One of the most difficult parts of creating an advanced filter class is figuring out how to parse the search data and turn it into a 2D array, which Excel can understand. I figured my user interface will be a string. So I started there. In my original implementation of the search algorithm it wasn’t very robust, so this time around I figured I would do it “right.”
Well, right is a relative term. The “right way” is using a recursive descent parser, which seems overly complicated to me. But a simpler approach that I found is a shunting yard algorithm. This will suit my purposes well.
Now, the reason I am building an AdvancedFilter class is to make it possible to have the user create a unique report – before I used it as a filter in my VBA code also, but since I am now using .NET I can just use LINQ. Although not a huge deal for the Excel Time Card add-in (but still nice to have), this tool will be a tremendous plus for the Spreadsheet Budget – if people like it enough I might even release it as its own project.
So this is how I imagine the user interacting with the Report Generator. A simple text box (I like to keep things as simple as possible :-) ) will be on a windows form. When the user types there will be helps for the user to choose from in an auto complete drop down box. Below would be a possible report requested from a user (called infix notation – you learn quite a bit of English when you code):
[Jobs]= Job1 AND [Time]\>2 OR ([Jobs]=Job2 OR [Jobs]=Job3 AND [Comments]=Project\*)
I need the parentheses since logic order of operations dictates that AND is before OR. Now I only need to worry about AND and ORs because Excel will figure out the rest for me, e.g., instead of putting
one could put
Now, this may look difficult for a user, but remember, there will be a drop down list for a user to choose from. Also, I will pull out the dates and supply a calendar to make it even easier on the user.
I found someone that put the shunting algorithm in Java. After programming for some time, it becomes much easier to look at the work of others in different languages and convert them to your own. Since not a whole lot is written in VB.NET this is a must for the VB programmer. After fixing the mistakes I made when transcribing the code, I was able to proceed.
The shunting algorithm I used puts the data in Reverse Polish notation (RPN). This notation is a bit confusing at first. But once you look at it for a while you get used to it. Let’s take the example shown in the Java code mentioned above. So this is the equation you need to parse:
The result after putting the code through the shunting algorithm is:
1 2 + 3 4 / 5 6 + \^ \*
So, to get the answer from the RPN notation we need to start from left to right. If there are two numbers in a row then perform operation, else goto next. If there are two operators in a row then get previous solutions and perform operation.
- Store 3
- Store 0.75
- Store 11
- Get previous two solutions.
- Get previous solution.
- Return 0.127
A little strange at first blush but after staring at it for a little while it starts to make sense.