How do you copy the contents of an array to a std vector in C without looping

Question

I have an array of values that is passed to my function from a different part of the program that I need to store for later processing   Since I don t know how many times my function will be called before it is time to process the data  I need a dynamic storage structure  so I chose a std  vector   I don t want to have to do the standard loop to push back all the values individually  it would be nice if I could just copy it all using something similar to memcpy

User · Answer

In addition to the methods presented above, you need to make sure you use either std::Vector.reserve(), std::Vector.resize(), or construct the vector to size, to make sure your vector has enough elements in it to hold your data. if not, you will corrupt memory. This is true of either std::copy() or memcpy().

This is the reason to use vector.push_back(), you can't write past the end of the vector.

User · Answer

If you can construct the vector after you ve gotten the array and array size  you can just say   std  vector lt ValueType gt  vec a  a   n        assuming a is your array and n is the number of elements it contains   Otherwise  std  copy   w resize   will do the trick   I d stay away from memcpy   unless you can be sure that the values are plain-old data  POD  types   Also  worth noting that none of these really avoids the for loop--it s just a question of whether you have to see it in your code or not   O n  runtime performance is unavoidable for copying the values   Finally  note that C-style arrays are perfectly valid containers for most STL algorithms--the raw pointer is equivalent to begin    and  ptr   n  is equivalent to end

User · Answer

Since I can only edit my own answer  I m going to make a composite answer from the other answers to my question   Thanks to all of you who answered   Using std  copy  this still iterates in the background  but you don t have to type out the code   int foo int  data  int size       static std  vector lt int gt  my data    normally a class variable    std  copy data  data   size  std  back inserter my data       return 0      Using regular memcpy   This is probably best used for basic data types  i e  int  but not for more complex arrays of structs or classes   vector lt int gt  x size   memcpy  amp x 0   source  size sizeof int

User · Answer

Assuming you know how big the item in the vector are    std  vector lt int gt  myArray  myArray resize  item count  0   memcpy   amp myArray front    source  item count   sizeof int      http   www cppreference com wiki stl vector start

User · Answer

Since I can only edit my own answer  I m going to make a composite answer from the other answers to my question   Thanks to all of you who answered   Using std  copy  this still iterates in the background  but you don t have to type out the code   int foo int  data  int size       static std  vector lt int gt  my data    normally a class variable    std  copy data  data   size  std  back inserter my data       return 0      Using regular memcpy   This is probably best used for basic data types  i e  int  but not for more complex arrays of structs or classes   vector lt int gt  x size   memcpy  amp x 0   source  size sizeof int

User · Answer

Assuming you know how big the item in the vector are    std  vector lt int gt  myArray  myArray resize  item count  0   memcpy   amp myArray front    source  item count   sizeof int      http   www cppreference com wiki stl vector start

User · Answer

avoid the memcpy  I say  No reason to mess with pointer operations unless you really have to  Also  it will only work for POD types  like int  but would fail if you re dealing with types that require construction

User · Answer

If all you are doing is replacing the existing data  then you can do this  std  vector lt int gt  data     evil global     void CopyData int  newData  size t count       data assign newData  newData   count

User · Answer

If you can construct the vector after you ve gotten the array and array size  you can just say   std  vector lt ValueType gt  vec a  a   n        assuming a is your array and n is the number of elements it contains   Otherwise  std  copy   w resize   will do the trick   I d stay away from memcpy   unless you can be sure that the values are plain-old data  POD  types   Also  worth noting that none of these really avoids the for loop--it s just a question of whether you have to see it in your code or not   O n  runtime performance is unavoidable for copying the values   Finally  note that C-style arrays are perfectly valid containers for most STL algorithms--the raw pointer is equivalent to begin    and  ptr   n  is equivalent to end

User · Answer

avoid the memcpy  I say  No reason to mess with pointer operations unless you really have to  Also  it will only work for POD types  like int  but would fail if you re dealing with types that require construction

User · Answer

std  copy is what you re looking for

User · Answer

Assuming you know how big the item in the vector are    std  vector lt int gt  myArray  myArray resize  item count  0   memcpy   amp myArray front    source  item count   sizeof int      http   www cppreference com wiki stl vector start

User · Answer

If all you are doing is replacing the existing data  then you can do this  std  vector lt int gt  data     evil global     void CopyData int  newData  size t count       data assign newData  newData   count

