why does the window show me x3=27 when x3=1:2:50?

I wrote this code ,and the c3 can be a vector ,the elements in this vector is the result of every loops,however,i found that the window show me an error ,here is my code

XX3=1:2:50  
AXX3=zeros(1,length(XX3))
a3=5
b3=-3
for x3=1:2:50 
    y3=a3*x3^2+b3 
       c3(x3)=AXX3(x3)+y3
end

And here is the error

Attempted to access AXX3(27); index out of bounds
because numel(AXX3)=25.

Error in wqeqwe (line 7)
       c3(x3)=AXX3(x3)+y3

I found that the x3 = 27.i wonder why? x3 is 1:2:50.there should be only 25 elements,i mean ,x3 should be as same as XX3,does anyone know where is my mistake

Answer 1

[madhan ravi]

No loop's needed:

XX3=1:2:50;
a3=5;
b3=-3;
x3=1:2:50;
y3=a3*x3.^2+b3;
AXX3=zeros(size(XX3)); 
c3=AXX3+y3; % if AXX3 is zero then why add it with y3 to get c3?

[madhan ravi]

Reason for your error:

AXX3 - has 25 elements but inside the loop you try to access 27th element which is out of bounds.

Your loop would be:

XX3=1:2:50;
AXX3=zeros(size(XX3));
a3=5
b3=-3
x3=1:2:50;
c3=zeros(size(XX3)); % pre-allocate
for k=1:numel(x3);
    y3=a3*x3(k)^2+b3;
    c3(k)=AXX3(k)+y3;
end

[yang-En Hsiao]

but where is 27 from in my code?the window show me x=x3=1:2:50,and x3=27,it is strange

[madhan ravi]

>> x3 = 1:2:50

x3 =

  Columns 1 through 13

     1     3     5     7     9    11    13    15    17    19    21    23    25

  Columns 14 through 25

    27    29    31    33    35    37    39    41    43    45    47    49
   %^^-------?????????????????? 

>> 

I suggest you to do Matlab onramp course and learn the basics of MATLAB.

When you try to access the 27th element MATLAB stops the execution showing the warning, so the last value of x3 is 27, that being stored in the workspace when x3 is typed in command window you get a scalar 27 since you run it a loop, clear?

[yang-En Hsiao]

ok i understand,but why will the matlab stop and warn when when the x3 is 27?

[madhan ravi]

Jeez..,What is Indexing and why it is the first and foremost important thing to learn before making tasks difficult?

% example
x = 1:25 % 25 elements
x(27) % will throw error why?
% ^^---- there exists no such element in the 27th place in x because x has only 25 elements not 27, get it?