#### Can you interpolate a non-linear set of values?

```I am going crazy with the following problem.  Can anyone help?

I have a range from 0 to 10 on columns A1 to A11.  A user enters a se
of 10 values that are not necessarily linear on columns B1 to B11 (fo
example: 15,16,17,19,22,25,28,33,34,35,36).  Then, another user enter
a value on another cell, for example: 20.  I need to introduce
formula that can tell me what number 20 corresponds to in column A.  I
other words, 20 would correspond to approximately 3.3 because 20 fall
between 19 and 22 in column B, which in turn falls between 3 and 4 i
column A.  Is there any way I can get that ~3.3 with some kind o
formula

--
Piffa
-----------------------------------------------------------------------
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```
 0
11/18/2005 7:30:31 PM
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```Excel doesn't have a built in linear interpolation function (some othe
spreadsheets like Quattro Pro do).  I have seen complex workshee
functions posted to the newsgroups that will do a linear interpolation
but they are pretty complex functions.  If you want to search th
newsgroups for one, feel free, but I can't build one.
One of the first things I did when I converted from QP to Excel was t
write my own Interpolation function in VBA for use in my spreadsheets.
Not very complicated, looks something like:

Function interpolation(knownys,knownxs,newx)
i=0
do
i=i+1
loop until knownxs(i)>newx
interpolation=(knownys(i)-knownys(i-1))/(knownxs(i)-knownxs(i-1))*(newx-knownxs(i-1))+knownys(i-1)
end function

Of course, you need to decide how to handle cases where newx is smalle
than the smallest knownx or larger than the largest knownx (whether t
return an error value, or extrapolate based on the end interval o
what).  My search algorithm assumes that the knownxs are in ascendin
order.  You could use any search algorithm you like to locate th
interval containing newx

--
MrShort
-----------------------------------------------------------------------
MrShorty's Profile: http://www.excelforum.com/member.php?action=getinfo&userid=2218

```
 0
11/18/2005 9:50:35 PM
```First use MATCH() to find the pair of values around the given value.  Then
use FORECAST() to get a linear interpolation between 19 and 22.
--
Gary''s Student

"MrShorty" wrote:

>
> Excel doesn't have a built in linear interpolation function (some other
> spreadsheets like Quattro Pro do).  I have seen complex worksheet
> functions posted to the newsgroups that will do a linear interpolation,
> but they are pretty complex functions.  If you want to search the
> newsgroups for one, feel free, but I can't build one.
> One of the first things I did when I converted from QP to Excel was to
> write my own Interpolation function in VBA for use in my spreadsheets.
> Not very complicated, looks something like:
>
> Function interpolation(knownys,knownxs,newx)
> i=0
> do
> i=i+1
> loop until knownxs(i)>newx
> interpolation=(knownys(i)-knownys(i-1))/(knownxs(i)-knownxs(i-1))*(newx-knownxs(i-1))+knownys(i-1)
> end function
>
> Of course, you need to decide how to handle cases where newx is smaller
> than the smallest knownx or larger than the largest knownx (whether to
> return an error value, or extrapolate based on the end interval or
> what).  My search algorithm assumes that the knownxs are in ascending
> order.  You could use any search algorithm you like to locate the
> interval containing newx.
>
>
> --
> MrShorty
> ------------------------------------------------------------------------
> MrShorty's Profile: http://www.excelforum.com/member.php?action=getinfo&userid=22181
>
>
```
 0
GarysStudent (1572)
11/18/2005 10:20:03 PM

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