MATLAB Answers

## How to make a function

Asked by NIMA RAHMANI MEHDIABADI

### NIMA RAHMANI MEHDIABADI (view profile)

on 16 Apr 2019
Latest activity Answered by Geoff Hayes

### Geoff Hayes (view profile)

on 17 Apr 2019
So i have a equation of y = x sin(x) - cos (x)
i have done a code which takes the derivative of the function and evaluates at point where x = 2
my code:
clear;
syms x a;
f = inline('x * sin(x) - cos(x)','x');
Df = diff(f(x),x);
double(subs(Df,x,2))
however now i want to make a function called math_Derivative and evaluate at point where x = a
my code:
function y = math_Derivative(x)
y = x * sin(x) - cos(x)
Df = diff(Df,x);
double(subs(Df,x,a))
end
it gives me an error of Not enough input arguments

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R2018b

## 1 Answer

Answer by Geoff Hayes

### Geoff Hayes (view profile)

on 17 Apr 2019

Nima - what is giving the error Not enough input arguments? How are you calling your function? If I just call it like
>> math_Derivative
either from the command line (like above) or from within a function, then I will observe the same error message... because I am not providing an input parameter. Since your math_Derivative function signature expects an input parameter, then you need to provide one like
>> y = math_Derivative(42);
In the above, the input parameter is 42 and the output of this function is assigned to the variable y. Even if this is the cause of the error and so is resolved, you may still have other problems with the code. For example,
Df = diff(Df,x);
What is Df? You are trying to make use of it before it has been initialized....Is this the Df from
f = inline('x * sin(x) - cos(x)','x');
Df = diff(f(x),x);
?

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