How do I erase an element from std vector by index

Question

I have a std  vector lt int   and I want to delete the n th element  How do I do that   std  vector lt int gt  vec   vec push back 6   vec push back -17   vec push back 12    vec erase

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the fastest way  for programming contests by time complexity     constant    can erase 100M item in 1 second       vector lt int gt  it    vector lt int gt   iterator   amp vec pos       vec erase it      and most readable way   vec erase vec begin     pos

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Actually  the erase function works for two profiles    Removing a single element  iterator erase  iterator position    Removing a range of elements  iterator erase  iterator first  iterator last      Since std  vec begin   marks the start of container and if we want to delete the ith element in our vector  we can use   vec erase vec begin     index     If you look closely  vec begin   is just a pointer to the starting position of our vector and adding the value of i to it increments the pointer to i position  so instead we can access the pointer to the ith element by    amp vec i    So we can write   vec erase  amp vec i       To delete the ith element

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template  lt typename T gt  void remove std  vector lt T gt  amp  vec  size t pos        std  vector lt T gt   iterator it   vec begin        std  advance it  pos       vec erase it

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If you have an unordered vector you can take advantage of the fact that it s unordered and use something I saw from Dan Higgins at CPPCON  template lt  typename TContainer  gt  static bool EraseFromUnorderedByIndex  TContainer amp  inContainer  size t inIndex         if   inIndex  lt  inContainer size                   if   inIndex    inContainer size   - 1               inContainer inIndex    inContainer back            inContainer pop back            return true            return false      Since the list order doesn t matter  just take the last element in the list and copy it over the top of the item you want to remove  then pop and delete the last item

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To delete a single element  you could do   std  vector lt int gt  vec   vec push back 6   vec push back -17   vec push back 12       Deletes the second element  vec 1   vec erase vec begin     1     Or  to delete more than one element at once      Deletes the second through third elements  vec 1   vec 2   vec erase vec begin     1  vec begin     3

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The erase method on std  vector is overloaded  so it s probably clearer to call  vec erase vec begin     index     when you only want to erase a single element

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Erase an element with index   vec erase vec begin     index    Erase an element with value  vec erase find vec begin   vec end   value

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How about this   void squeeze vector lt int gt   amp v        int j   0      for  int i   1  i  lt  v size    i            if  v i     v j   amp  amp    j    i              v j    v i       v resize j   1

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To delete an element use the following way      declaring and assigning array1  std vector lt int gt  array1  0 2 3 4       erasing the value in the array array1 erase array1 begin   n     For a more broad overview you can visit  http   www cplusplus com reference vector vector erase

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here is one more way to do this if you want to delete a element by finding this with its value in vector you just need to do this on vector   vector lt int gt  ar n   ar erase remove ar begin    ar end      place your value here from vector array      it will remove your value from here  thanks

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It may seem obvious to some people  but to elaborate on the above answers  If you are doing removal of std  vector elements using erase in a loop over the whole vector  you should process your vector in reverse order  that is to say using for  int i   v size   - 1  i  gt   0  i--  instead of  the classical  for  int i   0  i  lt  v size    i    The reason is that indices are affected by erase so if you remove the 4-th element  then the former 5-th element is now the new 4-th element  and it won t be processed by your loop if you re doing i    Below is a simple example illustrating this where I want to remove all the odds element of an int vector   include  lt iostream gt   include  lt vector gt   using namespace std   void printVector const vector lt int gt   amp v        for  size t i   0  i  lt  v size    i                  cout  lt  lt  v i   lt  lt   quot   quot             cout  lt  lt  endl     int main             vector lt int gt  v1  v2      for  int i   0  i  lt  10  i                  v1 push back i           v2 push back i                 print v1     cout  lt  lt   quot v1   quot   lt  lt  endl      printVector v1            cout  lt  lt  endl              print v2     cout  lt  lt   quot v2   quot   lt  lt  endl      printVector v2               Erase all odd elements     cout  lt  lt   quot --- Erase odd elements --- quot   lt  lt  endl              loop with decreasing indices     cout  lt  lt   quot Process v2 with decreasing indices   quot   lt  lt  endl      for  int i   v2 size   - 1  i  gt   0  i--                if  v2 i    2    0                        cout  lt  lt   quot    quot               v2 erase v2 begin     i                     else                       cout  lt  lt  v2 i   lt  lt   quot   quot                       cout  lt  lt  endl      cout  lt  lt  endl              loop with increasing indices     cout  lt  lt   quot Process v1 with increasing indices   quot   lt  lt  endl      for  int i   0  i  lt  v1 size    i                  if  v1 i    2    0                        cout  lt  lt   quot    quot               v1 erase v1 begin     i                     else                       cout  lt  lt  v1 i   lt  lt   quot   quot                                 return 0     Output  v1  0 1 2 3 4 5 6 7 8 9  v2  0 1 2 3 4 5 6 7 8 9 --- Erase odd elements --- Process v2 with decreasing indices    8   6   4   2   0  Process v1 with increasing indices  0            Note that on the second version with increasing indices  even numbers are not displayed as they are skipped because of i

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If you work with large vectors  size   100 000  and want to delete lots of elements  I would recommend to do something like this   int main int argc  char   argv         vector  lt int gt  vec      vector  lt int gt  vec2       for  int i   0  i  lt  20000000  i             vec push back i         for  int i   0  i  lt  vec size    i                  if vec at i   3    0              vec2 push back i              vec   vec2      cout  lt  lt  vec size    lt  lt  endl      The code takes every number in vec that can t be divided by 3 and copies it to vec2  Afterwards it copies vec2 in vec  It is pretty fast  To process 20 000 000 elements this algorithm only takes 0 8 sec   I did the same thing with the erase-method  and it takes lots and lots of time   Erase-Version  10k elements     0 04 sec Erase-Version  100k elements    0 6  sec Erase-Version  1000k elements   56   sec Erase-Version  10000k elements      still calculating   gt 30 min

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The previous answers assume that you always have a signed index  Sadly  std  vector uses size type for indexing  and difference type for iterator arithmetic  so they don t work together if you have  -Wconversion  and friends enabled  This is another way to answer the question  while being able to handle both signed and unsigned   To remove   template lt class T  class I  class   typename std  enable if lt std  is integral lt I gt   value gt   type gt  void remove std  vector lt T gt   amp v  I index        const auto  amp iter   v cbegin     gsl  narrow cast lt typename std  vector lt T gt   difference type gt  index       v erase iter       To take   template lt class T  class I  class   typename std  enable if lt std  is integral lt I gt   value gt   type gt  T take std  vector lt T gt   amp v  I index        const auto  amp iter   v cbegin     gsl  narrow cast lt typename std  vector lt T gt   difference type gt  index        auto val    iter      v erase iter        return val

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The erase method will be used in two ways    Erasing single element   vector erase  vector begin     3       Deleting the fourth element  Erasing range of elements   vector erase  vector begin     3  vector begin     5       Deleting from fourth element to sixth element

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I suggest to read this since I believe that is what are you looking for https   en wikipedia org wiki Erase E2 80 93remove idiom  If you use for example    vec erase vec begin     1  vec begin     3     you will erase n -th element of vector but when you erase second element  all other elements of vector will be shifted and vector sized will be -1  This can be problem if you loop through vector since vector size   is decreasing  If you have problem like this provided link suggested to use existing algorithm in standard C   library  and  remove  or  remove if     Hope that this helped

[c++] How do I erase an element from std::vector<> by index?

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