User · Answer

Assuming you know how big the item in the vector are    std  vector lt int gt  myArray  myArray resize  item count  0   memcpy   amp myArray front    source  item count   sizeof int      http   www cppreference com wiki stl vector start

User · Answer

avoid the memcpy  I say  No reason to mess with pointer operations unless you really have to  Also  it will only work for POD types  like int  but would fail if you re dealing with types that require construction

User · Answer

If you can construct the vector after you ve gotten the array and array size  you can just say   std  vector lt ValueType gt  vec a  a   n        assuming a is your array and n is the number of elements it contains   Otherwise  std  copy   w resize   will do the trick   I d stay away from memcpy   unless you can be sure that the values are plain-old data  POD  types   Also  worth noting that none of these really avoids the for loop--it s just a question of whether you have to see it in your code or not   O n  runtime performance is unavoidable for copying the values   Finally  note that C-style arrays are perfectly valid containers for most STL algorithms--the raw pointer is equivalent to begin    and  ptr   n  is equivalent to end

User · Answer

In addition to the methods presented above, you need to make sure you use either std::Vector.reserve(), std::Vector.resize(), or construct the vector to size, to make sure your vector has enough elements in it to hold your data. if not, you will corrupt memory. This is true of either std::copy() or memcpy().

This is the reason to use vector.push_back(), you can't write past the end of the vector.

User · Answer

Since I can only edit my own answer  I m going to make a composite answer from the other answers to my question   Thanks to all of you who answered   Using std  copy  this still iterates in the background  but you don t have to type out the code   int foo int  data  int size       static std  vector lt int gt  my data    normally a class variable    std  copy data  data   size  std  back inserter my data       return 0      Using regular memcpy   This is probably best used for basic data types  i e  int  but not for more complex arrays of structs or classes   vector lt int gt  x size   memcpy  amp x 0   source  size sizeof int

User · Answer

There have been many answers here and just about all of them will get the job done     However there is some misleading advice   Here are the options   vector lt int gt  dataVec   int dataArray       1  2  3  4  5  6  7  8  9  10    unsigned dataArraySize   sizeof dataArray    sizeof int       Method 1  Copy the array to the vector using back inserter        copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 2  Same as 1 but pre-extend the vector by the size of the array using reserve       dataVec reserve dataVec size     dataArraySize       copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 3  Memcpy       dataVec resize dataVec size     dataArraySize       memcpy  amp dataVec dataVec size   - dataArraySize    amp dataArray 0   dataArraySize   sizeof int          Method 4  vector  insert       dataVec insert dataVec end     amp dataArray 0    amp dataArray dataArraySize          Method 5  vector   vector       vector lt int gt  dataVec2  amp dataArray 0    amp dataArray dataArraySize        dataVec insert dataVec end    dataVec2 begin    dataVec2 end         To cut a long story short Method 4  using vector  insert  is the best for bsruth s scenario     Here are some gory details   Method 1 is probably the easiest to understand   Just copy each element from the array and push it into the back of the vector   Alas  it s slow   Because there s a loop  implied with the copy function   each element must be treated individually  no performance improvements can be made based on the fact that we know the array and vectors are contiguous blocks   Method 2 is a suggested performance improvement to Method 1  just pre-reserve the size of the array before adding it   For large arrays this might help   However the best advice here is never to use reserve unless profiling suggests you may be able to get an improvement  or you need to ensure your iterators are not going to be invalidated    Bjarne agrees   Incidentally  I found that this method performed the slowest most of the time though I m struggling to comprehensively explain why it was regularly significantly slower than method 1     Method 3 is the old school solution - throw some C at the problem   Works fine and fast for POD types   In this case resize is required to be called since memcpy works outside the bounds of vector and there is no way to tell a vector that its size has changed   Apart from being an ugly solution  byte copying   remember that this can only be used for POD types   I would never use this solution   Method 4 is the best way to go   It s meaning is clear  it s  usually  the fastest and it works for any objects   There is no downside to using this method for this application   Method 5 is a tweak on Method 4 - copy the array into a vector and then append it   Good option - generally fast-ish and clear     Finally  you are aware that you can use vectors in place of arrays  right   Even when a function expects c-style arrays you can use vectors   vector lt char gt  v 50      Ensure there s enough space strcpy  amp v 0    prefer vectors to c arrays      Hope that helps someone out there

