Fun with List.Generate() – Tips and producing a Date Range


There have been a couple of blog posts about List.Generate()one by Chris Webb and another recently by Andrew Todd on the PowerPivotPro blog.

I recommend both of these articles as starting points for exploring List.Generate().  This blog post is not an introductory one.

List.Generate() makes lists.  It can make a straight list of scalar values, or a list of records, or a list of tables, or a list of records that contain lists etc.

The list that is returned is defined by the algorithm that you enter.  That is, you can, with a bit of work get it to produce pretty much any list that you like.

To achieve this, List.Generate iteratively produces records (well, they don’t have to be records, but they usually are).  These records are basically its mental scratchpad, which it (you) uses to decide both what item to add to the list that it ultimately generates and how to produce the next record (to produce the next item in its list and so on).

List.Generate accepts 4 parameters:

  1. A starting condition.  Usually this is a function returning a record.  For example, ()=>[A=1,B=1].  This means that on the first iteration, you will have a record containing two fields, A and B, where A=1 and B=1.
  2. When to stop.  This is some kind of conditional statement telling Power Query when to stop iterating.  e.g. each [A] < 5.  Note that when checking this condition, we will have received a record of the same format as the starting condition e.g. [A=something, B=something].  It then checks the value of A within this record and while it is less than 5 it keeps going.
  3. How to make the next record.  This is where the work happens.  It receives a copy of the current iteration’s record (think mental scratchpad, current values) and returns the record for the next iteration e.g.:

    Now, the [A] and [B] inside the outer square brackets are the values of A and B in the current iteration‘s record.  The A and B without the square brackets refer to A and B in the next iteration‘s record that we are making here.  One really important thing to note is that, in producing the next iteration’s record we must produce a record with the same fields (perhaps with rare exceptions).    If you don’t, then List.Generate will fail because A or B will not be defined in the next iteration.  So you couldn’t, for example, return a record here such as each [B=7]
  4. What to put into the list.  This the actual item we’re going to add to the list this iteration.  It’s not the record we’ve been passing around, at least not necessarily. It could literally be anything e.g. each 6. But usually it is based on the current record e.g. each [A]. Would return a list with the value of A from every iteration.  Or we could return a record with both values e.g. [A=[A], B=[B]] (this means we are, in this instance, returning the same record we’ve been using as our mental scratchpad).  It returns a record with the values of A and B for each iteration.

Note – while you can get away with returning a record of [[A],[B]], you can’t return a record of [[A], [B], [A]+[B]].  Why?  Because records need fields to have names and [A]+[B] doesn’t have a name unless you give it one:

Let’s look at a more complex example of using List.Generate().

I have some data where staff on a graduate programme move between teams. The data is a table of name, date, team:

2016-03-06 10_57_28-Graduates.xlsx - Excel

We will use List.Generate to turn this into date ranges.

2016-03-06 10_57_50-Employee Range.xlsx - Excel

Our starting condition is this:

So the record we’ll be using as our mental scratchpad will start with a blank Employee, Team, Date and a Counter starting at 0.

End condition:

Keep going while our Counter variable is less than the number of rows in our InputData (i.e. our list of Employees)

OK, here’s where the real work occurs.  This is how the next record in each iteration is produced:

First thing to note, we use our Counter to refer to fields in our InputData table.  So Employee=InputData{[Counter]}[#”Employee Name”], grabs the Employee Name from the Counterth row in our InputData.

Next thing to be aware of.  To calculate the value of Date in our next iteration, we use a function.  Date will be equal to whatever this function produces.  You can see that within the function we use the same approach as above to refer to elements within our InputData.  This function could do anything functions usually do (including calling other functions), what matters is that it returns a value.

Now what the function returns isn’t actually a date.  It’s a record.

So it will set the Date value in the record we are iterating to be equal to a record (so we’ve got a record within a record).

The rest of the code takes the generated list of records, expands it out to a table.  It then adds a special Index to allow us to clean pivot the output into the required output.

Here’s the full code:

There are ways to get this same output without using List.Generate(). The point here is to explore some other ways you can employ List.Generate() in your own projects.