i    number in            std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end    Eraser number in    myNumbers end      Instead of writing your own functor in this case you could use std  remove  std  vector lt int gt  myNumbers  myNumbers erase std  remove myNumbers begin    myNumbers end    number in   myNumbers end      In C  11 you could use a lambda instead of a functor  std  vector lt int gt  myNumbers  myNumbers erase std  remove if myNumbers begin    myNumbers end     number in  int number   return number    number in      myNumbers end      In C  17 std  experimental  erase and std  experimental  erase if are also available  in C  20 these are  finally  renamed to std  erase and std  erase if  note  in Visual Studio 2019 you ll need to change your C   language version to the latest experimental version for support   std  vector lt int gt  myNumbers  std  erase if myNumbers  Eraser number in       or use lambda  or  std  vector lt int gt  myNumbers  std  erase myNumbers  number in

[c++] Erasing elements from a vector

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