Copying a "vector of vector" into" array of array"

Question

First I had an int array called Myarray[51][4] filled by data.

For sorting it based on the second column of Myarray, I used the following code (using the conversion of array into a vector of vector: my_vector[51][4]) :

int Myarray [51][4];
vector< vector<int> > my_vector ;
for( const auto& row : Myarray ) 
    my_vector.push_back( vector<int>( begin(row), end(row) ) ) ;

sort( begin(my_vector), end(my_vector),
       []( const vector<int>& a, const vector<int>& b ) { return a[1] < b[1] ; } ) ;

This code has sorted my_vector. Now I want to copy the sorted vector into Myarray again for using it as array of integer with dimension of [51][4]. How do I do it?

Just a quick question: Are you certain that you want to compare the second elements rather than the first ones? — stefan
– stefan, Commented Jul 30, 2013 at 12:48
int Myarray [51][4]; sort( begin(Myarray), end(Myarray), []( int(&a)[4], int(&b)[4] ) { return a[1] < b[1] ; } ) ; — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Jul 30, 2013 at 12:56
@R.MartinhoFernandes did you try compiling that? It doesn't for me with g++ (4.7 and 4.8) — stefan
– stefan, Commented Jul 30, 2013 at 12:58

sasha.sochka · Accepted Answer · 2013-07-30 20:19:40Z

4

The simplest and the most obvious one:

for (size_t row = 0; row < my_vector.size(); ++row) {
  for (size_t col = 0; col < my_vector[row].size(); ++col) {
    Myarray[row][col] = my_vector[row][col];
  }
}

Or another solution without the inner loop:

for (size_t row = 0; row < my_vector.size(); ++row) {
  copy(my_vector[row].begin(), my_vector[row].end(), Myarray[row]);
}

But better stop using C-style arrays in C++ and switch to std::vector or std::array completely!

For your request in the comment here is an example with std::vector:

vector<vector<int>> my_vector =  {{3, 8, 7, 2}, {9, 12, 0, 4}, {12, 2, 14, 1}};
sort(begin(my_vector), end(my_vector),
     [](const vector<int>& a, const vector<int>& b) { return a[1] < b[1]; }
) ;

edited Jul 30, 2013 at 20:19

answered Jul 30, 2013 at 12:54

sasha.sochka

14.8k10 gold badges46 silver badges68 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Mehdi Tanhatalab Over a year ago

if I want to sort the following array what is the code?int Myarray[3][4] = { {3, 8, 7, 2}, {9, 12, 0, 4}, {12, 2, 14, 1}. I want to have: {12, 2, 14, 1} , {3, 8, 7, 2} , {9, 12, 0, 4}.

sasha.sochka Over a year ago

@MehdiTanhatalab, it is harder than I initially thought as you cannot copy array. You would have to write you own sort function. But better just stick with std containers, e.g. std::vector. Why do you prefer C-style arrays?

Mehdi Tanhatalab Over a year ago

I have been programming using Visal Foxpro till now and a beginner in C++. There, there are only arrays but not vectors. That is why i know arrays better. bur if you recommend working with data using vector, as it may be more speed in the run time, I will shift to it.

sasha.sochka Over a year ago

@MehdiTanhatalab, in run-time the speed difference is almost negligible. And yes - I do recommend using std::vector or std::array instead of C-style arrays - in C++ you should want C-style arrays very-very rarely.

Mehdi Tanhatalab Over a year ago

Thank you. I do not know what do you mean C-style array is there a C++-style array? Do you mean that why I omitted the "std::" ? I used the "using namespace std" which i did not bring here.

|

Collectives™ on Stack Overflow

Copying a "vector of vector" into" array of array"

1 Answer 1

7 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Your Answer

Sign up or log in

Post as a guest

Related