

SSCommitted
Group: General Forum Members
Last Login: Wednesday, July 30, 2014 11:38 AM
Points: 1,836,
Visits: 479


You earler post had a commet about the function only handles one expression at a time. That was kind of the point. In a select statement the function could be called for multiple expressions. That is actually the case in my situation. A table column contains a simple math expression. So using a function would allow me to get the answer to as many lines as would get returned in the select statement.
Thanks for your posts.
Ben




Valued Member
Group: General Forum Members
Last Login: Monday, January 7, 2013 2:28 PM
Points: 51,
Visits: 158


The method for evaluating algebraic equations is properly done through the use of reverse polish notation or more formally known as postfix notation. (It's referred to a reverse Polish notation because a Polish fellow figured it out) I've written these programs in COBOL, never thought about doing it in in TSQL, maybe VB.NET. Could be a challenge. Through the use of cursors it might not be that difficult. I'll see if I can come up with something in a few days. You can use this method with a few modifications to evaluate formal logic as well. i.e is (A > B) or (C > B & D = B) a true statement. That can get very complex, trust me. So can algebra.




Valued Member
Group: General Forum Members
Last Login: Monday, January 7, 2013 2:28 PM
Points: 51,
Visits: 158


My earlier post was used in a software package to evaluare formulas in a table. The table was comprised of three "columns". The first column had the user friendly version of the forumla, the second column had the postfix form of that equation, the third was a key field used by an evaluation program to find the proper formula. The the evaluater program would read the table based on the key column and evaluate the second column streight away. Prettly slick, really.
When I say user friendly think of (a + b) / c. I don't have access to my programs right now to show the exact form of that in postfix but t is something like: c!/!a!b!+ (the ! is used as a separator similar to a , in a .csv file). To get the user friendly formula into postfix you have to use stack processing which lends itself to cursor processing (I think).




SSCommitted
Group: General Forum Members
Last Login: Wednesday, July 30, 2014 11:38 AM
Points: 1,836,
Visits: 479


I would stay away from cursors if you decide to do it in tsql. It might be better to use vb.net or c# and then use the clr. Although I believe you could still use a recursive function call if you were able to properly parse the equation.
Ben




Valued Member
Group: General Forum Members
Last Login: Monday, January 7, 2013 2:28 PM
Points: 51,
Visits: 158


The key to this method is the formulas have already been converted to postfix notation and all the evaluator function has to do is peddle down the string evaluating the variables as you go and then applying the op code (+  * /,etc.) to the variable. If you use an HP12c calculator you would be familiar with postfix notation. Formulas are generally prethought out and entered via key strokes. Once the operator exits that field the program would immediately format the formula into postfix noation ready for the processing program to select it and evaluate it. You have to think how something like this would be used. At a data entry level you don't have to worry about speed.




Forum Newbie
Group: General Forum Members
Last Login: Thursday, March 13, 2014 4:18 PM
Points: 3,
Visits: 18


I wonder if it would be fair to call this code a simplified RPN processor. Essentially you are taking infix notation and putting into a form that used to be required on some calculators.




Valued Member
Group: General Forum Members
Last Login: Monday, January 7, 2013 2:28 PM
Points: 51,
Visits: 158


Sort of. The code for evaluating a simple formula like mentioned early in this thread could be called a simplified RPN evaluator, I guess. I really didn't spend much time on it because I know intemately what an RPN process is all about. A real RPN processor can evaluate any depth algebraic equation.




Forum Newbie
Group: General Forum Members
Last Login: Tuesday, June 17, 2014 4:40 PM
Points: 5,
Visits: 63


considering execution for many rows
DECLARE @ExprCollection Table(expr VARCHAR(100) ) INSERT INTO @ExprCollection(expr) SELECT '2+1' UNION ALL SELECT '5*2' UNION ALL SELECT '6/3' DECLARE @genSql AS VARCHAR(MAX)
CREATE TABLE #ExpValueTable ([value] INT,[Id_Exp] [VARCHAR](100) PRIMARY KEY) SELECT @genSql = CASE WHEN @genSql Is Null THEN ' INSERT INTO #ExpValueTable ([value],[Id_Exp]) ' + ' SELECT ' + expr + ' AS ExpValue,''' + expr +''' As Exp' ELSE @genSql + ' UNION SELECT ' + expr + ' AS ExpValue,''' + expr +''' As Exp' END FROM @ExprCollection GROUP BY expr
EXEC(@genSql) SELECT * FROM #ExpValueTable
 OR JOIN YOUR TABLE TO THE TEMPORARY TABLE... AND BETTER USING THE TABLE PRIMARY KEY DROP TABLE #ExpValueTable




