# 1 Array Reshape, Repeat and Expand Examples

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## 1.1 Basic Examples of Reshape

a = [1,2,3,4,5,6]';
b = reshape(a, [3,2])

b = 3x2
1     4
2     5
3     6

b(:)

ans = 6x1
1
2
3
4
5
6

a = [1,2,3;4,5,6;7,8,9;10,11,12]'

a = 3x4
1     4     7    10
2     5     8    11
3     6     9    12

b = reshape(a, [6,2])

b = 6x2
1     7
2     8
3     9
4    10
5    11
6    12

## 1.2Stack Two Matrix of Equal Column Count Together

a = [1,2;3,4];
a_stacked = [a;a;a];
disp(a_stacked);

1     2
3     4
1     2
3     4
1     2
3     4

## 1.3Repeat/Duplicate Matrix Downwards

There is a 2 by 3 matrix, to be repeated 4 times, downwards. This is useful for replicating data matrix for say counterfactual purposes.

Below, we have two ways of repeating a matrix downwards. Copy as whole, or copy row by row.

row_count = 2;
col_count = 3;
repeat_mat_count = 2;

data_vec = 1:(row_count*col_count);
searchMatrix = reshape(data_vec,row_count,col_count);

% To repeat matrix downwards
rep_rows_idx = [1:row_count]'*ones(1,repeat_mat_count);
rep_rows_idx = rep_rows_idx(:);

rep_cols_idx = [1:col_count];
rep_cols_idx = rep_cols_idx(:);

searchMatrixRep_stack = searchMatrix(rep_rows_idx, rep_cols_idx);

% To insert repeated rows following original rows
rep_rows_idx = ([1:row_count]'*ones(1,repeat_mat_count))';
rep_rows_idx = rep_rows_idx(:);

searchMatrixRep_dup = searchMatrix(rep_rows_idx, rep_cols_idx);

disp(searchMatrix)

1     3     5
2     4     6

disp(searchMatrixRep_stack)

1     3     5
2     4     6
1     3     5
2     4     6

disp(searchMatrixRep_dup)

1     3     5
1     3     5
2     4     6
2     4     6

## 1.4 Index Dimension Transform

it_inner_fin = 5; it_outter_fin = 3;

it_inner_cur = it_outter_fin it_outter_cur = it_inner_fin

ar_it_cols_idx = 1:1:(it_inner_fin*it_outter_fin) ar_it_cols_inner_dim = repmat(1:it_inner_cur, $it_outter_cur, 1$) ar_it_cols_inner_dim(:)’

mt_it_cols_idx = reshape(ar_it_cols_idx, $it_inner_cur, it_outter_cur$)’ mt_it_cols_idx(:)’

it_inner_fin = 5;
it_outter_fin = 3;

ar_it_cols_idx = 1:1:(it_inner_fin*it_outter_fin)

ar_it_cols_idx = 1x15
1     2     3     4     5     6     7     8     9    10    11    12    13    14    15

mt_it_cols_idx = reshape(ar_it_cols_idx, [it_outter_fin, it_inner_fin])'

mt_it_cols_idx = 5x3
1     2     3
4     5     6
7     8     9
10    11    12
13    14    15

mt_it_cols_idx(:)'

ans = 1x15
1     4     7    10    13     2     5     8    11    14     3     6     9    12    15