Reshape Matlab table - matlab

Question

I have the following table
name = ['A' 'A' 'A' 'B' 'B' 'C' 'C' 'C' 'C' 'D' 'D' 'E' 'E' 'E']';
value = randn(14, 1);
T = table(name, value);
i,e.
T =
name value
____ _________
A 0.0015678
A -0.76226
A 0.98404
B -1.0942
B 0.71249
C 1.688
C 1.4001
C -0.9278
C -1.3725
D 0.11563
D 0.076776
E 1.0568
E 1.1972
E 0.29037
I want to transform it in the following way: take the first two cells in value corresponding to different values in name and put it in the 5x2 matrix. This matrix would have rows corresponding to different names A,B,C,D,E and columns corresponding to values, e.g. the first two rows are
0.0015678 -0.76226
-1.0942 0.71249

Answer 1

This can be done with accumarray using a custom function. The first step is to convert the name column of T into a numeric vector; and then accumarray can be applied.
This approach requires T being sorted according to column 1, because only in this case is accumarray guaranteed to preserve order (as indicated in its documentation). So if T may not be sorted (although it is in your example), sort it first using sortrows.
T = sortrows(T, 1); %// you can remove this line if T is guaranteed to be sorted
[~, ~, names] = unique(T(:,1)); %// names as a numeric vector
result = cell2mat(accumarray(names, T.value, [], #(x) {x([1 2]).'}));

Answer 2

First figure out where each name has values located in the table, then cycle through each name and place the first two values encountered for each name into individual cell arrays. Once you're done, reshape the matrix to 5 x 2 as you have said. As such, do something like this:
names = unique(T.name); %// 1
ind = arrayfun(#(x) find(T.name == x), names, 'uni', 0); %// 2
vals = cellfun(#(x) T.value(x(1:2)), ind, 'uni', 0); %// 3
m = [vals{:}].'; %// 4
Let's go through each line of code slowly.
Line #1
The first line finds all unique names through unique and we store them into names.
Line #2
The next line goes through all of the unique names and finds those locations / rows in the table that share that particular name. I use arrayfun and go through each name in names, find those rows that share the same name as one we are looking for, and place those row locations into individual cells; these are stored into ind. To find the locations of each valid name in our table, I use find and the locations are placed into a column vector. As such, we will have five column vectors where each column vector is placed into an individual cell. These column vectors will tell us which rows match a particular name located in your table.
Line #3
The next line uses cellfun to go through each of the cells in ind and extracts the first two row locations that share a particular name, indexes into the value field for your table to pull those two values, and these are placed as two-element vectors into individual cells for each name.
Line #4
The last line of code simply unrolls each two-element vector. The first two elements of each name get stored into columns. To get them into rows, I simply transpose the unrolling. The output matrix is stored into m.
If you want to see what the output looks like, this is what I get when I run the above code with your example table:
m =
0.0016 -0.7623
-1.0942 0.7125
1.6880 1.4001
0.1156 0.0768
1.0568 1.1972
Be advised that I only showed the first 5 digits of precision so there is some round-off at the end. However, this is only for display purposes and so what I got is equivalent to what your expect for the output.
Hope this helps!

Answer 3

If you want use the tables, you could try something like this:
count = 1;
U = unique(table2array(T(:,1)));
for ii = 1:size(U,1)
A = find(table2array(T(:,1)) == U(ii));
A = A(1:2);
B(count,1:2) = table2array(T(A,2));
count = count + 1;
end
Personally, I would find this simpler to do with your name and value arrays and forget about the table. If it is a requirement then I understand, however I will provide my solution still. It may provide some insight either way.
count = 1;
U = unique(name);
for ii = 1:size(U,1)
A = find(name == U(ii));
A = A(1:2);
B(count,1:2) = value(A);
count = count + 1;
end
Quick and dirty, but hopefully it's good enough. Good luck.

Answer 4

Another solution that is more manageable and easily scalable exists. Since MATLAB R2013b you can use a specialized function for pivoting a table (which is what you want to do): unstack.
In order to get exactly what you wanted, you need to add an extra variable to your table that will indicate replications:
name = ['A' 'A' 'A' 'B' 'B' 'C' 'C' 'C' 'C' 'D' 'D' 'E' 'E' 'E']';
value = randn(14, 1);
rep = [1, 2, 3, 1, 2, 1, 2, 3, 4, 1, 2, 1, 2, 3];
T = table(name, value, rep);
T =
name value rep
____ _________ ___
A 0.53767 1
A 1.8339 2
A -2.2588 3
B 0.86217 1
B 0.31877 2
C -1.3077 1
C -0.43359 2
C 0.34262 3
C 3.5784 4
D 2.7694 1
D -1.3499 2
E 3.0349 1
E 0.7254 2
E -0.063055 3
Then you just use unstack like this:
pivotTable = unstack(T, 'value','name')
pivotTable =
rep A B C D E
___ _______ _______ ________ _______ _________
1 0.53767 0.86217 -1.3077 2.7694 3.0349
2 1.8339 0.31877 -0.43359 -1.3499 0.7254
3 -2.2588 NaN 0.34262 NaN -0.063055
4 NaN NaN 3.5784 NaN NaN
Afterwards, it's a matter of re-arranging the table if you still want to.

Answer 5

The easiest way is to first convert the table into a matrix form and then reshape it by using the "reshape" function in Matlab.
matrix = t{:,:};% t-- your table variable
reshape_matrix = reshape(matrix ,[2,3]) % [2,3]--> the size of the matrix you desire
These two steps can be done by one line of code
reshape_matrix = reshape(t{:,:},[2,3]);