User · Answer

Yet another answer  since the person said  I don t know how many times my function will be called   you could use the vector insert method like so to append arrays of values to the end of the vector   vector lt int gt  x   void AddValues int  values  size t size       x insert x end    values  values size       I like this way because the implementation of the vector should be able to optimize for the best way to insert the values based on the iterator type and the type itself   You are somewhat replying on the implementation of stl   If you need to guarantee the fastest speed and you know your type is a POD type then I would recommend the resize method in Thomas s answer   vector lt int gt  x   void AddValues int  values  size t size       size t old size x size        x resize old size   size  0      memcpy  amp x old size   values  size   sizeof int

User · Answer

In addition to the methods presented above, you need to make sure you use either std::Vector.reserve(), std::Vector.resize(), or construct the vector to size, to make sure your vector has enough elements in it to hold your data. if not, you will corrupt memory. This is true of either std::copy() or memcpy().

This is the reason to use vector.push_back(), you can't write past the end of the vector.

User · Answer

int dataArray       1  2  3  4  5  6  7  8  9  10     source  unsigned dataArraySize   sizeof dataArray    sizeof int    std  vector lt int gt  myvector  dataArraySize     target  std  copy   myints  myints dataArraySize   myvector begin         myvector now has 1 2 3    10  -

User · Answer

std  copy is what you re looking for

User · Answer

Yet another answer  since the person said  I don t know how many times my function will be called   you could use the vector insert method like so to append arrays of values to the end of the vector   vector lt int gt  x   void AddValues int  values  size t size       x insert x end    values  values size       I like this way because the implementation of the vector should be able to optimize for the best way to insert the values based on the iterator type and the type itself   You are somewhat replying on the implementation of stl   If you need to guarantee the fastest speed and you know your type is a POD type then I would recommend the resize method in Thomas s answer   vector lt int gt  x   void AddValues int  values  size t size       size t old size x size        x resize old size   size  0      memcpy  amp x old size   values  size   sizeof int

User · Answer

There have been many answers here and just about all of them will get the job done     However there is some misleading advice   Here are the options   vector lt int gt  dataVec   int dataArray       1  2  3  4  5  6  7  8  9  10    unsigned dataArraySize   sizeof dataArray    sizeof int       Method 1  Copy the array to the vector using back inserter        copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 2  Same as 1 but pre-extend the vector by the size of the array using reserve       dataVec reserve dataVec size     dataArraySize       copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 3  Memcpy       dataVec resize dataVec size     dataArraySize       memcpy  amp dataVec dataVec size   - dataArraySize    amp dataArray 0   dataArraySize   sizeof int          Method 4  vector  insert       dataVec insert dataVec end     amp dataArray 0    amp dataArray dataArraySize          Method 5  vector   vector       vector lt int gt  dataVec2  amp dataArray 0    amp dataArray dataArraySize        dataVec insert dataVec end    dataVec2 begin    dataVec2 end         To cut a long story short Method 4  using vector  insert  is the best for bsruth s scenario     Here are some gory details   Method 1 is probably the easiest to understand   Just copy each element from the array and push it into the back of the vector   Alas  it s slow   Because there s a loop  implied with the copy function   each element must be treated individually  no performance improvements can be made based on the fact that we know the array and vectors are contiguous blocks   Method 2 is a suggested performance improvement to Method 1  just pre-reserve the size of the array before adding it   For large arrays this might help   However the best advice here is never to use reserve unless profiling suggests you may be able to get an improvement  or you need to ensure your iterators are not going to be invalidated    Bjarne agrees   Incidentally  I found that this method performed the slowest most of the time though I m struggling to comprehensively explain why it was regularly significantly slower than method 1     Method 3 is the old school solution - throw some C at the problem   Works fine and fast for POD types   In this case resize is required to be called since memcpy works outside the bounds of vector and there is no way to tell a vector that its size has changed   Apart from being an ugly solution  byte copying   remember that this can only be used for POD types   I would never use this solution   Method 4 is the best way to go   It s meaning is clear  it s  usually  the fastest and it works for any objects   There is no downside to using this method for this application   Method 5 is a tweak on Method 4 - copy the array into a vector and then append it   Good option - generally fast-ish and clear     Finally  you are aware that you can use vectors in place of arrays  right   Even when a function expects c-style arrays you can use vectors   vector lt char gt  v 50      Ensure there s enough space strcpy  amp v 0    prefer vectors to c arrays      Hope that helps someone out there

