Vector of Vectors to create matrix

Question

I am trying to take in an input for the dimensions of a 2D matrix  And then use user input to fill in this matrix  The way I tried doing this is via vectors  vectors of vectors   But I have encountered some errors whenever I try to read in data and append it to the matrix     cin gt  gt CC  cin gt  gt RR  already done vector lt vector lt int gt   gt  matrix  for int i   0  i lt RR  i          for int j   0  j lt CC  j              cout lt  lt  Enter the number for Matrix 1            cin gt  gt matrix i  j             Whenever I try to do this  it gives me a subscript out of range error  Any advice

User · Answer

Vector needs to be initialized before using it as cin>>v[i][j]. Even if it was 1D vector, it still needs an initialization, see this link

After initialization there will be no errors, see this link

User · Answer

I m not familiar with c    but a quick look at the documentation suggests that this should work     cin gt  gt CC  cin gt  gt RR  already done vector lt vector lt int gt   gt  matrix  for int i   0  i lt RR  i          vector lt int gt  myvector      for int j   0  j lt CC  j                  int tempVal   0          cout lt  lt  Enter the number for Matrix 1           cin gt  gt tempVal          myvector push back tempVal             matrix push back myvector

User · Answer

I did this class for that purpose  it produces a variable size matrix   expandable  when more items are added        pragma once  include lt vector gt   include lt iostream gt   include lt iomanip gt  using namespace std  template  lt class T gt class Matrix   public      Matrix     default      bool AddItem unsigned r  unsigned c  T value                if  r  gt   Rows count                        Rows resize r   1               Rows count   r   1                    else                       Rows resize Rows count                     if  c  gt   Columns Count                         for  std  vector lt T gt  amp  row   Rows                                row resize c   1                             Columns Count   c   1                    else                       for  std  vector lt T gt  amp  row   Rows                                row resize Columns Count                                   if  r  lt  Rows size                if  c  lt  static cast lt std  vector lt T gt  gt  Rows at r   size                                   Rows at r   at c    value                            else                               cout  lt  lt  Rows at r  size    lt  lt    greater than    lt  lt  c  lt  lt  endl                        else             cout  lt  lt   ERROR   lt  lt  endl          return true            void Show                 std  cout  lt  lt                      lt  lt std  endl          for  std  vector lt T gt  r   Rows                        for  auto amp  c   r                  std  cout  lt  lt       lt  lt setw 5  lt  lt  c              std  cout  lt  lt  std  endl                    std  cout  lt  lt                       lt  lt  std  endl            void Show size t n                std  cout  lt  lt                       lt  lt  std  endl          for  std  vector lt T gt  r   Rows                        for  auto amp  c   r                  std  cout  lt  lt       lt  lt  setw n   lt  lt  c              std  cout  lt  lt  std  endl                    std  cout  lt  lt                       lt  lt  std  endl             Matrix    public      std  vector lt std  vector lt T gt  gt  Rows      unsigned Rows count      unsigned Columns Count

User · Answer

You have to initialize the vector of vectors to the appropriate size before accessing any elements  You can do it like this      assumes using std  vector for brevity vector lt vector lt int gt  gt  matrix RR  vector lt int gt  CC      This creates a vector of RR size CC vectors  filled with 0

User · Answer

As it is  both dimensions of your vector are 0   Instead  initialize the vector as this   vector lt vector lt int gt   gt  matrix RR   for   int i   0   i  lt  RR   i        matrix i  resize CC     This will give you a matrix of dimensions RR   CC with all elements set to 0

User · Answer

try this  m   row  n   col  vector lt vector lt int gt  gt  matrix m  vector lt int gt  n     for i   0 i  lt  m  i         for j   0  j  lt  n  j               cin  gt  gt  matrix i  j           cout  lt  lt  endl    cout  lt  lt     matrix     lt  lt  endl  for i   0  i  lt  m  i          for j   0  j  lt  n  j                  cout  lt  lt  matrix i  j   lt  lt                 cout  lt  lt  endl

