# How to Code This Using filter?

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Kareem Elgindy 2020 年 7 月 18 日

I don't know how to use filter and unfortunately the Help Documentation of filter is not useful enough for me. I'd like to use filter to code the following recursive formula
x(i+1) = a(i)*x(i), i = 1, 2 , ..., n,
where a(i) is some function of i. Assume x(1) = 2. I don't want to use a for loop for possibly faster implementations.

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### 採用された回答

Devineni Aslesha 2020 年 7 月 21 日
Assuming that 'a' is input data and 'x' is the output data, you can use the filter function as shown below
a = 1:5;
num = 1;
den = [1 -1];
x = filter(num,den,a)
for the linear filter equation x(i) - x(i-1) = a(i). However, in your case, it appears like the filter equation is a nonlinear equation. i.e. x(i+1)/x(i) = a(i). So, make sure if your filter equation is correct.

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