# using the find function to find intersection of two lines

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shobhit mehrotra on 15 Apr 2015
Commented: Renato Agurto on 16 Apr 2015
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!
##### 2 CommentsShowHide 1 older comment
shobhit mehrotra on 16 Apr 2015
Yes every time it crosses 18 so the answer would be 2 & 10

Titus Edelhofer on 15 Apr 2015
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 on 16 Apr 2015
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))

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 on 15 Apr 2015
That's better than mine, because my code always selects the point to the left, Adam's picks the one that's nearer ...

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