Functions

Hy, i'm trying to create a function to calculate some polynomials for a given value (taken as keyboard input). I wrote this code:

function[]= tchebychef(n)
n= keyboard;
C(1,:)=[zeros(1, n-1) -1 0];
S(1, :)=[zeros(1, n) 1];
P =[zeros(1, n-2) -1 0 1];
  for k=2:n
  C(k,:)= conv(C(k-1,:),C(1,:))-conv(S(k-1,:),P);
  S(k,:)=-polyder(C(k,:))/k;
end;

but I get this error:

Error :Function definitions are not permitted in this context.

How can I solve this? Do I have to write my function differently? THX

Answer 1

[Oleg Komarov]

You should use a dedicated .m file for a function, i.e. the first line of a function is always (except comments):

function out = foo(in1,in2m,...) .

You cannot mix a script with a function.

More details in Scripts and Functions

[Alejo Corsani]

THX...this is my first time with a function!!! XD How should I write it? Thank you very much!!!

[Sargondjani]

- make an .m file

- with content:

function [output1,output2,...]=my_function_name(input1,input2,....)

%then calculations:

output1=input1*input2...

end

Save this file as my_function_name.m somewhere that matlab calls it. That's it!!

Now you call the function by simply with:

[y1,y2,...]=my_function_name(x1,x2,...);

for some examples, check out the file exchange

[Alejo Corsani]

THX!!!

Answer 2

[Alejo Corsani]

Hy, I wrote a function file (tchebyshef.m) containing this:

function [ C, S ] = tchebyshef( n )
%UNTITLED Summary of this function goes here
%   Detailed explanation goes here
% function[n]= tchebychef;
C(1,:)=[zeros(1, n-1), -1, 0];
S(1,:)=[zeros(1, n), 1];
P =[zeros(1, n-2), -1, 0, 1];
for k=2:n
  C(k,:) = conv(C(k-1,:),C(1,:))-conv(S(k-1,:),P);
  S(k,:)=-polyder(C(k,:))/k;
end; 

 end

But when i write this:

   C= zeros(4,5);
   S= zeros(4,5);
   [C,S]= tchebyshef(4)

I get this error:

Error in tchebyshef (line 9)
  C(k,:) = conv(C(k-1,:),C(1,:))-conv(S(k-1,:),P);

What is my error?

[Walter Roberson]

You need to show us your error message.

[Alejo Corsani]

I entered my error message at the end of my answer. However, this is my errror message:

In an assignment A(I) = B, the number of elements in B and I must be the same.

Error in tcheby (line 7)

C(1)=[zeros(1, n-1), -1, 0];

[Alejo Corsani]

Sorry..I wrote the wrong error message...the correct one is this:

Subscripted assignment dimension mismatch.

Error in tchebyshef (line 9)

C(k,:) = conv(C(k-1,:),C(1,:))-conv(S(k-1,:),P);

[Walter Roberson]

Earlier you showed the line the error occurred, but not the error message. Now you show a different line, and that is a line of code that does not appear in the code you have shown us, and is related to a different function name than you used before.

We need the relevant function's code, and the error message.

Answer 3

[Alejo Corsani]

Hi, as I said, I wrote a function file (tchebybyshef.m) in which I wrote:

function [ C, S ] = tchebyshef( n )
%UNTITLED Summary of this function goes here
%   Detailed explanation goes here
% function[n]= tchebychef;
C(1,:)=[zeros(1, n-1), -1, 0];
S(1,:)=[zeros(1, n), 1];
P =[zeros(1, n-2), -1, 0, 1];
for k=2:n
  C(k,:) = conv(C(k-1,:),C(1,:))-conv(S(k-1,:),P);
  S(k,:)=-polyder(C(k,:))/k;
end; 
end

then in my Matlab Command Window I wrote these three lines:

C= zeros(4,5); % Inizialize C as a matrix of all zeros
S= zeros(4,5); %Inizialize S as a matrix of all zeros
[C,S]= tchebyshef(4)

and this is my Matlab response:

Subscripted assignment dimension mismatch.

 Error in tchebyshef (line 9) 
 C(k,:) = conv(C(k-1,:),C(1,:))-conv(S(k-1,:),P);

Thx for your time!!! :D

[Walter Roberson]

Use the 'same' option to conv()

[Alejo Corsani]

Sorry I'm very new to Matlab...how and where should I use the "same" option? Thank you very much!!!

[Oleg Komarov]

conv(C(k-1,:),C(1,:),'same')

For future reference, try to keep one post per question, otherwise it becomes increasingly harder (not yet in this specific case) to follow the evolution of the post and keeps other contributors away.