Error using plot Vectors must be the same length.

So I've got an error in plot:

Error using plot
Vectors must be the same length.

Error in lesson5_1 (line 25)
     plot(X,Y,'*k',xi,zi);grid
     % Построение графика

Here is the code;

X=[1910 1920 1930 1940 1950 1960 1970 1980 1990]-1909; %госдолг сша
Y=[120.7 26.0 162.0 50.7 256.9 290.5 380.9 90.9 20.6];
xzv=1955-1909;
xi=min(X):0.05:max(X);            % Вспомогательный вектор абсциссы
yi=spline(X,Y,xi);
AS=[2 1 0 0 0 0 0 0 0;
    1 4 1 0 0 0 0 0 0;
    0 1 4 1 0 0 0 0 0;
    0 0 1 4 1 0 0 0 0;
    0 0 0 1 4 1 0 0 0;
    0 0 0 0 1 4 1 0 0;
    0 0 0 0 0 1 4 1 0;
    0 0 0 0 0 0 1 4 1;
    0 0 0 0 0 0 0 1 2];
BS=3.*[26.0-120.7;162.0-120.7;50.7-26.0;
       256.9-162.0;290.5-50.7;380.9-256.9;
       90.9-290.5;20.6-380.9;20.6-90.9];
S=AS\BS;
 dx=(max(X)-min(X))/50;             % Вычисление шага

     for i=(1:50),
        xi(i)=min(X)+dx*i;              % Вспомогательные точки абсциссы
        zi(i)=form2(X,Y,S,xi(i));          % Значение S3 в точке xi(i)
     end;

     plot(X,Y,'*k',xi,zi);grid          % Построение графика

And here is the function form2:

     function y=form2(X,Y,S,z);
       i=max(find(X<z));
       h=X(i+1)-X(i);
       p=(X(i+1)-z)/h;
       q=(z-X(i))/h;

   y=p^2*(2*q+1)*Y(i)+p^2*q*S(i)+...
    q^2*(2*p+1)*Y(i+1)-q^2*p*S(i+1);

What could be the problem?

Answer 1

[KALYAN ACHARJYA]

No Error

[Valeria Pinus]

that's the first plot, i'm asking about the second (edited the code, now there won't be misunderstanding)

[KALYAN ACHARJYA]

To plot the two vectors must be same length , see xi and zi, both are different length

>> whos xi
  Name      Size              Bytes  Class     Attributes
  xi        1x1601            12808  double              
>> whos zi
  Name      Size            Bytes  Class     Attributes
  zi        1x50              400  double   

You have defined xi varible in two places

  xi=min(X):0.05:max(X); 

Other

within for loop

[Valeria Pinus]

I understand that, but I don't know why these vectors have different length and how to fix it.

[KALYAN ACHARJYA]

Here I have changed the later xi variable name to xi_new

X=[1910 1920 1930 1940 1950 1960 1970 1980 1990]-1909; %госдолг сша
Y=[120.7 26.0 162.0 50.7 256.9 290.5 380.9 90.9 20.6];
xzv=1955-1909;
xi=min(X):0.05:max(X);            % Вспомогательный вектор абсциссы
yi=spline(X,Y,xi);
AS=[2 1 0 0 0 0 0 0 0;
    1 4 1 0 0 0 0 0 0;
    0 1 4 1 0 0 0 0 0;
    0 0 1 4 1 0 0 0 0;
    0 0 0 1 4 1 0 0 0;
    0 0 0 0 1 4 1 0 0;
    0 0 0 0 0 1 4 1 0;
    0 0 0 0 0 0 1 4 1;
    0 0 0 0 0 0 0 1 2];
BS=3.*[26.0-120.7;162.0-120.7;50.7-26.0;
       256.9-162.0;290.5-50.7;380.9-256.9;
       90.9-290.5;20.6-380.9;20.6-90.9];
S=AS\BS;
 dx=(max(X)-min(X))/50;             % Вычисление шага
     for i=(1:50)
        xi_new(i)=min(X)+dx*i;              % Вспомогательные точки абсциссы
        zi(i)=form2(X,Y,S,xi_new(i));          % Значение S3 в точке xi(i)
     end
     plot(X,Y,'*k',xi_new,zi,'r');grid          % Построение графика 
     

[KALYAN ACHARJYA]

The reason, the later xi just replaced the previous 50 elements of the initial xi vector and maintained the same length.

Lets example you defined xi as following

xi=0:1:10   % Length eleven 
for i=1:5 
xi(i)=i*3;
y(i)=..
end

In the second loop, it replace the initial 5 elements of the xi, but it maintained the initial length of xi, and y reflects the length of equal to the loop iteration. So you cant plot xi and yi

Always assign the new variable name to avoid such clashing the variables name, if memory allows.

Hope it helps

[Valeria Pinus]

now it works, thank you!