How to convert vector to array

Question

How do I convert a std  vector lt double gt  to a double array

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vector lt double gt  thevector        double  thearray    amp thevector 0     This is guaranteed to work by the standard  however there are some caveats  in particular take care to only use thearray while thevector is in scope

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There s a fairly simple trick to do so  since the spec now guarantees vectors store their elements contiguously   std  vector lt double gt  v  double  a    amp v 0

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We can do this using data   method   C  11 provides this method   Code Snippet   include lt bits stdc   h gt  using namespace std    int main       ios  sync with stdio false      vector lt int gt v    7  8  9  10  11     int  arr   v data       for int i 0  i lt v size    i            cout lt  lt arr i  lt  lt             return 0

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Vectors effectively are arrays under the skin  If you have  a function   void f  double a       you can call it like this   vector  lt double gt  v  v push back  1 23   f   amp v 0       You should not ever need to convert a vector into an actual array instance

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std  vector lt double gt  vec  double  arr   vec data

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For C  11  vector data   will do the trick

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As to std  vector lt int gt  vec  vec to get int   you can use two method    int  arr    amp vec 0   int  arr   vec data      If you want to convert any type T vector to T  array  just replace the above int to T   I will show you why does the above two works  for good understanding   std  vector is a dynamic array essentially   Main data member as below   template  lt class T  class Alloc   allocator lt T gt  gt  class vector      public          typedef T          value type          typedef T          iterator          typedef T          pointer                        private          pointer start           pointer finish           pointer end of storage        public          vector   start  0   finish  0   end of storage  0                     The range  start   end of storage   is all the array memory the vector allocate   The range start   finish   is all the array memory the vector used   The range finish   end of storage   is the backup array memory   For example  as to a vector vec  which has  9  9  1  2  3  4  is pointer may like the below     So  amp vec 0    start   address    start  is equivalent to int  array head   In c  11  the data   member function just return start   pointer data           return start      equivalent to  value type    array head

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If you have a function  then you probably need this foo  amp array 0   array size      If you managed to get into a situation where you need an array then you need to refactor  vectors are basically extended arrays  you should always use them

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You can do some what like this  vector  lt int gt  id  vector  lt double gt  v   if id size    gt  0        for int i   0  i  lt  id size    i                  for int j   0  j  lt  id size    j                          double x   v i  j               cout  lt  lt  x  lt  lt  endl

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What for  You need to clarify  Do you need a pointer to the first element of an array  or an array   If you re calling an API function that expects the former  you can do do something  amp v 0   v size     where v is a vector of doubles  The elements of a vector are contiguous   Otherwise  you just have to copy each element   double arr 100   std  copy v begin    v end    arr     Ensure not only thar arr is big enough  but that arr gets filled up  or you have uninitialized values

[c++] How to convert vector to array

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