create a matrix with condition in C++ - c++

Question

I want to create a matrix B from a matrix A, in C++.
First column of A is distance D1, second column is distance D2. Matrix B copies the same columns (and rows) of A, except when in A it happens that D2-D1=delta exceeds a threshold. In this case, the row of A is break in two rows in B.
I wrote an algorithm, but the problem is that it gives segmentation fault. Someone can help, why it happens?
std::vector<float> newD1(10), newD2(10);
float box=5.;
int j=0;
for(auto i=0;i<D1.size();i++){
float delta=D2[i]-D1[i];
if (delta>box){ //break row i in two rows: j and j+1
//first half of row i goes in row j
newD1[j]=D1[i];
newD2[j]=(D1[i]+D2[i])/2.;
//second half of row i goes in j+1
D1[j+1]=(D1[i]+D2[i])/2.;
D2[j+1]=D2[i];
j=j+2; //we skip two row because we break up the original row in 2 rows
}
else{
newD1[j]=(D1[i]);
newD2[j]=D2[i];
j=j+1; //we skip one row because the original row is unchanged
}
}
Here I give you an example of matrix A and B; I also specify delta beside each line of the matrix.
Matrix A:
#D1 D2 delta
|0 5 | 5
A= |5 15 | 10 }--> exceed the threshold, delta>5. Must break in 2 rows in `B`
|15 17 | 2
B is created breaking the second line in two lines, because delta>5 :
#D1 D2 delta
|0 5 | 5
B= |5 10 | 5 }--> created from row #2 of `A`. `D2` is midpoint between`D1` and `D2` in row #2 in `A`
|10 15 | 5 }--> created from row #2 of `A`. `D1` is midpoint between`D1` and `D2` in row #2 in `A`
|15 17 | 2
EDIT:
What if I want to recursively break up the rows (e.g. suppose that at row #2 in A, delta>3*box, meaning that I need to break up that row in 3 rows in B). Any suggestions?

Answer 1

1.You can use push_back to avoid the size definition
2.You updated D1 and D2 instead of newD1 and newD2
#include <vector>
#include <iostream>
using namespace std;
int main()
{
std::vector<float> D1 = { 0,5,15 };
std::vector<float> D2 = { 5,15,17 };
std::vector<float> newD1, newD2;
float box = 5.;
for (auto i = 0; i < D1.size(); i++) {
float delta = D2[i] - D1[i];
if (delta > box) { //break row i in two rows: j and j+1
//first half of row i goes in row j
newD1.push_back(D1[i]);
newD2.push_back((D1[i] + D2[i]) / 2.);
//second half of row i goes in j+1
newD1.push_back ((D1[i] + D2[i]) / 2.);
newD2.push_back (D2[i]);
}
else {
newD1.push_back(D1[i]);
newD2.push_back(D2[i]);
}
}
}