Why do i get the error "inconsistent row/column dimensions" for line a=X\y

i=3.5
j=10.0 
k=14.5 
x=[0.0:0.5:i]' 
y=[20.4;17.8;15.5;13.4;11.3;9.2;6.9;5.9] 
X=[x ones(x)] 
a=X\y 
xx=[0.0:0.5:i+0.5]' 
yy=a(1)*xx+a(2) 
plot(xx,yy) 
plot2d(x,y,-1)
x=[i+0.5:0.5:j]'
y=[5.9+0.5;6.1;6.3;6.6;6.9;7.2;7.5;7.8;8.0;8.3;8.5;8.6;8.9;9.6]
X=[x ones(x)]
a=X\y
xx=[i-1:0.5:j+1]'
yy=a(1)*xx+a(2) 
plot(xx,yy) 
plot2d(x,y,-1)
x=[j+0.5:0.5:k]' 
y=[9.6+0.5;10.5;11.4;12.2;13.1;13.8;14.5;15.2;16.0;16.6] 
X=[x ones(x)] 
a=X\y 
xx=[j-1:0.5:k+1]' 
yy=a(1)*xx+a(2) 
plot(xx,yy) 
plot2d(x,y,-1)

[Dyuman Joshi]

You have defined X incorrectly, ones() requires scaler input and for doing using mldivide (\), both X and y need to have same number of rows.

[Himani]

I'm sorry, I'm new to this, what exactly should I change in the code

Answer 1

[dpb]

This time I formatted your code for you; in future use the "CODE" button to do so (or select code and use CTRL-E).

Please add the trailing semicolons so don't overwhelm output screen with echo of every line.

The above code doesn't get that far even

i=3.5;
j=10.0; 
k=14.5;
x=[0.0:0.5:i].';
y=[20.4;17.8;15.5;13.4;11.3;9.2;6.9;5.9];
X=[x ones(x)];
a=X\y;

You were probably looking for

X=[x ones(size(x))];

instead.

Then

p=X\y
plot(x,y,'*-')
hold on
yhat=polyval(p,x);
plot(x,yhat,'r-')

works as intended.

However, NB that

b=polyfit(x,y,1)

produces the same result with less user code.

And also note you can evaluate polynomials with polyval or polyconf

[Himani]

Thank you so much!!!

[dpb]

Glad to help...

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