Calculations that have variable cells in Excel - excel

Question

I thought I understood Excel well enough until my boss asked me to do something in excel that involved MATCH and INDEX and for the life of me I can't figure out how those functions work. Perhaps more appropriately, I can't figure out how they're working in the spreadsheet I'm looking at. I'll just walk you through carefully what I'm trying to do.
I start by creating a drop down list which is no problem. The drop down list is a list of what is essentially probability tables. To skip past some irrelevant math, each table will have a single number that is generated from all of the numbers in the table that will be used in some calculations in other parts of the spreadsheet. Each of these tables will also have a name (top cell) that matches an entry in the drop down list.
[If anyone knows how to format this question so that it is a little easier to follow my question please, please feel free to tell me how. I don't know how to format stackexchange questions for excel worth anything.]
So ultimately, I have two main tables that are preforming calculations. The calcuations themselves are irrelevant. What is relevant is that I need to be able to add in a value into the calculation that can change depending on what is selected from the drop down list. So let me try to give an example.
Lets say I have this table:
| Month | Balance | Interest Rate |
|:--------:|:-------:|:-------------:|
| January | 100.00 | 1% |
| February | 101.00 | 1% |
| March | 102.01 | 1% |
| April | 103.03 | 1% |
| May | 104.06 | 1% |
| June | 105.10 | 1% |
I want the interest rate to be dependent on the drop down list so that perhaps I have set up where my drop down lists is generated from a table of cells that is something like this:
| Interest Rate |
|:-------------:|
| Low |
| Medium |
| High |
And I have three tables labeled "Low", "Medium", and "High". Each of these tables will preform some calculation to get the final Interest Rate result that will be the number that shows up in the top table. So that if I select "High", my table will look like this instead.
| Month | Balance | Interest Rate |
|:--------:|:-------:|:-------------:|
| January | 100.00 | 5% |
| February | 105.00 | 5% |
| March | 110.25 | 5% |
| April | 115.76 | 5% |
| May | 121.55 | 5% |
| June | 127.63 | 5% |
I'm pretty sure I need index and match functions to do this. I'll even put in a formula that is doing something close to what I'm doing but I can't seem to decipher how everything is working.
=INDEX($U$13:$BM$416,MATCH(D12,$T$13:$T$416,0),MATCH($A$13,$U$11:$BM$11,0) + 1) * SUM(P:P)
I know that that doesn't mean much when you can't see the worksheet, but that formula is doing very close to what I need to do. I guess my ultimately question is just if anyone will help walk me through how I could accomplish this in Excel?
EDIT: Here's a better glimpse
Lets say I have these 3 tables that show interest rates for various different things (e.g. Auto Loan, Mortage, Credit Cards). The "######"s are just showing that there are values in those cells that are used to calculate the numbers at the bottom (0.01, 0.03, etc.). Lets also say that the range in excel for these 3 Data Tables is A1:I6.
| | DataTable 1 | | | DataTable 2 | | | DataTable 3 | |
|:------------:|:---------------:|:-------------:|:------------:|:---------------:|:-------------:|:------------:|:---------------:|:-------------:|
| Low,Interest | Medium,Interest | High,Interest | Low,Interest | Medium,Interest | High,Interest | Low,Interest | Medium,Interest | High,Interest |
|--------------|-----------------|---------------|--------------|-----------------|---------------|:-------------|-----------------|---------------|
| ####### | ####### | ####### | ####### | ####### | ####### | ####### | ####### | ####### |
| ####### | ####### | ####### | ####### | ####### | ####### | ####### | ####### | ####### |
| 0.01 | 0.03 | 0.05 | 0.02 | 0.04 | 0.06 | 0.10 | 0.20 | 0.30 |
I have a drop down list in A8 that contains the values Data Table 1, Data Table 2, and Data Table 3.
Lets say I have another table (Range is K1:M14) that looks like the 1st table in this question.
| Month | Balance | Medium Interest |
|:---------:|:-------:|:---------------:|
| January | $100.00 | 3% |
| February | $103.00 | 3% |
| March | $106.09 | 3% |
| April | $109.27 | 3% |
| May | $112.55 | 3% |
| June | $115.93 | 3% |
| July | $119.41 | 3% |
| August | $122.99 | 3% |
| September | $126.68 | 3% |
| October | $130.48 | 3% |
| November | $134.39 | 3% |
| December | $138.42 | 3% |
I wrote a formula that would determine how the 3% gets into the Medium Interest column.
=INDEX($A$6:$I$6,MATCH($A$8,$A$1:$I$1,0),MATCH($M$2,$A$2:$I$2,0))
It works when I choose Data Table 1 in the drop down list. It correctly places 3% which is the medium interest rate for Data Table 1 but when I choose either of the other 2 data tables, I get an invalid cell reference error. This is essentially what I need to do in my real spreadsheet.

