logical indexing

imagine you have a two matrixes:

a=[1 2 3 4 5 6 7 8 9];
b=[1 0 1 0 1];

how do i use the b matrix as a logical index? I'd expect:

a(b)
ans =
[1 3 5]

but instead i get the error: "Subscript indices must either be real positive integers or logicals."

if I try 
a(~b) 
ans =
[2 4]

now I could use a(~~b) which does what i want but this seems inelegant. Can anyone suggest a better solution?

Answer 1

[Jacob Halbrooks]

Use LOGICAL to do the type conversion:

a(logical(b))

Answer 2

[Aldin]

Here:

for i = 1:5
if b(i) == 1
disp(a(i))
end
end

:)

[Aldin]

If you want i can put the result 1,3,5 in an array ???

[Thijs]

using a for loop seems even MORE inelegant

[Aldin]

Thanks

[Aldin]

index = 0;

for i = 1:length(b)

if b(i) == 1

index = index + 1;

result(index) = a(i);

end

end

[Aldin]

result =

1 3 5

[Aldin]

Here is another solution:

a(b(1:5)==1)

Answer 3

[Thijs]

a(logical(b)) does what I want. In case anyone else is wondering

[Aldin]

See above it's similiar with my solution :)

[Thijs]

true, thanks

Answer 4

[Onomitra Ghosh]

Your "b" matrix is in double. You need to convert that to logical values for logical indexing:

>> a(boolean(b))

ans =

     1     3     5

Answer 5

[Aldin]

but what if you haven't only '1' and '0' in b array. I think it's better my first solution or second &Onomitra Ghosh his code with logical work correctly