Writing a basic formula - need assisstance

Hi all,

I am really struggling to right a basic formula. I am hoping someone can aid me with this.

I have:

Var1 = 40000 x 1 matrix

and i would like to create a matrix, Var2 with a formula, which i am struggling with. The formula i need to transpose onto matlab is as follows:

Var2_n = (Var1_n - Var1_n+1) + Var2_n-1`

I hope that is clear.

I do know the first part of the equation can be quantified utilising the diff() function, however I am unsure how to incorporate the second part of the equation.

Any help would be much appreciated.

[KSSV]

Var2_n = (Var1_n - Var1_n+1) + Var2_n-1` ?? this means Var2(n) = Var1(n)-Var1(n+1)+Var2(n-1)???If so you should be having intital condition for Var2, do you have it?

[Theodore Bowen]

There you go, i made a mistake in the explanation already.

No! I have not computed Var2 yet. Should i split this up into two formulae?

[Rik]

You need a starting condition, because the calculation of the next value depends on the previous. In my answer I assumed the starting condition to be 0.

Therefor, it will not be possible to split this up into two formulas. Recursive formulas are easiest to solve with a loop, but sometimes they can be linearized.

Answer 1

[Rik]

You may be able to play around with functions like cumsum, but I'm afraid you will have to use a loop:

Var1=rand(4000,1);%just so I have some data to work with
Var2=zeros(size(Var1));%pre-allocate
for n=2:(length(Var1)-1)
   Var2(n)=(Var1(n)-Var1(n+1))+Var2(n-1);
end

[Rik]

This is of course a massive waste of time if you can reduce this problem mathematically, so do try that.