Forum Discussion
calculate "around" a variable
I am attempting to use variables within a measure, but not getting the desired results. I've looked around at all kinds of DAX reference material and can't figure out why this doesn't work. I know there are other ways to accomplish why I'm trying to do, but I'd like to know specifically why this doesn't work.
Basically, I'm setting a variable and then trying to recalculate that variable within the same measure. The recalculation doesn't work.
Data (table name is PD):
| Dt | Value |
| 1/1/2018 | 100 |
| 1/2/2018 | 200 |
| 1/3/2018 | 250 |
| 1/4/2018 | 300 |
| 1/5/2018 | 400 |
Dt has a relationship to a Date table (DateTab)
Creating a measure that finds the difference in Value from the previous day:
Delta = var __TodayVal = SUM(PD[Value]) var __PrevVal = CALCULATE(__TodayVal, PREVIOUSDAY(DateTab[Date])) var __Delta = __TodayVal -__PrevVal return __Delta
Here is what I would expect
| Dt | Value | Delta |
| 1/1/2018 | 100 | |
| 1/2/2018 | 200 | 100 |
| 1/3/2018 | 250 | 50 |
| 1/4/2018 | 300 | 50 |
| 1/5/2018 | 400 | 100 |
But this is what I get
I'm sure it has something to do with evaluation context inside the measure, but I'm wracking my brain to be able to explain it. I know I can work around it by creating multiple measures, but I'd like to know a concrete reason for why this can't be done in a single measure.
Thanks,
David
A variable in DAX is actually a constant. Unlike variables in other programming languages, once you declare a variable in DAX by assigning it a value, that value is immutable. Your variable __TodayVal in the CALCULATE for __PrevVal will not be affected at all by the filter argument.
11 Replies
A variable in DAX is actually a constant. Unlike variables in other programming languages, once you declare a variable in DAX by assigning it a value, that value is immutable. Your variable __TodayVal in the CALCULATE for __PrevVal will not be affected at all by the filter argument.
Hi AlB - so what you are saying is that once __TodayVal is evaluated it is the value itself, not the code that populated it. And applying a different filter context to a constant does nothing.
Thanks!
David
Hi dedelman_clng,
In order to make this work you need to have your variable to be based on the context so it will move accordingly to your value in each line for this you should use redo your measure to this:
Delta = VAR first_date = CALCULATE ( MIN ( Table1[Dt] ); ALL ( Table1[Dt] ) ) VAR date_selection = CALCULATE ( MAX ( Table1[Dt] ); FILTER ( ALL ( Table1[Dt] ); Table1[Dt] < MAX ( Table1[Dt] ) ) ) RETURN IF ( MIN ( Table1[Dt] ) = first_date; BLANK (); SUM ( Table1[Value] ) - CALCULATE ( SUM ( Table1[Value] ); FILTER ( ALL ( Table1[Dt] ); Table1[Dt] = date_selection ) ) )Explanation
First_Date = Used only to return blank on the first row, if place only the last part of the measure on the first row value of Delta will be the same as on the column value
Date_selection = This is a variable that calculates the maximum date for all dates below the row context so for january the second this measure give january first, and so on
After the return what I'm calculating is the sum for the current date (row context) and then picking up the Date Selection variable and get that specific day.
Having the measure like this will give you a variable based on row context and not a constant value, since I'm changing the context of the variable within itself.
The measure is also created in this way allows you to have leaps on your dates in order to always get the latest number and not have some lines with 0 or blanks and others with the full value, As you can see I go from january 5th to February 20 and the delta is calculated so if you have weekends or not continuous dates it will always return delta value for previous row.
dedelman_clng and AlB sorry for entering in this discussion but the context of a measure can be change at different levels not only in the visuals, and the variables within the measures can themselve have a change in context based on the way you create them so the variables are not constants depends on the way you build them.
Regards,
MFelix
Thanks MFelix - this makes sense.
Looking at your code and explanation, it still looks like you're not "recalculating" the variable in a different context within the same call, which is what I was trying to accomplish. The solution you provide is very elegant and definitely much more flexible than assuming a continuum of days.
David
Hi dedelman_clng,
When defining the variable and comparing it with the maximum value it can or not recalculate the variable.
If you are using a card it will pick up the full dataset and return the maximum date value so "table context", however when having a table visual you get into the row context of the visual and for each line the maximum value of date is "recalculated" for each interaction of the date.
So it's recalculating in a context enviroment.
Glad I could help.
Regards,
MFelix
