Question

Is there an easier way to construct a 5 Diagonal matrix in Eigen? I can probably run for loops and allocate diagonals and zeros, but I did come across Diagonal<> just not sure how to use it for 5 diagonals, instead of one. Any ideas? EDIT: Figured this one out! For those wondering; you can use

matrix.diagonal(+n) = vector;
matrix.diagonal(-n) = vector;

to access super/sub diagonals of a matrix and write over them with vectors.

General side question: Is there a way I can skip an allocation when running a for loop in C++? For example:

int n; //size of matrix
MatrixXd m(n,n); //nxn matrix

for(int i=0; i<n; i++)
{ 
   m(i,i) = 5; 
   m(i,i+1) = 6;
   m(i,i-1) = 4;
   m(i,i+2) = 7;
   m(i,i-2) = 3;
}


for (int i=0; i<n; i++)
{
    for(int j=0; j<n; j++)
    {
         if(m(i,j) = something) //I want the loop to skip m(i,j) where 
            break;              //i have already allocated values to m(i,j)
                                //How do I do that, in general, in C++?      
         else
       { m(i,j) = 0;}
    }
 }

Thanks

Answer 1

听起来好像您想跳过对角线，因为它们已被初始化（此处分配的术语不正确）。
查看设置对角线的循环，您会看到所设置的每个（i，j）都遵循abs(i-j) <= 2。例如，当您设置元素(i, i+2) -> abs(i-(i+2)) -> abs(-2) -> 2小于或等于2时。

因此，第二个循环中的条件应为：

if (abs(i-j) <= 2)
  continue;//break will exit the loop, continue will skip to the next iteration

5 Diagonal Matrix and for loops

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