Recursive Method returning answers backwards

Question

Chas 2023-5-2

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1956854-recursive-method-returning-answers-backwards

评论： Torsten 2023-5-7

Consider my code for Euler's Method below. If you run the code, you will see the correct values be stored in the matrix "vals", but after the return statement, the "vals" that is returned is from the first iteration. Why does MATLAB do this?

f = @(x,y)(x^2*(2+y));
ans = euler(1, 0.1, f, 0.0, 1.0, [])
function vals = euler(y,h,f,start,finish,vals)
    if (finish - h < start)
        y;
        return
    else
        y = y + h*f(start,y)
        start = start + h
        vals = [vals,y]
        euler(y,h,f,start,finish, vals)
    end
end

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

John D'Errico 2023-5-2

编辑：John D'Errico 2023-5-2

MATLAB does what you tell it to do. It is not DOING anything to you. Just your code, code written in error. Why you are usign recursion, I don't know, since Euler is simple to write with a loop. Read the answer from James.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

James Tursa 2023-5-2

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1956854-recursive-method-returning-answers-backwards#answer_1227929

编辑：James Tursa 2023-5-2

在 MATLAB Online 中打开

This is a very convoluted way to program the Euler method. There is no need for recursion here, just use simple loops instead.

Regarding why you aren't getting the results you expected, it has to do with how you update vals. Your very first call to Euler is this:

ans = euler(1, 0.1, f, 0.0, 1.0, [])

which returns the vals from the very first call via this line:

vals = [vals,y]

but then in all your subsequent calls to euler( ) you throw away the returned vals:

% you don't save the returned vals, so this line essentially accomplishes nothing
euler(y,h,f,start,finish, vals) 

Maybe changing this euler( ) call to this might get you what you expected:

vals = euler(y,h,f,start,finish, vals)

But, seriously, this code is convoluted and hard to follow because of the recursion. It would be best to rewrite it as a simple loop.

2 个评论
显示无隐藏无

Chas 2023-5-7

I already had done it with a loop and was curious to figure out how recursion worked in MATLAB. What is a fix to this code? I can't think of one based on your explination.