The lack of details on the real function makes it difficult to be
precise. As a dummy function, consider =A6*2+A3. Then in D1, put
=C1*2+A$3
and copy the formula down. Note the "$" associated with every row
reference other than the hypothesized input. "$" prevents the reference
from floating relative to each pasted location, so that you can enter
the equation only once.

If your function supports array processing, you can eliminate column C
in the process by selecting D1:D100 an array entering (Ctrl-Shift-Enter)
=ROW(C1:C100)*2+A$3

Jerry

Steven wrote:

OK I think of myself as an experienced excel user but
cant figure something out which should be very basic.

Say I have a formula where the input is cell A6, the
formula in cell A1 is A6*2 and the output is written in
A8.

I now want to compare the answers as if A6=1 through to
A6=100. So I write out 1 through 100 in cells C1 down to
c100. Then in the corresponding cell next to each number
(D1-D100) I want to know the answer of that number
inserted into the function A6*2, without repeating the
function in every single D cell. (In reality the function
in question is actually a very complicated one that I
cant express in one cell, and need to go through the
complicated function everytime).

Therefore what I am trying to do is write something like,
=set(A6=C1,A8) in the cell d2, and have an answer 2. then
in d3 have an answer of 4 and so on......

to translate =set(a6=c1,a8) set a6 to equal c1, so the
input a6 would be 1 in this case, the output should be
the answer in cell a8. In this case 2.

Thanks for help, and my apologies for the long winded
question.