Reliability Calculation for N-Modular Redundant Systems

Hi All,

I need some help with the following formula for Excel Macros. I need to calculate a problem with the following equation and I am having trouble inputting the formula correctly into Excel. Please see attached pic.

I need to have variables for "M", "N", "i" and "Rm(t)", which I can retrieve from a separate Excel sheet.

Thanks in advance!

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I usually find that I must first understand the math before I can program Excel/VBA to perform the calculation, and I find myself having trouble understanding your formula and example.

1) As given, the formula has R on both sides of the equation. Is this indicative of a recursive type function? If the R on the right hand side is the same R as the left hand side, the notation is a little different. How does the notation for R on the right hand side relate to the notation on the left hand side?

2) Other than "I am having trouble", can you explain in more detail what you are stuck on? To this end, a copy of some of your failed attempts might be instructive. Are you having trouble with the internal For..Next loop? If this is supposed to be a recursive function, is it the recursion that is causing you trouble?

3) Do you have a more detailed worked example? A lot of programming is being able to "translate" a sequence of operations into the desired programming language, which means that we need to understand, in sufficient detail, the steps being taken.

We may be better able to help you if you address these questions.

A few thoughts.

I think you misunderstood what I meant by recursive. If I am understanding now, this is not a recursive function, because the R(t) on the right side is Not the same as the R(t) that the function is defining. This function will require a loop, but it is not really recursive.

Another clarification, You ask for the "summation" and "factorial" functions in Excel, but this is in the VBA forum. Are you intending to solve this using Excel's spreadsheet functions, or in VBA? Either "programming language" (because I view a spreadsheet as a different kind of programming language) is capable of solving the problem, one just needs to decide which programming language to use.

In a spreadsheet, I would start by designating cells for M, N, and R(t) where I can enter those values. Then a column for i, and additional columns for the individual addends in the summation. Finally, the "summation" part becomes a simple =SUM() function.

This might look something like:

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Note how I used absolute and relative references to make copying easy and efficient. Also, contrary to almost every trend I see in suggestions made in almost every spreadsheet forum, note that I broke the function up into several small, simple parts that fit easily into a single cell. One could certainly combine these into fewer cells, but I often find -- especially when I am first starting a project, that I prefer to break the calculations up into smaller pieces so each cell is easy to code and debug.

If you intend to use VBA to do this, the "summation" function in VBA is usually a loop (I would probably use a For..Next loop for this one). VBA does not have a built in Factorial functiion, so you would need to either code your own factorial function, or use Excel's FACT() function via the "worksheet function" object.

Does that help?

As I indicated, yes, it can be done in VBA or any other programming language you want to use (VBA is usually convenient because it is built into Office, but it is certainly not the only choice).

In just about any of these programming languages, a For..Next loop for a summation like this will look like (pseudocode using similar notation to the spreadsheet example):

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In essence, it is the same thing I did in the spreadsheet example.

Are there specific parts of this that you don't understand and need help with?