[R] finding the intersection of two vectors

David Winsemius dwinsemius at comcast.net
Wed Jul 6 23:50:38 CEST 2011

```On Jul 6, 2011, at 4:43 PM, Data Analytics Corp. wrote:

> Hi,
>
> Suppose I have two vectors, not necessarily the same length (in
> fact, they usually are different lengths): y.1 that has increasing
> values between 0 and 1; y.2 that has decreasing values between 1.0
> and 0.  You can picture these as being supply (= y.1) and demand (=
> y.2) curves from economics.  I typically plot these vectors on the
> same graph against a common x variable, which happens to be price
> for what I do.  The price variable runs from, say, \$0 to \$25.  When
> I plot y.1 and y.2, I've been eye-balling a vertical line at a price
> point where y.1 intersects y.2.  I'm now tired of eye-balling a line
> through the intersection -- takes too much time to get it right or
> just close enough.  I can't figure out how to find the price value
> at which the two curves intersect.  Going back to the economics
> interpretation, I want the price where supply equals demand.  Any
> suggestions as to how I can find that price point in R?  Any
> functions that help?

?approxfun # or..
?splinefun  #  should allow you to make two functions and then solve
for the X taht minimizes the difference.

--

David Winsemius, MD
West Hartford, CT

```