How can I repeatedly put a function result into itself, and get a matrix of the results?

I want to take the output of a function and put it back into the function to get my next output, then get a matrix of all the results. For example, if my function was b=3*a+1, I would want to get the matrix [4, 13, 40, 121...] etc, but a very large size, eg. 500 numbers.

Here's how I'm currently doing it. I have the function:

function [ y ] = test( x )
%UNTITLED2 Summary of this function goes here
%   Detailed explanation goes here
y=3*x+1
assignin('base', 'B', evalin('base', 'B')+1)
end

The purpose of the assignin bit is to increase B (a variable in my main workspace) by 1 every time I use the function.

I start by setting B as 0. Then in my command window I do:

M(1,B)=test(1)

This sets M as 4.

Then I do:

M(1,B)=test(M(1,B))

This makes M a 1x2 matrix [4, 13].

I have to keep repeating this last command to get more values in matrix M.

Is there a way I could get, let's say, 500 values of M all at once, giving me a 1x500 matrix in one go?

P.S. 3X+1 is just an example - the equation I'm actually using is a bit more complicated, so finding a general equation for the nth term won't help.

I'm on 2016a

[Stephen23]

Using assignin and evalin is not a good practice for such trivial code:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Answer 1

[Thorsten]

 N = 500;
 Y = zeros(1, N); % preallocate for speed
 Y(1) = myfun(1);
 for i = 2:N
   Y(i) = myfun(Y(i-1); 
 end

with myfun.m defined as

 function y = myfun(x) % use a name for the function that is not a Matlab command
 y = 3*x+1;