How to use 'loop' in function for column iteration?

[F75]

Hi, I have series of columns (40) for which I want to calculate variance with prior column. How do i use 'loop' in a function to get what I need? Is that a best approach, I may have 1M+ records.

My original columns are in A thru E, I was planning to merge them in Column G (thought it was logical to perform loop on a string separated by comma). 

Maybe use a function to get Column N and then split it to get desired Columns P thru S.

The values are incremant (Col B thru E)

Thank You

[AlB · Accepted solution]

Hi @F75 

Place the following M code in a blank query to see the steps.See it all at work in the attached file.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8nNU0lEyNAASUBSrE63k6gsWNUIV9QELmgAJI0MkYf+SjNQikIwZSMYYRFjC5MCmG5tiM90Em+nmIIeYm2Mx3QJkiCVIj6UFihZLkBZLc7AvQJYbGpoqxcYCAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Region = _t, Column2 = _t, Column3 = _t, Column4 = _t, Column5 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Region", type text}, {"Column2", Int64.Type}, {"Column3", Int64.Type}, {"Column4", Int64.Type}, {"Column5", Int64.Type}}),
    #"Unpivoted Other Columns" = Table.UnpivotOtherColumns(#"Changed Type", {"Region"}, "Attribute", "Value"),
    #"Grouped Rows" = Table.Group(#"Unpivoted Other Columns", {"Region"}, {{"Grouped", each [Value] }}, GroupKind.Local),
    #"Added Custom1" = Table.AddColumn(#"Grouped Rows", "Custom", each let aux1_ = [Grouped], aux2_ = {0} & List.RemoveLastN(aux1_,1),  aux_ = List.Transform(List.Zip({aux1_, aux2_}), each _{0} - _{1})  in aux_),
    #"Extracted Values" = Table.TransformColumns(#"Added Custom1", {"Custom", each Text.Combine(List.Transform(_, Text.From), ","), type text}),
    #"Split Column by Delimiter" = Table.SplitColumn(#"Extracted Values", "Custom", Splitter.SplitTextByDelimiter(",", QuoteStyle.Csv), {"Custom.1", "Custom.2", "Custom.3", "Custom.4"}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Split Column by Delimiter",{{"Custom.1", Int64.Type}, {"Custom.2", Int64.Type}, {"Custom.3", Int64.Type}, {"Custom.4", Int64.Type}}),
    #"Removed Columns" = Table.RemoveColumns(#"Changed Type1",{"Grouped"})
in
    #"Removed Columns"

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