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working with an array pointer in a recursive function

Asked by Itzik Ben Shabat on 8 Nov 2012

H, I want to create a recursive function that gets an image matrix and turns it into a quadtree array. so i figured the best way to do this is with a recursive function. so this is what i came up with:

 function [void] = ConstrucQuadtree( A,n,QuadtreeArray,index )
%ConstrucQuadtree constructs a quadtree array from a greyscale image matrix
%A recursively
if (n=1)
    QuadtreeArray(i)=A;
    index=index+1;
else
    ConstrucQuadtree(A(0:n/2,0:n/2),length(A(0:n/2,0:n/2),QuadtreeArray,index);
    ConstrucQuadtree(A(0:n/2,n/2:),length(A(0:n/2,n/2:n),QuadtreeArray,index);
    ConstrucQuadtree(A(n/2:n,0:n/2),length(A(n/2:n,0:n/2),QuadtreeArray,index);
    ConstrucQuadtree(A(n/2:n,n/2:n),length(A(n/2:n,n/2:n),QuadtreeArray,index);
end
end

since i never worked with recursive functions in MATLAB i have a few questions : 1. how do i return void ? (i guess what i wrote in my code is illegal) 2. how do i work with pointers? (both index and QuadtreeArray are supposed to change from one recursion to another.

thanks

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Itzik Ben Shabat

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1 Answer

Answer by Daniel on 8 Nov 2012
Accepted answer

If you want to return "void" you simply do not return anything ...

function ConstrucQuadtree( A,n,QuadtreeArray,index )

MATLAB is lazy and passes pointers around when it can and only allocates new memory when necessary. For your purposes if instead you return QuadtreeArray and index, MATLAB will not allocate new memory. It i important that you return a variable of the same name as the input ...

function [QuadtreeArray, index] = ConstrucQuadtree( A,n,QuadtreeArray,index )

and then modify your code to accept the returned arguments. Something like ...

[QuadtreeArray, index] = ConstrucQuadtree(A(0:n/2,0:n/2),length(A(0:n/2,0:n/2),QuadtreeArray,index);

1 Comment

Itzik Ben Shabat on 8 Nov 2012

thanks! worked great! (aside from a small indexing problem, forgot that the array starts from 1 and not 0... ) :)

Daniel

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