why am i getting left and right sides unequal error in matlab

One possible reason for getting a "left and right sides are unequal" error in Matlab is when attempting to assign a value to a non-existent or non-allocated element of an array. For instance, consider the following code that assigns a value to an element of a 2D array based on some condition:

main.m
A = zeros(3,3);
for i = 1:size(A,1)
    for j = 1:size(A,2)
        if A(i,j) == 0
            A(i,j) = i+j;
        end
    end
end
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This code should work as expected and populate the elements of the array A with the sum of their indices whenever they're 0. However, if you mistakenly use a different operator such as = instead of == in the if statement, then Matlab will attempt to assign the result of the expression A(i,j) = i+j (which is a scalar) to the logical value of the expression A(i,j), which is another scalar. This will lead to a "left and right sides are unequal" error because scalar-to-scalar assignment is not allowed in Matlab.

To avoid this error, you can use the disp function to print the values of the operands and find out which one is causing the issue. You can also use a breakpoint to step through the code and inspect the values of the variables at runtime. Additionally, you can use the dbstop if error command to automatically stop the program whenever an error occurs, which can help narrow down the cause of the error. Finally, you can use the try-catch construct to gracefully handle errors and provide custom error messages to the user.