matlab code for lagrange methode

i getting this error :

Operator '-' is not supported for operands of type 'cell'.
Error in untitled (line 20)

code

clear all;
close all;
clc;
%%
syms x;
%Data
%Input data
n=input('Enter no. of node points:');                %no. of node points
%enter x values
a=input('Enter the values of x in braces:');
if length(a)==n
    b=input('Enter the values of y in braces:');
    %calculation of coeffiecients
    for i=1:n
        for j=1:n
            c.c(i,j)=(a(1,i)-a(1,j));
        end
    end
    c=c.c;
    q=size(c);
    c=c';
    c(c==0)=[];                  %deleting the zero values from the matrix
    c=reshape(c,q(2)-1,q(1));
    c=c';
    j=1;
    for i=1:n
        d.d(i,j)=prod(c(i,:));
    end
    d=d.d;
    d=d';
    for i=1:n
        coff.coff(i,j)=b(1,i)./d(1,i);
    end
    coff=coff.coff;
    coff=coff';                                  %coefficients
    %%
    for i=1:n
        for j=1:n
            p.p(i,j)=(x-a(1,j));
        end
    end
    p=p.p;
    m=size(p);
    p=p';
    p(1:n+1:end)=[];                     %deleting the diagonal elements
    p=reshape(p,m(2)-1,m(1));
    %%
    p=prod(p);
    pol=coff.*p;
    pol=sum(pol);                                %Lagrange polynomial
    %%
    l=input('Enter a value to compute the b value:');
    output=subs(pol,l);
    %%
    %Displaying output
    disp('');
    fprintf('Value of input x:=%f\n',double(output));
    %%
    if l>max(a)
        x=min(a):0.1:l;
        y=subs(pol,x);
        plot(x,y,'r');
        xlabel('X')
        ylabel('Y')
    else
        x=min(a):0.1:max(a);
        y=subs(pol,x);
        plot(x,y,'r');
        xlabel('X')
        ylabel('Y')
    end
else
    display('Error: Values of x exceeds the number of node points')
end
%End of code

[madhan ravi]

That’s why preallocation is really important. Preallocate variable before the loop to avoid ambiguities. You’re creating struct inside loops.

[darova]

I have a problem

Error: Values of x exceeds the number of node points

Answer 1

[Nagasai Bharat]

Hi Razan,

It is my understanding that you are performing some numerical computation using a cell array taken as input. To access an element of type cell array, curly brackets ‘{}’ to be used instead to parenthesis ‘()’. Or the cell array should be converted to matrices using cell2mat function.

Changes to your code as below should work.

if length(a)==n
    b=input('Enter the values of y in braces:');
    %calculation of coeffiecients
    for i=1:n
        for j=1:n
            c.c(i,j)=(a{1,i}-a{1,j});
        end
    end
    c=c.c;
    q=size(c);
    c=c';
    c(c==0)=[];                  %deleting the zero values from the matrix
    c=reshape(c,q(2)-1,q(1));
    c=c';
    j=1;
    for i=1:n
        d.d(i,j)=prod(c(i,:));
    end
    d=d.d;
    d=d';
    for i=1:n
        coff.coff(i,j)=b{1,i}./d(1,i);
    end
    coff=coff.coff;
    coff=coff';                                  %coefficients
    %%
    for i=1:n
        for j=1:n
            p.p(i,j)=(x-a{1,j});
        end
    end
    p=p.p;
    m=size(p);
    p=p';
    p(1:n+1:end)=[];                     %deleting the diagonal elements
    p=reshape(p,m(2)-1,m(1));
    %%
    p=prod(p);
    pol=coff.*p;
    pol=sum(pol);                                %Lagrange polynomial
    %%
    l=input('Enter a value to compute the b value:');
    output=subs(pol,l);
    %%
    %Displaying output
    disp('');
    fprintf('Value of input x:=%f\n',double(output));
    %%
    if l>max(cell2mat(a))
        x=min(cell2mat(a)):0.1:l;
        y=subs(pol,x);
        plot(x,y,'r');
        xlabel('X')
        ylabel('Y')
    else
        x=min(cell2mat(a)):0.1:max(cell2mat(a));
        y=subs(pol,x);
        plot(x,y,'r');
        xlabel('X')
        ylabel('Y')
    end