How can I assign zeros to a particular value in an expression

Is it possible to assign zeros to following entries: u(N-N), u(N) i.e u(0) and u(4) in the loop below?

N =4;
u = (1:N);
v = (N:2*N-2);
for i=2:N
    rhsode(i,1)=(u(i)-2*u(i-1)+u(i-2))+(v(i)-v(i-2)) 
end 

[dpb]

No. MATLAB arrays are 1-based indexing (1:N) and that is inviolate (unlike C which is 0-based (0:N-1) and also inviolate). Fortran has the facility to define arrays with arbitrary indexing (although the default is also 1-based); unfortunately, The MathWorks chose to not bring that faciility over.

You can only address elements u(1) through u(4) and v(1) through v(3) based on N=4 above.

Answer 1

[Bruno Luong]

test

rhsode = 2×1

0 5

rhsode = 3×1

0 5 2

rhsode = 4×1

0 5 2 -9

function test
N =4;
u = (1:N);
v = (N:2*N-2);
function ui = getu(i)
if i <= 0 | i >= 4
    ui = 0;
else
    ui = u(i);
end
end
function vi = getv(i)
if i <= 0 | i >= 4
    vi = 0;
else
    vi = v(i);
end
end
for i=2:N
    rhsode(i,1)=(getu(i)-2*getu(i-1)+getu(i-2))+(getv(i)-getv(i-2)) 
end 
end

Answer 2

[Matt J]

N =8;
u = [0,(1:N-1),0];
v=N:N-1+numel(u);

rhsode=diff(u,2)+v(3:end)-v(1:end-2)

rhsode = 1×7

2 2 2 2 2 2 -6

[Matt J]

Or,

N = 8;
u = (1:N-1);
v=N+(0:numel(u)+1);

rhsode=conv(u,[1,-2,1],'same') + conv(v,[1,0,-1],'valid')

rhsode = 1×7

2 2 2 2 2 2 -6

[University Glasgow]

Sorry, I still didn't understand this. How can I relate this to my for loop to give me 3 polynomials since I want u_1, u_2 and u_3?

[dpb]

What do you expect the the results to be?

Internally with the various builtin functions like <polyval> and friends, MATLAB represents a polynomial as a row vector with coefficients in descending powers with missing terms explicitly represented by 0 in the appropriate position. But, vectors in MATLAB are still 1-based addressing so

p=[1 -4 4];

would be the representation of quadratic x.^2 - 4*x + 4

But, it's totally unclear what your code is intended to produce...

[Matt J]

Sorry, I still didn't understand this. How can I relate this to my for loop to give me 3 polynomials since I want u_1, u_2 and u_3?

The code you've posted aims to generate an output rhsode of length N-1. That is what my code does.

It is not clear however what boundary conditions you intended on v, so I just assumed in my examples that it was to be extrapolated linearly.