Executing loop on certain numbers

i have two vectors L=1:12; idx=[3 5 9]; I wanted to execute for loop for i=12:-1:1 now i want execute vector idx, starting from last elemet to first i.e 9,5,3 something like

    if i==9
        do stuff
    end

    if i==5
        do stuff 
    end
    .
    .
    if i==3
        do stuff
    end
end
i don't know how many elements in vector idx before hand.

Answer 1

[Stephen23]

You can execute over the vector backwards by using basic MATLAB indexing:

idx = [3,5,9];
for k = idx(end:-1:1)
    ... code here
end

[Raghavendra Reddy P]

Thank you sir.. its great answer idx = [3,5,9]; for k = idx(end:-1:1) here i wanted to do stuff on each elements like if k==9 do stuff end . . if k==3 .... end how can i do this..???

[Stephen23]

For a start you could use switch inside the loop:

for k = idx(end:-1:1)
    switch k
        case 9
            ... code
        case 5
            ... code
        case 3
            ... code
        otherwise
            error('This values is not known: %d',k)
    end
end

But, depending on what those calculations are, there might be much easier and faster ways of achieving this. If you tell us exactly what the do stuff operations actually are, then we might be able to tidy this up too. Can the the vector idx change, or is it always the same?

[Raghavendra Reddy P]

Thank you sir, soon i will write what calculations and stuff to do. vector idx will change.