Trouble with my function

I want to run this code:

syms x

syms y(x)

y(x) = 5*(x^2);

values = 4;

h = 1;

derivative(5,1)

function yd = derivative(values, h)

syms y(x)

yd = (y(values + h) - y(values - h)) / (2*h);

end

But instead of getting the real value ( in this case 40 ), I got :

ans =

y(6)/2 - y(4)/2

How can I solve this please.

Answer 1

[Star Strider]

You need to tell your ‘derivative’ function what ‘y’ is.

Changing ‘derivative’ to an anonymous function (for my convenience) and otherwise leaving it unchanged (except to add ‘y’ as an argument):

syms x y(x)

derivative = @(values, h, y) (y(values + h) - y(values - h)) / (2*h);

y(x) = 5*(x^2);
values = 4;
h = 1;
Result = derivative(5,1,y)

produces:

Result =
50

Note —

Check = diff(y)
CheckTest = Check(values)

produces:

CheckTest =
40

I will let you troubleshoot that discrepancy.

[lilo moutila]

but it is neccessery for me to use

function

yd = derivative(values, h)

end

[Star Strider]

O.K., then do so. I used the anonymous funciton version because it is more convenient for me. The results will be the same.

It will still be necessary to include ‘y’ as an argument, regardless of the function format.

[lilo moutila]

Please how can we do it using

function

yd = derivative(values, h)

end

And you said that i have to include 'y' as an argument.

[Star Strider]

It must be some version of:

function yd = derivative(values, h, y)
yd = (y(values + h) - y(values - h)) / (2*h);
end

The order of the arguments are not important, so long as the call to ‘derivative’ has them in the same order.

‘And you said that i have to include 'y' as an argument.’

I did, because ‘derivative’ does not otherwise have a definition of ‘y’ (in most instances, however I will not go into the exceptions here), so it must be passed as an argument.

It is also not necessary (and is likely not appropriate) to declare:

syms y(x)

inside ‘derivative’, since it already has been declared (and therefore exists) as a symbolic function.

[lilo moutila]

So the code should be like this:

syms x y(x)

y(x) = 5*(x^2);

values = 4;

h = 1;

derivative(5,1)

function yd = derivative(values, h, y)

yd = (y(values + h) - y(values - h)) / (2*h);

end

If yes, I still got the error message

Thank you for your help

[Star Strider]

I am not getting any error messages when I run the code I posted.

You need to include ‘y’ as an argument in your call to it. Currently, you are not doing that. See my original Answer in order to unbderstand how to do that correctly.

Other than that, I have no idea what the problem is with your implementation of it.