How to expand my cell without losing information - matlab

Question

I have this a cell that holds 4 vectors, and I'd like to expand it so it could hold another vector. Thanks in advance.
a = cell(4, 1);
a{1} = [1, 2, 3];
a{2} = [1, 4, 9];
a{3} = [1, 4, 9];
a{4} = [1; 5];
And I would like to add a new vector [2, 7] so that I will get this
a{1} = [2, 7];
a{2} = [1, 2, 3];
a{3} = [1, 4, 9];
a{4} = [1, 4, 9];
a{5} = [1; 5];
How could I do that ?

Answer 1

a = cell(4, 1);
a{1} = [1, 2, 3];
a{2} = [1, 4, 9];
a{3} = [1, 4, 9];
a{4} = [1; 5];
a = [[2, 7];a]
Yielding:
a =
[1x2 double]
[1x3 double]
[1x3 double]
[1x3 double]
[2x1 double]

Answer 2

The name for what you are trying to do is concatenation. In MATLAB, square brackets are an implicit method of concatenation. While you typically think about these being used to concatenate numbers to form a vector or matrix:
x = [1, 2, 3, 4] % 1 x 4
You can also use them to concatenate other data types (including cell arrays)
y = [{1,2}, 5, {4, 5}]; % 1 x 3 Cell Array
It is also important to keep track of the dimension of concatenation. With square brackets you can either concatenate across the columns (2nd dimension) using a comma or across the rows (1st dimension using a semi-colon).
size([1,2,3])
1 x 3
size([1;2;3])
3 x 1
While square brackets are a quick and easy solution, I prefer to be a little more explicit. For this, MATLAB has the following functions: cat, horzcat, and vertcat. I personally prefer cat since you can specify concatenation in any arbitrary dimension.
x = cat(2, 100, 200, 300); % 1 x 3
100 200 300
x = cat(1, 100, 200, 300); % 3 x 1
100
200
300
x = cat(3, 100, 200, 300) % 1 x 1 x 3
x(:,:,1) =
100
x(:,:,2) =
200
x(:,:,3) =
300
So back to your original question, you want to concatenate in the 1st dimension (rows) so you could simply do the following:
a = cell(4, 1);
a{1} = [1, 2, 3];
a{2} = [1, 4, 9];
a{3} = [1, 4, 9];
a{4} = [1; 5];
a = cat(1, [2, 7], a);
Also remember that this form of using concat or the square brackets can be applied to most datatypes within MATLAB including structs among others.