Recursively reversing large vector efficiently

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Rayyan Ramrajkar
Rayyan Ramrajkar on 12 Aug 2021
Commented: Rayyan Ramrajkar on 14 Aug 2021
I am trying to write a code that will recursively compute reverse of given large vector . I have tried 2 codes one simple recursive code and other one iterative reversal code which flips 2 elements in one loop hence efficient
I want to replicate second code below in recursive way how can I approach that? Thank you for your time
% First code with simple recursion that flips end element one by one
function w=reversal_v2(v)
if length(v)==1
w= v;
else
w= [v(end),reversal_v2(v(1:end-1))];
end
% Second code with simple recursion that flips end element and first
% element together hence half the required time
b=1:1e4; %I/P vector
x=length(b); % To get length of vector
a=zeros(x);
for i=1:floor(length(b)/2) %Loop uses simple for loop and temp to substitute the values
tmp=b(i);
a(i)=b(x+1-i);
a(x+1-i)=tmp;
end

Answers (1)

per isakson
per isakson on 12 Aug 2021
Five ways to flip a row vector. The last one, reversal_v3(), answers your question. Recursion is by two order of magnitude slower than the for-loop.
v = 1:1e4;
tic, v(end:-1:1); toc
Elapsed time is 0.000255 seconds.
tic, flip(v); toc
Elapsed time is 0.000254 seconds.
tic, reversal_v2(v); toc
Elapsed time is 0.274852 seconds.
tic, SecondCode(v); toc
Elapsed time is 0.001037 seconds.
tic, reversal_v3(v); toc
Elapsed time is 0.097940 seconds.
function w = reversal_v2(v)
% First code with simple recursion that flips end element one by one
if length(v)==1
w = v;
else
w = [v(end),reversal_v2(v(1:end-1))];
end
end
function a = SecondCode( b )
% Second code with simple recursion that flips end element and first
% element together hence half the required time
% b=1:1e4; % I/P vector
x = length(b); % To get length of vector
a = zeros(1,x); % Create row vector
% Loop uses simple for loop and temp to substitute the values
for ii = 1:floor(length(b)/2)
tmp=b(ii);
a(ii)=b(x+1-ii);
a(x+1-ii)=tmp;
end
end
function w = reversal_v3(v)
% ... replicate second code below in recursive way ...
if length(v) <= 2
w = flip( v );
else
w = [ v(end), v(end-1), reversal_v3(v(1:end-2)) ];
end
end
  1 Comment
Rayyan Ramrajkar
Rayyan Ramrajkar on 14 Aug 2021
Thank you for your elaborate codes and I got the logic need to be implemented.
Regarding for loop vs recursion, I understand that simple loop approach is far more uperior than Recursion, but Recusrsion is must have requirement

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