Matlab - Subscript indices must either be real positive integers or logicals.

function S = valles(M)

%    M=[5  5  7  8   3   1; 5  8  4  9   8   3;5  7  9  6   1   7;2  1  7   3  4   6;2  4  5   1  6   9]

 if(nargin==0)

     error('Ningun valor de entrada detectado')

end

[f,c]=size(M);

for i=1:f

    for j=1:c

        if(M(i,j)>9 || M(i,j)<1 || floor(M(i,j))~=M(i,j))

            error('La matriz contiene numeros no válidos')

        end

    end

end

if(c<=2 || f<=2)

    S=[];

end

a=1;

b=1;

for i=1:f-1

    for j=1:c-1

        if((i>1 && j>1))

            if(M(i,j)<M(i-1,j) && M(i,j)<M(i+1,j) && M(i,j)<M(i,j+1) && M(i,j)<M(i,j-1) && M(i,j)<M(i-1,j-1) && M(i,j)<M(i+1,j+1))

                v(a)=i;

                d(b)=j;

                a=a+1;

                b=b+1;

            end

        end

    end

end

S=ones(length(v),2);

for i=1:length(v)

    S(i,1)=v(i);

    S(i,2)=d(i);

end

end

>> M=[5  5  7  8   3   1; 5  8  4  9   8   3;5  7  9  6   1   7;2  1  7   3  4   6;2  4  5   1  6   9];

>> S = valles(M)

S =

     2     3

     3     5

     4     2