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add different number of zeros in the beginning of a matrix

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Praveen Verma
Praveen Verma on 20 Jul 2021
Answered: Jan on 20 Jul 2021
suppose i have two matrix
A=[1
3
2]
the row index of 1 is 1, 3 is 2 and 2 is 3.
S=[2 3 4 5 6
4 6 8 9 1
2 3 4 5 6
6 5 4 2 1
4 6 8 9 1
2 3 4 5 6 ]
Now i want to insert zeros in the beginning of each row of S. The number of zeros in each row depends upon the index of matrix A and number of rows in which i want to insert zeros depends upon element value matrix A.
For example in matrix A row index of 1 is 1 and value of element is 1 so i want to insert one zero at the beginning of first row of matrix S. Similary in matrix A the row index of 3 is 2 and element value is 3 so i want to insert 2 zeros in next 3 rows (second third and fourth ) in the beginning of matrix S. again in matrix A the row index of 2 is 3 and element value is 2 so i want to insert 3 zeros in next 2 rows (fifth and sixth) in the beginning of matrix S.
finally the result will look like this
S=[0 2 3 4 5 6
0 0 4 6 8 9 1
0 0 2 3 4 5 6
0 0 6 5 4 2 1
0 0 0 4 6 8 9 1
0 0 0 2 3 4 5 6 ]
my actual matrix is very large so plz provide a general solution.
  1 Comment
Dyuman Joshi
Dyuman Joshi on 20 Jul 2021
It is not possible to make such a numeric array (of irregular size). You can check it by actually copy-pasting on matrix S and it will give you an error.
However, You can make a cell array (if that is an acceptable answer)

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Answers (1)

Jan
Jan on 20 Jul 2021
The output cannot be a matrix, as Dyuman Joshi told you already, because in a matrix all rows must have the same number of elements. With a cell vector:
A = [1; 3; 2];
S = [2 3 4 5 6; ...
4 6 8 9 1 ; ...
2 3 4 5 6 ; ...
6 5 4 2 1; ...
4 6 8 9 1 ; ...
2 3 4 5 6 ];
nZ = repelem(1:numel(A), A);
nS = size(S, 1);
Result = cell(nS, 1);
for k = 1:nS
Result{k} = [zeros(1, nZ(k)), S(k, :)];
end

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