Loop Computation in Power Query - mysql

Question

I'm trying to merge several columns in a table to columns of another table. Each column in the primary table contains texts while the PrimaryAnalysis table contains indexes for the texts. I'd like to create columns of indexes for the primary table but I'm having to do that one at a time for each table thus:
#"Merged Queries" = Table.NestedJoin(#"Changed Type2",{"Text.1"},PrimaryAnalysis,{"Letter"},"NewColumn"),
#"Expanded NewColumn" = Table.ExpandTableColumn(#"Merged Queries", "NewColumn", {"Index"}, {"Index"}),
#"Renamed Columns2" = Table.RenameColumns(#"Expanded NewColumn",{{"Index", "First"}}),
#"Merged Queries1" = Table.NestedJoin(#"Renamed Columns2",{"Text.2"},PrimaryAnalysis,{"Letter"},"NewColumn"),
#"Expanded NewColumn1" = Table.ExpandTableColumn(#"Merged Queries1", "NewColumn", {"Index"}, {"Index"}),
#"Renamed Columns3" = Table.RenameColumns(#"Expanded NewColumn1",{{"Index", "2nd"}}),
#"Merged Queries2" = Table.NestedJoin(#"Renamed Columns3",{"Text.3"},PrimaryAnalysis,{"Letter"},"NewColumn"),
#"Expanded NewColumn2" = Table.ExpandTableColumn(#"Merged Queries2", "NewColumn", {"Index"}, {"Index"}),
#"Renamed Columns4" = Table.RenameColumns(#"Expanded NewColumn2",{{"Index", "3rd"}}),
Now I have to do that for 23 columns. Is there a way to implement DO...Repeat or any other loop in Power Query to perform this task?
Thanks in advance.

Answer 1

One way to loop in Power Query is to use a recursive function.
In the code below I read an Excel file with a table that should be similar to your primary table (so the step #"Changed Type2" in the code below should be similar to your step #"Changed Type2").
Next a function AddIndices is defined in which 1 column with an Index is added in each iteration. After 23 iterations the function stops, otherwise it calls itself.
An important point of attention with such recursive functions is that it MUST include a Table.Buffer (see step "Expanded"), otherwise in each iteration the code tries to evaluate all former iterations again and gets stuck. Table.Buffer prevents this.
In the last step of the query, the function is invoked.
let
Source = Excel.Workbook(File.Contents("C:\Users\Marcel\Documents\Forum bijdragen\StackOverflow Power Query\Loop Computation in Power Query.xlsx"), null, true),
Tabel1_Table = Source{[Item="Tabel1",Kind="Table"]}[Data],
#"Changed Type2" = Table.TransformColumnTypes(Tabel1_Table,{{"Text.1", type text}, {"Text.2", type text}, {"Text.3", type text}, {"Text.4", type text}, {"Text.5", type text}, {"Text.6", type text}, {"Text.7", type text}, {"Text.8", type text}, {"Text.9", type text}, {"Text.10", type text}, {"Text.11", type text}, {"Text.12", type text}, {"Text.13", type text}, {"Text.14", type text}, {"Text.15", type text}, {"Text.16", type text}, {"Text.17", type text}, {"Text.18", type text}, {"Text.19", type text}, {"Text.20", type text}, {"Text.21", type text}, {"Text.22", type text}, {"Text.23", type text}}),
// Recursive function:
AddIndices = (TableSoFar as table, optional Iteration as number) as table =>
let
CurrentIteration = if Iteration = null then 1 else Iteration,
CurrentColumn = "Text."&Text.From(CurrentIteration),
NewIndexColumn = "Index."&Text.From(CurrentIteration),
MergedTable = Table.NestedJoin(TableSoFar,{CurrentColumn},PrimaryAnalysis,{"Letter"},"NewColumn"),
Expanded = Table.Buffer(Table.ExpandTableColumn(MergedTable, "NewColumn", {"Index"}, {NewIndexColumn})),
Result = if CurrentIteration = 23 then Expanded else #AddIndices(Expanded, CurrentIteration + 1)
in
Result,
// Call recursive function:
AddedIndices = AddIndices(#"Changed Type2")
in
AddedIndices