Use a recognized coding pattern to assign cell array elements

Assign values to the elements of the cell array using a coding pattern that the code generator can recognize. If you use a for-loop to assign initial values to the cell array elements and the cell array is variable-size, make sure that you:

To assign a scalar value to each element of a 1-by-n cell array, follow this pattern.

function out = codingPatternExample1(n) %#codegen
ca = cell(1,n);   
for i = 1:n
    ca{i} = sqrt(i);
end
out = ca{n};
end

To assign an array of values to each element of a 1-by-n cell array, assign arrays of values to each element in a single statement. Do not use subscripted or dot assignments.

function out = codingPatternExample2(n) %#codegen
ca = cell(1,n);   
for i = 1:n
    ca{i} = [sqrt(i) i i^2];
end
out = ca{n};
end

To assign a value to each element of a multidimensional cell array, use nested loops. For example, use this coding pattern for a three-dimensional (m-by-n-by-p) array.

function out = codingPatternExample3(m,n,p) %#codegen
ca = cell(m,n,p);
for i = 1:m
    for j =1:n
        for k = 1:p
            ca{i,j,k} = i+j+k;
        end
    end
end
out = ca{m,n,p};
end

Use the repmat function

The repmat function takes an input array and replicates it a specified number of times in the row and column dimensions to construct an output array. In certain cases where your cell array contains repeating elements, you can rewrite your code to define the elements of a cell array by using repmat. For example, consider the function repmatAssignmentError, which creates a 1-by-(n*2) cell array using the cell function. repmatAssignmentError assigns 1 to the odd elements of ca and 0 to the even elements. Code generation fails for this function because initial values are assigned to the cell array elements in different for-loops and because the loop counters increment by a value other than 1.

function out = repmatAssignmentError(n) %#codegen
ca = cell(1,n*2);
for i = 1:2:n*2
    ca{i} = 1;
end
for i = 2:2:n*2
    ca{i} = 0;
end
out = ca{n};
end

To resolve this issue, first use the cell function to create a temporary 1-by-2 cell array whose first element is 1 and whose second element is 0. Then, use the repmat function to create a 1-by-(n*2) cell array whose element values alternate between 1 and 2.

function out = repmatAssignmentExample(n) %#codegen
tmp = cell(1,2);
tmp{1} = 1;
tmp{2} = 0;
ca = repmat(tmp,1,n);
out = ca{n};
end

Use coder.nullcopy

You can use the coder.nullcopy function to preallocate memory for a homogenous cell array without specifically assigning values to the cell array elements. In general, homogenous cell arrays contain elements that are all of the same type, such as double or string. However, under certain circumstances, the code generator can classify the same cell array as homogenous in some contexts but as heterogeneous in other contexts. See Code Generation for Cell Arrays (MATLAB Coder).

If you use coder.nullcopy to define a homogenous cell array, you can generate code without having to adhere to the recognized coding patterns described above. For example, consider the function nullcopyExample. Because this function uses coder.nullcopy to preallocate memory for homogenous cell array ca, you can assign values to the cell array in two separate loops.

function out = nullcopyExample(n) %#codegen
tmp = cell(1,n);
ca = coder.nullcopy(tmp);
for i = 1:4
    ca{i} = 0;
end
for i = 5:n
    ca{i} = i;
end
out = ca{n};
end