Valued Member
Group: General Forum Members
Last Login: Monday, January 7, 2013 2:28 PM
Points: 51,
Visits: 158


In this example the size of the table of literals is unlimited. However aditional coding is needed tu use variable names i.e. A, B, C, etc. A little fooling around with this thing and it might serve the most elementary uses.
DECLARE @ExprCollection Table(expr VARCHAR(100) )
INSERT INTO @ExprCollection(expr) SELECT '2+1' UNION ALL SELECT '5*2' UNION ALL SELECT '6/3'
drop table #ExpValueTable CREATE TABLE #ExpValueTable ([value] decimal(18,4),[Id_Exp] [VARCHAR](100) PRIMARY KEY) declare ExprCursor cursor for select * from @ExprCollection
declare @Expr as varchar(100), @SQL varchar(max)
open ExprCursor fetch next from ExprCursor into @Expr
while @@Fetch_Status = 0 BEGIN set @SQL = ' INSERT INTO #ExpValueTable ([value],[Id_Exp]) SELECT ' + @expr + ' AS ExpValue,''' + @expr +''' As Exp'
fetch next from ExprCursor into @Expr
exec (@SQL) END
close ExprCursor deallocate ExprCursor
select * from #ExpValueTable




Grasshopper
Group: General Forum Members
Last Login: Wednesday, August 20, 2014 10:39 AM
Points: 13,
Visits: 24


When I ran Ben's code, I had some problems if I didn't leave spaces between the operators and the values. i.e., fn_simplemath('3  4') worked while fn_simplemath('34') failed... I suspect this would require a minor tweaking of some of the substring arguments. But trying to debug a recursive function can be a bit troublesome.
It also occurred to me that since were were not concerned with operator precedence and simply calculating from left to right, we should be able to simply loop through the string without needing to be recursive.
My approach was to create a string that duplicates the input except for replacing all the "acceptable" operator values with a single unique character, in my case I used a tilde(~). This second string can then be used to identify the position of all the operators in the original string and make it easier to parse out the individual operand values
Here is my code:
CREATE Function [dbo].[fn_simplemath2] ( @Expression1 varchar(255)) returns numeric(18,6) As BEGIN   @Expression2 will duplicate @Expression1 with all operators replaced with ~ Declare @Expression2 varchar(255) Set @Expression2 = REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(@Expression1,'','~'),'+','~'),'*','~'),'/','~'),'%','~')   Local variables Declare @PrevOpLoc int  Location of previous operator Declare @NextOpLoc int  Location of next operator Declare @OpChar Char(1)  Current operator character Declare @Result numeric(18,6)  Hold running calculation and final return value Declare @NextVal numeric(18,6)  the next substring value to be used to modify result base on operator   Find the first operator Set @NextOpLoc = CHARINDEX('~',@Expression2)   Initialize @Result to the first substring, If there are no operators, move entire string to @Result Set @Result = CASE When @NextOpLoc = 0 then CAST(@Expression1 as numeric(18,6)) Else CAST(LEFT(@Expression1,@NextOpLoc1) as numeric(18,6)) END   Now we will loop until we run out of operators While @NextOpLoc <> 0 BEGIN  Get the actual operator from @Expression1, pull out the next substring value Set @OpChar = SUBSTRING(@Expression1,@NextOpLoc,1) Set @PrevOpLoc = @NextOpLoc Set @NextOpLoc = CHARINDEX('~',@Expression2, @NextOpLoc + 1) Set @NextVal= Cast( SUBSTRING(@Expression1,@PrevOpLoc+1, Case When @NextOpLoc = 0 then LEN(@Expression1) Else @NextOpLoc1 End  @PrevOpLoc) as numeric(18,6))   Perform the appropriate operation Set @Result = Case @OpChar When '' then @Result  @NextVal When '+' then @Result + @NextVal When '*' then @Result * @NextVal When '/' then @Result / @NextVal When '%' then @Result % @NextVal Else null End END Return @Result END



