Recursive function to reverse a vector

Question

katherine keogh 2020-10-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/621353-recursive-function-to-reverse-a-vector

回答： SUTITHI 2022-11-22

采纳的回答： John D'Errico

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The question is to write a function to reverse a vector so v=[1 2 3] becomes v= [3 2 1]

It must be recursive

So my idea how to write this function is start off with d=1 the last digit of w, the output, is equal to the first digit of v the input. so thats what I wrote. As the function develops d eventually equals the length of the original input symoblising the function has reached an end and it will return w. I just don't know how to keep d growing everytime, as currently with every time I recall the function d will rewrite itself to be 1. I think it has something to do with the base case but I don't know. I'm not looking for the answer on a plate but more like a discussion of someone talking me through it

function [w]=reversal(v)
d=1
e=length(v)
w(end-d+1)=v(d)
if d==e
w=w
else 
   [w]=reversal(v) 
end
end

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Answer 1

John D'Errico 2020-10-21

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Hmm. Suppose you strip off the last digit of the vector on each pass? Can you do that? You already know how to find the last element of a vector. That is just v(end). Your logic would look something like the logic in your digit_sum problem.

function vrev = reversal(v)
  if length(v) <= 1
    vrev = v;
  else
    % here, you need to break v into two pieces. 
    % one of those peieces will be the final element of v.
    % The other will be everything else.
    % then you can build up the vector, appending
    % the last element to the BEGINNING of the result.
    
    % take a shot at it, and I will look back in...
  end
end

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John D'Errico 2020-10-21

Yep. In fact, that looks very much like the code I wrote when I tested my algorithm.

The only thing I would add are semi-colons at the ends of your lines. they get rid of the crapola that overwhelms your command window. So you only need to see output that you really want to see.

I think you are getting the hang of recursion. Or perhaps I should say - you will understand recursion when you understand recursion.

Image Analyst 2020-10-21

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Answer 2

David Hill 2020-10-21

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https://ww2.mathworks.cn/matlabcentral/answers/621353-recursive-function-to-reverse-a-vector#answer_520303

编辑：David Hill 2020-10-21

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function w=reversal(v)
if length(v)==1
    w= v;
else
    w= [v(end),reversal(v(1:end-1))];
end

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David Hill 2020-10-21

Just like John explained below. You stop the recursion when the length of the vector is equal to 1, otherwise you keep calling the function and growing the layers of recursion. For example: [1 2 3 4] becomes

w=[4 , reversal([1 2 3])]

w=[3, reversal([1 2])]

w=[2, reversal([1])]

w=1

You go 4 layers into the recursion. When the recursion ends, it all comes together.

w=[4 3 2 1]

katherine keogh 2020-10-21