eval(sprintf('left = X+X%d+X%d+​X%d+X%d+X%​d+X%d', i==1:6)); throws error Undefined function or variable 'X0'. why

In my script I try to automate the additon of the variables but it is throwing error. please help

[James Tursa]

Use 1:6 instead of i==1:6

But, seriously, you should not be doing this at all. Heed all of the advice below.

Answer 1

[Daniel M]

Forget about it. <https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval>

[ARAVINTH DESIKAN]

what are the alternatives to eval function then can you tell me I wnat dynamic variables to be created from a cell

[Daniel M]

You should spend some time and read through that link a little deeper.

Answer 2

[Walter Roberson]

i==1:6

returns a vector of values that are all either 0 (not equal) or 1 (equal) . You slide those 0 and 1 into variable names. If i is a scalar, you can be sure that at most one of the locations is 1; if i is a row vector of length 6 then there might be more than one location with 1.

Therefore you are constructing an expression with variable X, and variable X0, and possibly also variable X1. You would get an error if variable X0 did not exist.

It is far from clear why you would want to use that code.

[ARAVINTH DESIKAN]

how to use then

[Walter Roberson]

We do not know what you are trying to do. Are you trying to do the assignment

left = X + X1 + X2 + X3 + X4 + X5 + X6

if so then create a vector

Xvals = [X, X1, X2, X3, X4, X5, X6 ... up to maximum possible]

after which you can sum(Xvals(1:6)),

If you are not using scalars but you are using arrays that are the same size, then

nd = ndims(X);
Xvals = cat(nd+1, X, X1, X2, X3, X4, X5, X6 ... up to maximum possible);

after which you can sum(Xvals,nd+1) if you are doing all of them. If you are not doing all of them then it is possible to do, but it is easier if you know the maximum dimension ahead of time, such as sum(Xvals(:,:,1:6), 3) for the case the values are 2D.

[ARAVINTH DESIKAN]

left = X + X1 + X2 + X3 + X4 + X5 + X6

I want to automate this for n number of variables

Answer 3

[Bhaskar R]

I support Daniel M answer. But if you need answer anyway these are the resolutions

From your question it is clear that X0 is not initializied that means you need to initialize each and every variable initialized before the eval function.

Even if your all initializations are correct, the evaluation is giving error as non-ASCII characters contained string.

>> txt = sprintf('left = X+X%d+X%d+​X%d+X%d+X%​d+X%d', i==1:6)    % took sprintf seperate as seperate string
>> txt =

    'left = X+X0+X1+​X0+X0+X'
>> ASCII_values = double(txt);    % ASCII values of the string
>> ASCII_values(16)                % this is out of ASCII chacter
    ans =

        8203
>> txt(16)                % this is empty string in the sprintf string
    ans =

        '​'
 >> txt(16) = [];        % you need to remove that non ASCII from the string so that you can apply eval cammand over there

For this you need to make additional programming

[ARAVINTH DESIKAN]

I have values coming from array ao.

the length of ao is 6

for i=1:length(ao)

eval(sprintf('X%d = [A%d(1)]', i,i));

eval(sprintf('Y%d = [A%d(2)]', i,i));

eval(sprintf('W%d = [A%d(3)]', i,i));

eval(sprintf('H%d = [A%d(4)]', i,i));

end

now I want to add x1 to length of ao automatically can you help

[Walter Roberson]

The second last % in 'left = X+X%d+X%d+​X%d+X%d+X%​d+X%d' is followed by char(8203) "zero width space" .

Sometimes the mechanism that creates titles for here introduces odd characters that are not in the original text.