User · Answer

There have been many answers here and just about all of them will get the job done     However there is some misleading advice   Here are the options   vector lt int gt  dataVec   int dataArray       1  2  3  4  5  6  7  8  9  10    unsigned dataArraySize   sizeof dataArray    sizeof int       Method 1  Copy the array to the vector using back inserter        copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 2  Same as 1 but pre-extend the vector by the size of the array using reserve       dataVec reserve dataVec size     dataArraySize       copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 3  Memcpy       dataVec resize dataVec size     dataArraySize       memcpy  amp dataVec dataVec size   - dataArraySize    amp dataArray 0   dataArraySize   sizeof int          Method 4  vector  insert       dataVec insert dataVec end     amp dataArray 0    amp dataArray dataArraySize          Method 5  vector   vector       vector lt int gt  dataVec2  amp dataArray 0    amp dataArray dataArraySize        dataVec insert dataVec end    dataVec2 begin    dataVec2 end         To cut a long story short Method 4  using vector  insert  is the best for bsruth s scenario     Here are some gory details   Method 1 is probably the easiest to understand   Just copy each element from the array and push it into the back of the vector   Alas  it s slow   Because there s a loop  implied with the copy function   each element must be treated individually  no performance improvements can be made based on the fact that we know the array and vectors are contiguous blocks   Method 2 is a suggested performance improvement to Method 1  just pre-reserve the size of the array before adding it   For large arrays this might help   However the best advice here is never to use reserve unless profiling suggests you may be able to get an improvement  or you need to ensure your iterators are not going to be invalidated    Bjarne agrees   Incidentally  I found that this method performed the slowest most of the time though I m struggling to comprehensively explain why it was regularly significantly slower than method 1     Method 3 is the old school solution - throw some C at the problem   Works fine and fast for POD types   In this case resize is required to be called since memcpy works outside the bounds of vector and there is no way to tell a vector that its size has changed   Apart from being an ugly solution  byte copying   remember that this can only be used for POD types   I would never use this solution   Method 4 is the best way to go   It s meaning is clear  it s  usually  the fastest and it works for any objects   There is no downside to using this method for this application   Method 5 is a tweak on Method 4 - copy the array into a vector and then append it   Good option - generally fast-ish and clear     Finally  you are aware that you can use vectors in place of arrays  right   Even when a function expects c-style arrays you can use vectors   vector lt char gt  v 50      Ensure there s enough space strcpy  amp v 0    prefer vectors to c arrays      Hope that helps someone out there

User · Answer

Since I can only edit my own answer  I m going to make a composite answer from the other answers to my question   Thanks to all of you who answered   Using std  copy  this still iterates in the background  but you don t have to type out the code   int foo int  data  int size       static std  vector lt int gt  my data    normally a class variable    std  copy data  data   size  std  back inserter my data       return 0      Using regular memcpy   This is probably best used for basic data types  i e  int  but not for more complex arrays of structs or classes   vector lt int gt  x size   memcpy  amp x 0   source  size sizeof int

User · Answer

If all you are doing is replacing the existing data  then you can do this  std  vector lt int gt  data     evil global     void CopyData int  newData  size t count       data assign newData  newData   count

User · Answer

std  copy is what you re looking for

User · Answer

std  copy is what you re looking for

User · Answer

avoid the memcpy  I say  No reason to mess with pointer operations unless you really have to  Also  it will only work for POD types  like int  but would fail if you re dealing with types that require construction

User · Answer

Yet another answer  since the person said  I don t know how many times my function will be called   you could use the vector insert method like so to append arrays of values to the end of the vector   vector lt int gt  x   void AddValues int  values  size t size       x insert x end    values  values size       I like this way because the implementation of the vector should be able to optimize for the best way to insert the values based on the iterator type and the type itself   You are somewhat replying on the implementation of stl   If you need to guarantee the fastest speed and you know your type is a POD type then I would recommend the resize method in Thomas s answer   vector lt int gt  x   void AddValues int  values  size t size       size t old size x size        x resize old size   size  0      memcpy  amp x old size   values  size   sizeof int

User · Answer

Yet another answer  since the person said  I don t know how many times my function will be called   you could use the vector insert method like so to append arrays of values to the end of the vector   vector lt int gt  x   void AddValues int  values  size t size       x insert x end    values  values size       I like this way because the implementation of the vector should be able to optimize for the best way to insert the values based on the iterator type and the type itself   You are somewhat replying on the implementation of stl   If you need to guarantee the fastest speed and you know your type is a POD type then I would recommend the resize method in Thomas s answer   vector lt int gt  x   void AddValues int  values  size t size       size t old size x size        x resize old size   size  0      memcpy  amp x old size   values  size   sizeof int