Answer 1

Not sure I can say how to do what you want, but I can at least explain MATCH and INDEX and what your formula is doing with them - hopefully that will be enough!
MATCH(what,in_where,match_type) will return the index of what in the array/range in_where based on the match_type. The "best" match_type is 0 - like in your example - which means "exact match". The other options are 1 for "less than" and -1 for "greater than" - both requiring you in_where to be sorted...
So you first example MATCH(D12,$T$13:$T$416,0) is looking for the exact value that is in D12 in the range $T$13:$T$416.
INDEX(in_where,row,column) will return the value in the array/range in_where at row row and column column.
In your example, you are looking in the range $U$13:$BM$416 with the row/column given by the MATCHes... The first MATCH is looking for D12 roughly in column T and the second is looking for A13 roughly in row 11. (I.e. it looks like your "table" has headers in row 11 and "keys" in column T and you are searching for the intersection of their locations.) The +1 in the column will be to correctly align the index returned by the MATCH and the the column number for the INDEX...
Without seeing this (e.g. file somewhere like dropbox, or a screengrab) it is hard to say more - but I hope this helps!
Btw - INDEX & MATCH as a combination can also be a good replacement of VLOOKUP if your "key" column is to the right of the "value" column that you want (or if you have a wide table and any change within it force loads of recalculations)
UPDATE based on second part of question
The new example gives the formula: =INDEX($A$6:$I$6,MATCH($A$8,$A$1:$I$1,0),MATCH($M$2,$A$2:$I$2,0))
INDEX will return the value for the given row/column. In this example your "table" is a single row $A$6:$I$6 so you would need to give just row 1 here - you aren't looking in a grid, but just a list.
So, you are looking to find the interest value for the interest level (Low/Medium/High at the top of your table) in the right DataTable (selected from a drop down). There are a few ways to do this, depending on the control you have...
Create a real data grid with Low/Medium/High down the side, 1/2/3 along the top and the % inside - then use INDEX as originally planned... MATCH the selections to the row/column
If that is a bit much to do, how about creating a new "compound key" to your table... e.g. Row 2 could contain 1_Low, 1_Medium, 1_High, 2_Low, etc. You then use just one search, but with a concatenated key : =INDEX($A$6:$I$6,1,MATCH($A$8 & "_" & $M$1,$A$2:$I$2,0))
Otherwise, you would first need to find "DataTable 1" in the first header row and use that to restrict the range to search for the interest rate level header... It is a bit more complicated and would depend more on the details of your tables (e.g. are they all the same number of columns - Low/Medium/High - or do some have Very High)

Answer 2

The formula for the 3% in the Medium Interest column should be:
=INDEX(A6:I6,,MATCH(A8,A1:I1,0)+1)
This formula returns the value in the index array A6:I6. Since there is only one row of data in the array A6:I6, it is not necessary to specify the row position for the index function. That is why there are two commas together - the row reference is missing and is not necessary. MATCH supplies the column number reference for the function based on your drop down list, which will be the values 1, 4, or 7 and then +1 to move over one more column. MAKE SURE that in cell A1 you have "DataTable 1", in cell D1 have "DataTable 2", and cell G1 have "DataTable 3".