User · Answer

Assume we have the following class    include  lt vector gt   class Matrix    private    std  vector lt std  vector lt int gt  gt  data       First of all I would like suggest you to implement a default constructor    include  lt vector gt   class Matrix    public    Matrix    data          private    std  vector lt std  vector lt int gt  gt  data       At this time we can create Matrix instance as follows   Matrix one    The next strategic step is to implement a Reset method  which takes two integer parameters that specify the new number of rows and columns of the matrix  respectively    include  lt vector gt   class Matrix    public    Matrix    data           Matrix const int  amp rows  const int  amp cols        Reset rows  cols          void Reset const int  amp rows  const int  amp cols        if  rows    0    cols    0          data assign 0  std  vector lt int gt  0          else         data assign rows  std  vector lt int gt  cols                private    std  vector lt std  vector lt int gt  gt  data       At this time the Reset method changes the dimensions of the 2D-matrix to the given ones and resets all its elements  Let me show you a bit later why we may need this   Well  we can create and initialize our matrix   Matrix two 3  5     Lets add info methods for our matrix    include  lt vector gt   class Matrix    public    Matrix    data           Matrix const int  amp rows  const int  amp cols        Reset rows  cols          void Reset const int  amp rows  const int  amp cols        data resize rows       for  int i   0  i  lt  rows    i          data at i  resize cols                int GetNumRows   const       return data size           int GetNumColumns   const       if  GetNumRows    gt  0          return data 0  size               return 0        private    std  vector lt std  vector lt int gt  gt  data       At this time we can get some trivial matrix debug info    include  lt iostream gt   void MatrixInfo const Matrix amp  m      std  cout  lt  lt       rows       lt  lt  m GetNumRows                lt  lt       cols       lt  lt  m GetNumColumns    lt  lt        lt  lt  std  endl     int main       Matrix three 3  4     MatrixInfo three       The second class method we need at this time is At  A sort of getter for our private data    include  lt vector gt   class Matrix    public    Matrix    data           Matrix const int  amp rows  const int  amp cols        Reset rows  cols          void Reset const int  amp rows  const int  amp cols        data resize rows       for  int i   0  i  lt  rows    i          data at i  resize cols                int At const int  amp row  const int  amp col  const       return data at row  at col          int amp  At const int  amp row  const int  amp col        return data at row  at col          int GetNumRows   const       return data size           int GetNumColumns   const       if  GetNumRows    gt  0          return data 0  size               return 0        private    std  vector lt std  vector lt int gt  gt  data       The constant At method takes the row number and column number and returns the value in the corresponding matrix cell    include  lt iostream gt   int main       Matrix three 3  4     std  cout  lt  lt  three At 1  2      0 at this time     The second  non-constant At method with the same parameters returns a reference to the value in the corresponding matrix cell    include  lt iostream gt   int main       Matrix three 3  4     three At 1  2    8    std  cout  lt  lt  three At 1  2      8     Finally lets implement  gt  gt  operator    include  lt iostream gt   std  istream amp  operator gt  gt  std  istream amp  stream  Matrix  amp matrix      int row   0  col   0     stream  gt  gt  row  gt  gt  col    matrix Reset row  col      for  int r   0  r  lt  row    r        for  int c   0  c  lt  col    c          stream  gt  gt  matrix At r  c                return stream      And test it    include  lt iostream gt   int main       Matrix four     An empty matrix    MatrixInfo four        Example output               rows   0   cols   0      std  cin  gt  gt  four       Example input           2 3      4 -1 10      8 7 13    MatrixInfo four        Example output               rows   2   cols   3       Feel free to add out of range check  I hope this example helps you

User · Answer

What you have initialized is a vector of vectors  so you definitely have to include a vector to be inserted  Pushed  in the terminology of vectors  in the original vector you have named matrix in your example   One more thing  you cannot directly insert values in the vector using the operator  cin   Use a variable which takes input and then insert the same in the vector   Please try this out      int num  for int i 0  i lt RR  i            vector lt int gt inter mat          Intermediate matrix to help insert push  contents of whole row at a time        for int j 0  j lt CC  j                cin gt  gt num                Extra variable in helping push our number to vector            vin push back num       Inserting numbers in a row  one by one                     v push back vin              Inserting the whole row at once to original 2D matrix

[c++] Vector of Vectors to create matrix

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