User · Answer

If all you are doing is replacing the existing data  then you can do this  std  vector lt int gt  data     evil global     void CopyData int  newData  size t count       data assign newData  newData   count

User · Answer

In addition to the methods presented above, you need to make sure you use either std::Vector.reserve(), std::Vector.resize(), or construct the vector to size, to make sure your vector has enough elements in it to hold your data. if not, you will corrupt memory. This is true of either std::copy() or memcpy().

This is the reason to use vector.push_back(), you can't write past the end of the vector.

User · Answer

If you can construct the vector after you ve gotten the array and array size  you can just say   std  vector lt ValueType gt  vec a  a   n        assuming a is your array and n is the number of elements it contains   Otherwise  std  copy   w resize   will do the trick   I d stay away from memcpy   unless you can be sure that the values are plain-old data  POD  types   Also  worth noting that none of these really avoids the for loop--it s just a question of whether you have to see it in your code or not   O n  runtime performance is unavoidable for copying the values   Finally  note that C-style arrays are perfectly valid containers for most STL algorithms--the raw pointer is equivalent to begin    and  ptr   n  is equivalent to end

User · Answer

There have been many answers here and just about all of them will get the job done     However there is some misleading advice   Here are the options   vector lt int gt  dataVec   int dataArray       1  2  3  4  5  6  7  8  9  10    unsigned dataArraySize   sizeof dataArray    sizeof int       Method 1  Copy the array to the vector using back inserter        copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 2  Same as 1 but pre-extend the vector by the size of the array using reserve       dataVec reserve dataVec size     dataArraySize       copy  amp dataArray 0    amp dataArray dataArraySize   back inserter dataVec          Method 3  Memcpy       dataVec resize dataVec size     dataArraySize       memcpy  amp dataVec dataVec size   - dataArraySize    amp dataArray 0   dataArraySize   sizeof int          Method 4  vector  insert       dataVec insert dataVec end     amp dataArray 0    amp dataArray dataArraySize          Method 5  vector   vector       vector lt int gt  dataVec2  amp dataArray 0    amp dataArray dataArraySize        dataVec insert dataVec end    dataVec2 begin    dataVec2 end         To cut a long story short Method 4  using vector  insert  is the best for bsruth s scenario     Here are some gory details   Method 1 is probably the easiest to understand   Just copy each element from the array and push it into the back of the vector   Alas  it s slow   Because there s a loop  implied with the copy function   each element must be treated individually  no performance improvements can be made based on the fact that we know the array and vectors are contiguous blocks   Method 2 is a suggested performance improvement to Method 1  just pre-reserve the size of the array before adding it   For large arrays this might help   However the best advice here is never to use reserve unless profiling suggests you may be able to get an improvement  or you need to ensure your iterators are not going to be invalidated    Bjarne agrees   Incidentally  I found that this method performed the slowest most of the time though I m struggling to comprehensively explain why it was regularly significantly slower than method 1     Method 3 is the old school solution - throw some C at the problem   Works fine and fast for POD types   In this case resize is required to be called since memcpy works outside the bounds of vector and there is no way to tell a vector that its size has changed   Apart from being an ugly solution  byte copying   remember that this can only be used for POD types   I would never use this solution   Method 4 is the best way to go   It s meaning is clear  it s  usually  the fastest and it works for any objects   There is no downside to using this method for this application   Method 5 is a tweak on Method 4 - copy the array into a vector and then append it   Good option - generally fast-ish and clear     Finally  you are aware that you can use vectors in place of arrays  right   Even when a function expects c-style arrays you can use vectors   vector lt char gt  v 50      Ensure there s enough space strcpy  amp v 0    prefer vectors to c arrays      Hope that helps someone out there

User · Answer

int dataArray       1  2  3  4  5  6  7  8  9  10     source  unsigned dataArraySize   sizeof dataArray    sizeof int    std  vector lt int gt  myvector  dataArraySize     target  std  copy   myints  myints dataArraySize   myvector begin         myvector now has 1 2 3    10  -

[c++] How do you copy the contents of an array to a std::vector in C++ without looping?

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