How do I evaluate a user-input trigonometric function?

I'm not entirely sure how to explain this but here is the question:

My main issue is I don't know how to use eval when using a user-input trigonometric function. (I don't really need help with the graphing part).

This is my attempt:

n = input('Enter a positive number n: ');
x = linspace(0, n, 1000)

 trigfunc = input('Enter a trigonometric function: ', 's');
 eval('trigfunc(x)')

[Stephen23]

"Then, by using the function eval..."

Ugh, why teach impressionable students such awful programming practices?

Why not simply use a basic look-up, or str2func() or feval() ?

https://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

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

Answer 1

[Birdman]

n=input('enter a pos num\n');
x=linspace(0,n,1000);
trigfunc=input('Enter 1=sin(x), 2=cos(x), 3=tan(x), 4=cot(x)\n');
if(trigfunc==1)
    y=sin(x);
    plot(x,y);title(['sin(x) from [0 ' ,num2str(n), ']'])
elseif(trigfunc==2)
    y=cos(x);
    plot(x,y);title(['cos(x) from [0 ' ,num2str(n), ']'])
elseif(trigfunc==3)
    y=tan(x);
    plot(x,y);title(['tan(x) from [0 ' ,num2str(n), ']'])
elseif(trigfunc==4)
    y=cot(x);
    plot(x,y);title(['cot(x) from [0 ' ,num2str(n), ']'])
end

Answer 2

[Stephen23]

It is simpler to use str2func:

num = str2double(input('End value of vector: ','s'));
fun = str2func(input('Trig function (e.g. sin, cos, etc.): ','s'));
x = linspace(0,num,1000);
y = fun(x);
plot(x,y)

To which it is also easy to add input checking, etc.

Answer 3

[Prathap]

Plot the trigonometric curve as a function of time. Hint: Use ‘eval’ function within the script to accept different trigonometric functions (e.g., sin, cos, tan, etc.).

[Stephen23]

Much Better Hint: Use STR2FUNC() function within the script to accept different trigonometric functions:

https://www.mathworks.com/help/matlab/ref/str2func.html