Code for 'Reverse a Vector'

Question

Capulus_love 2020-8-13

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/579063-code-for-reverse-a-vector

回答： Rajith 2023-12-17

v = [1 2 3 4 5];
w = reversal(v)
%-----------------------------------------------------------
function w = reversal(v)
    persistent z;
    s = size(v);
    e = s(2);
    z(e) = v(1);
    if s(2) == 1
        return
    end
    v = v(2:end);
    reversal(v);
    w = z;
end
% The problem is that i can't pass the random vector. 
% Error says Variable w must be of size [1 9]. It is currently of size [1 15]. 
% Check where the variable is assigned a value.
% Test failed using v = [ -75 -22 57 13 -34 2 -94 -49 -11]
% Why does it work like this?

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Atif Penkar 2020-8-23

Did you solve this one? I am stuck on this too, if you could help me out would be great..

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Answer 1

Md Jawarul Moresalein Ayon 2020-8-28

3
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/579063-code-for-reverse-a-vector#answer_486236

在 MATLAB Online 中打开

I have done this one,you may check it ---

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

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Walter Roberson 2020-9-5

Hemanth:

There are a number of things that are wrong with that code.

Firstly:

This is a question about using recursion, so to implement it correctly, you need to define a function that calls itself recursively.

Recursive functions mostly have the same general structure:

they start by examining the inputs to determine whether the termination conditions have been met, and if so then return a direct computation of the inputs
they do a computation on a part of the input
they pass a modified version of the input to the function itself recursively (sometimes this is done before step 2)
they combine the result of the recursive call with the calculation on part of the input, and return that

Every recursive function will have a conditional test, needed to determine whether the recursion is finished. A function that is intended to be recursive but which does not have a conditional test, will run forever (or until it crashes because you accessed an element that does not exist)

Secondly:

indexing at end-1:-1:ceil(length(v)/2) is reversing in itself, and that is not supposed to be done directly. You should be extracting parts but not reversing them, and passing the parts to the function recursively to do the reversal of

Thirdly:

You are breaking into three pieces, one of which is the last element, and the other two of which are halves of what is remaining after removing the last element. You should not be doing that. You should be working with exactly two pieces. Those two pieces could be halves, or the two pieces could be the last element and the rest of the elements.

Hemanth Kumar Reddy Boreddy 2020-9-7

Hey Walter and Bruno, thanks for the help. I was almost there but confused in the end. Thanks again

Micheal Omojola 2020-11-9

编辑：Micheal Omojola 2020-11-9

在 MATLAB Online 中打开

@Bruno Luong. I have edited Walter's code, and it takes care of empty input:

function y=reversal(v) 
    y=[];
    if length(v)==1
        y=v(1);
    elseif length(v) > 1
        k=v(1:ceil(length(v)/2));     
        b=v(ceil(length(v)/2)+1 :end);              
        y=[reversal(b) reversal(k)];
    else
        return
    end         
end

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Answer 2

Serhii Tetora 2020-8-13

1
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https://ww2.mathworks.cn/matlabcentral/answers/579063-code-for-reverse-a-vector#answer_479160

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I had no this error with

v = [ -75 -22 57 13 -34 2 -94 -49 -11]

You can also use

w = flip(v)

It is same..

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Capulus_love 2020-8-13

编辑：Capulus_love 2020-8-13

i have no error too... but the answer says there is a problem.

i know the function 'flip' , but it needs to use recursive function so i made it.

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Answer 3

Mohamed Eid 2023-2-10

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/579063-code-for-reverse-a-vector#answer_1168850

编辑：Mohamed Eid 2023-2-14

在 MATLAB Online 中打开

This code solves the problem and passes all of test cases.

function v = reversal(v)
    len = length(v);
    
    if len > 1
        len = fix(len/2);
        left_be_right = reversal(v(1:len));
        right_be_left = reversal(v(len + 1:end));
        v = [right_be_left,left_be_right];
    end
end

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Answer 4

Walter Roberson 2020-8-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/579063-code-for-reverse-a-vector#answer_479166

That approach is wrong.

Reverse of A B is reverse of B, followed by reverse of A. When you let either A or B be a scalar then reverse of the scalar is the value itself. Therefore you can code each step with just a single recursive call and appending data.

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Answer 5

xin yi leow 2021-1-25

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/579063-code-for-reverse-a-vector#answer_605733

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function v=reversal(w)
    if length(w)==1
        v=w(1);
    else 
        v=[reversal(w(2:end)) w(1)];
    end 
end

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Rik 2021-1-25

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Your function will fail for empty inputs. The edit below fixed that and makes the function more compact.

function v=reversal(v)
if numel(v)>1
    v=[reversal(v(2:end)) v(1)];
end 
end

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Answer 6

Rajith 2023-12-17

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/579063-code-for-reverse-a-vector#answer_1372957

在 MATLAB Online 中打开

function v = reversal2(v)
    if length(v) > 1
        ii = round(length(v) / 2);
        v = [reversal2(v(ii+1:end)) reversal2(v(1:ii))];
    end
end