Resolve Error: Output Variable Must Be Assigned Before Run-Time Recursive Call

R2026b

Issue

When the code generator cannot determine recursion depth, it generates code that performs the recursive calls at run time. This is called run-time recursion. To use run-time recursion, the MATLAB® code must assign a value to the output variable before the first recursive call. Otherwise, the code generator produces this error:

All outputs must be assigned before any run-time recursive call. Output 'output_variable' is not assigned here.

Possible Solutions

To resolve this error, rewrite your code so that it assigns a value to the output variable before the recursive call. This solution can be applied whether the function uses direct or indirect recursion.

Direct Recursion

A directly recursive function calls itself. For example, this function calls itself in the if statement and assigns a value to the output variable out in the else statement. If the input array A is variable size, code generation fails because the code generator must use run-time recursion and the function assigns a value to out after the recursive call.

function out = directRecur_error(A)
if numel(A)>1
 out = A(1)+directRecur_error(A(2:end));
else
   out = A(1);
end
end

To resolve this error, rewrite the code so that assignment to the output variable occurs before the recursive call. In this example, assign a value to out in the if block and perform the recursive call in the else block.

function out = directRecur_example1(A)
if numel(A)==1
    out = A(1);
else
    out = A(1)+directRecur_example1(A(2:end));
end
end

Alternatively, assign a dummy value to out before the if block. 

function out = directRecur_example2(A)
out = 0;
if numel(A)>1
    out = A(1)+directRecur_example2(A(2:end));
else
    out = A(1);
end
end

Indirect Recursion

An indirectly recursive function calls itself through one or more other functions. For example, this function, indirectRecurA_error, calls indirectRecurB, which then calls indirectRecurA_error. Code generation fails because the code generator must use run-time recursion and the function assigns a value to out after the recursive call.

function out = indirectRecurA_error(x)
if x>=0
    out = indirectRecurB(x-1)+1;
else
    out = 0;
end
end

function out = indirectRecurB(x)
out = indirectRecurA_error(x-1)+2;
end

To resolve this error, rewrite the code so that assignment to the output variable occurs before the recursive call. In this example, assign a value to out in the if block and perform the recursive call in the else block.

function out = indirectRecurA_example1(x)
if x<0
    out = 0;
else
    out = indirectRecurB(x-1)+1;
end
end

function out = indirectRecurB(x)
out = indirectRecurA_example1(x-1)+2;
end

Alternatively, assign a dummy value to out before the if block. 

function out = indirectRecurA_error(x)
if x>=0
    out = indirectRecurB(x-1)+1;
else
    out = 0;
end
end

function out = indirectRecurB(x)
out = indirectRecurA_error(x-1)+2;
end

See Also

Topics