Why do i get an error saying "Unrecognized function or variable 'x'."

I am trying to solve a function with header [approx, true err, rel true err] = myDouble Exp(x, N), to compute for approximation of e^x² using the first N terms of the Taylor series expansion, also computing for true error and relative true error. The function myDoubleExp must take array inputs.

function [approx, true_err, rel_true_err] = MyDoubleExp(x,N)

approx = zeros(size(x));

true_err = zeros(size(x));

rel_true_err = zeros(size(x));

for n = 0:N

term = (x.^(2*n)) / factorial(n);

approx = approx + term;

end

true_value = exp(x.^2);

true_err = true_value - approx;

for i = 1:numel(x)

if true_value(i) == 0

rel_true_err(i) = 0;

else

rel_true_err(i) = abs(true_err(i)) / abs(true_value(i))*100;

end

end

end

If I run the above code I receive the following error

>> MyDoubleExp(x, N)

Unrecognized function or variable 'x'.

What correction should be done in order to get the code running? I am using MATLAB version R2023a

Answer 1

[Star Strider]

You have not defined ‘x’ specifically in your calling script prior to calling the function.

Try something like this —

x = pi;
N = 5;

[approx, true_err, rel_true_err] = MyDoubleExp(x,N)

approx = 1.3956e+03

true_err = 1.7938e+04

rel_true_err = 92.7817

function [approx, true_err, rel_true_err] = MyDoubleExp(x,N)
approx = zeros(size(x));
true_err = zeros(size(x));
rel_true_err = zeros(size(x));
for n = 0:N
    term = (x.^(2*n)) / factorial(n);
    approx = approx + term;
end
true_value = exp(x.^2);
true_err = true_value - approx;
for i = 1:numel(x)
    if true_value(i) == 0
        rel_true_err(i) = 0;
    else
        rel_true_err(i) = abs(true_err(i)) / abs(true_value(i))*100;
    end
end
end

It is almost always best to write and run scripts rather than executing code in the Command Window.

.

[Rejane Adelie]

It was not given in the problem so i didn't defined 'x', is it possible without it?

[Dyuman Joshi]

"It was not given in the problem so i didn't defined 'x', is it possible without it?"

You need to provide a value to run the code, it does not matter if you call the variable x, y, z or xyz or any other valid variable name.

You can also directly call the function -

%I have not defined x and N, but directly used the values
%                                              vv v
[approx, true_err, rel_true_err] = MyDoubleExp(pi,5)

approx = 1.3956e+03

true_err = 1.7938e+04

rel_true_err = 92.7817

function [approx, true_err, rel_true_err] = MyDoubleExp(x,N)
approx = zeros(size(x));
true_err = zeros(size(x));
rel_true_err = zeros(size(x));
for n = 0:N
    term = (x.^(2*n)) / factorial(n);
    approx = approx + term;
end
true_value = exp(x.^2);
true_err = true_value - approx;
for i = 1:numel(x)
    if true_value(i) == 0
        rel_true_err(i) = 0;
    else
        rel_true_err(i) = abs(true_err(i)) / abs(true_value(i))*100;
    end
end
end

[Star Strider]

@Rejane Adelie — The analysis by @Dyuman Joshi is correct. I cannot improve on it.