Create the cell array by using the cell function. For example:

function z = mycell(n, j)
%#codegen
assert (n < 100);
x = cell(1,n);   
for i = 1:n
    x{i} = i;
end
z = x{j};
end

For code generation, when you create a variable-size cell array by using cell, you must make sure that you assign values to all cell array elements. See Resolve Issue: Cell Array Elements Must Be Fully Defined Before Use (MATLAB Coder).

Grow the cell array. For example:

function z = mycell(n)
%#codegen
c = {1 2 3};
if n > 3
    c = {1 2 3 4};
end
z = c{n};
end

Force the cell array to be variable-size by using coder.varsize. Consider this code:

function y =  mycellfun()
%#codegen
c = {1 2 3};
coder.varsize('c', [1 10]);
y = c{1};
end

Without coder.varsize, c is fixed-size with dimensions 1-by-3. With coder.varsize, c is variable-size with an upper bound of 10.

Sometimes, using coder.varsize changes the classification of a cell array from heterogeneous to homogeneous. Consider this code:

function y =  mycell()
%#codegen
c = {1 [2 3]};
y = c{2};
end

The code generator classifies c as heterogeneous because the elements have different sizes. c is fixed-size with dimensions 1-by-2. If you use coder.varsize with c, it becomes homogeneous. For example:

function y =  mycell()
%#codegen
c = {1 [2 3]};
coder.varsize('c', [1 10], [0 1]);
y = c{2};
end

c becomes a variable-size homogeneous cell array with dimensions 1-by-:10.

To force c to be homogeneous, but not variable-size, specify that none of the dimensions vary. For example:

function y =  mycell()
%#codegen
c = {1 [2 3]};
coder.varsize('c', [1 2], [0 0]);
y = c{2};
end