using the find function to find intersection of two lines

Hi,

I have a vector A :

A = [ 10 20 30 40 50 60 50 40 30 20 10]

I want to find the nearest index where it crosses 18, so in this case it would be 2 and 10.

AA = find(A=18)

Thanks!

[Renato Agurto]

Do you want to find the index for every time it crosses 18:

for example if

 A = [10 20 30 40 50 60 50 40 30 21 10]

the answer would still be 2 and 10... or just 2?

[shobhit mehrotra]

Yes every time it crosses 18 so the answer would be 2 & 10

Answer 1

[Titus Edelhofer]

Hi,

you are looking for sign changes of A-18:

find((A(2:end)-18).*(A(1:end-1)-18) < 0)

Hope this helps,

Titus

[Renato Agurto]

I just want to complement the answer, so the right index is shown if is nearer as the left:

 tmp = find((A(2:end)-18).*(A(1:end-1)-18) < 0)
 B = tmp + (abs(A(tmp) - 18) > abs(A(tmp+1)-18))

Answer 2

[Adam]

find( A == 18 + min( abs( A - 18 ) ) )

is a one-liner to do what you want. Personally I would probably do it as multiple lines, but the idea is the same.

[Titus Edelhofer]

That's better than mine, because my code always selects the point to the left, Adam's picks the one that's nearer ...