find n-st element that meet a logical expression

I have 2 question: 1) find function can give the all n-th element (first or last) that meet a logical expression. i want only the second one, can i do it in one line. i need it for a for-loop, i want to start from the second one. now i do it by:

h = find(f==condition,2);
for i=h(end):length(g)
...

can we have a way to do it in 1 line (and not use variable h)?

2) i have a vector contain numbers, i want ignore all 0 and find the place of first one that change his sign from the previous ones.

for example, in array r=[0 0 -2 -3 0 0 4 0 5 -2], my result is the place of 4 (7th).

i use find and diff to find it but it give me 3 (the place in vector not include 0):

find(diff(sign(r~=0))~=0,1)+1

how can i find it?

Answer 1

[Henrik]

I can't think of a way to do your question 1 in one line of code, but is it really that important to remove one line of code?

For your question #2, this should do the trick. I suppose there's a way to do it with fewer lines, but this was the best I could come up with.

r=[0 0 -2 -3 0 0 4 0 5 -2];
r_no_zero=r(r~=0);
r_sign_change=find(diff(sign(r_no_zero),1))+1;
find(r==r_no_zero(r_sign_change(1)))

[Yona]

#1 is not important it only a question of aesthetic and i ask to see if MATLAB has a way that i don't know to do it.

#2 it's good. finally, i do it different:

r=[0 0 -2 -3 0 0 4 0 5 -2];
r_no_zero=r(find(r~=0,1));
find(sign(r)~=sign(r_no_zero) & r ~= 0,1)

i don't like the double use on find for this...

thank you, hope to get some more answer.