Resolve Error: Code Generation Does Not Support String and Object Arrays

Issue

Code generation does not support object arrays and string arrays. If you use a string or object array in your MATLAB^® code, or if you specify a string or object array as an input argument to your entry-point function, you see one of these error messages:

Code generation does not support object arrays. Use a cell array of className objects.

Code generation does not support string arrays. Use a cell array of character vectors.

Code generation does not support arrays of className objects. Instead of a string array, use a cell array of character vectors. Instead of an object array, use a cell array of objects.

Possible Solutions

If your code uses a string array, replace the array with a cell array of character vectors If your code uses an object array, replace the array with a cell array of objects. If you use an object array and assign values to object properties by using nonscalar indexing, use a cell array of objects and the deal function.

Use Cell Array of Character Vectors

If your MATLAB code uses a string array, rewrite your code to use a cell array of character vectors.

For example, consider the function useStrings_error, which creates a string array.

function out = useStrings_error(n) %#codegen
s = ":)";
for i = 2:n
    s(end+1) = ":)";
end
out = s;
end

Code generation for useStrings_error fails because s is a string array. To resolve this error, rewrite the function to use a cell array of character vectors. For example, code generation for the function useCharacters succeeds.

function out = useCharacters(n) %#codegen
s = {':)'};
for i = 2:n
    s{end+1} = ':)';
end
out = s;
end

Use Cell Array of Objects

If your MATLAB code uses an object array, rewrite your code to use a cell array of objects.

For example, consider the MATLAB function makeSquares_error, which uses the class MySquare. Code generation for makeSquares_error fails because mySquaresCollection is an object array.

classdef MySquare
    properties
        side
    end
end

function out = makeSquares_error(n) %#codegen
obj = MySquare;
obj.side = 1;
mySquaresCollection = [obj];
for i = 2:n
    obj.side = i^2;
    mySquaresCollection(end+1) = obj;
end
out = mySquaresCollection;
end

To resolve this error, define mySquares as a cell array. For example, code generation for makeSquares succeeds.

function out = makeSquares(n) %#codegen
obj = MySquare;
obj.side = 1;
mySquaresCollection = {obj};
for i = 2:n
    obj.side = i^2;
    mySquaresCollection{end+1} = obj;
end
out = mySquaresCollection;
end

Use Cell Array of Objects and deal

If your MATLAB class defines a class constructor method, you can assign values to that class by using indexed assignment. If you use this coding pattern, you can use the deal function to perform the same style of assignment in your MATLAB code for code generation.

For example, consider the class MyConstructorClass and the function useClass_error that uses the class. The class MyConstructorClass has a class constructor method that assigns a value to MyConstructorClass.prop. The useClass_error function creates an array of MyConstructorClass objects, and uses indexed assignment to assign values to a subset of objects in the array. Code generation for useClass_error fails because myObjs is an object array.

classdef MyConstructorClass
    properties
        prop
    end
    methods
        function obj = MyConstructorClass(val)
            obj.prop = val;
        end
    end
end

function out = useClass_error(n) %#codegen
obj = MyConstructorClass(0);
myObjs = [obj obj obj obj obj];
myObjs(2:3) = [MyConstructorClass(n^2) MyConstructorClass(n+2)]
out = myObjs;
end

To resolve this error without converting the indexed assignment into a loop, use a cell array and the deal function. For example, code generation for the function useClass succeeds.

function out = useClass(n) %#codegen
obj = MyConstructorClass(0);
myObjs = {obj obj obj obj obj};
[myObjs{2:3}] = deal(MyConstructorClass(n^2),MyConstructorClass(n+2));
out = myObjs;
end