4
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i have a matrix

A =
{{3., -1., 0, -1., 0, -1., 0},
 {-1., 2., -1., 0, 0, 0, 0},
 {0, -1.,1., 0, 0, 0, 0},
 {-1., 0, 0, 2., -1., 0, 0},
 {0, 0, 0, -1., 1., 0, 0},
 {-1., 0, 0, 0, 0, 2., -1.},
 {0, 0, 0, 0, 0, -1., 1.}}

I want to partition in the form

{{A11,A12,A13,A14},
 {A21,A22,A23,A24},
 {A31,A32,A33,A34},
 {A41,A42,A43,A44}}

where

A11={3};
A12=A13=A14={-1,0};
A21=A31=A41={{-1},{0}};
A22=A33=A44={{2,-1},{-1,1}}

and rest of the blocks are {{0,0},{0,0}}

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1 Answer 1

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You can use the undocumented internal function: Internal`PartitionRagged:

A = 
 {{3, -1, 0, -1, 0, -1, 0},
 {-1, 2, -1, 0, 0, 0, 0},
 {0, -1, 1, 0, 0, 0, 0},
 {-1, 0, 0, 2, -1, 0, 0},
 {0, 0, 0, -1, 1, 0, 0},
 {-1, 0, 0, 0, 0, 2, -1},
 {0, 0, 0, 0, 0, -1, 1}}

output = Internal`PartitionRagged[A, {{1, 2, 2, 2}, {1, 2, 2, 2}}]
{
 {{{3}},{{-1,0}},{{-1,0}},{{-1,0}}},
 {{{-1},{0}},{{2,-1},{-1,1}},{{0,0},{0,0}},{{0,0},{0,0}}},
 {{{-1},{0}},{{0,0},{0,0}},{{2,-1},{-1,1}},{{0,0},{0,0}}},
 {{{-1},{0}},{{0,0},{0,0}},{{0,0},{0,0}},{{2,-1},{-1,1}}}
}

A corresponding Grid:

Grid[A, Dividers -> {{True, {True, False}}, {True, {True, False}}}]

enter image description here

You can recombine the partitions with ArrayFlatten if needed:

ArrayFlatten[output] == A
True
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