Power Query Transform a Column based on Another Column

I keep thinking this should be easy but the answer is evading me. In Excel Power Query, I would like to transform the value in each row of a column based on another column's value. For example, assume I have Table1 as follows:

Column A | Column B
-------------------
X        | 1
Y        | 2

I would like to transform the values in Column A based on the values in Column B, without having to add a new column and replace the original Column A. I have tried using TransformColumns but the input can only be the target column's value - I can't access other field values in the row/record from within the TransformColumns function. I would like to be able to do something like this:

=Table.TransformColumns(Table1, {"Column A", each if [Column B]=1 then "Z" else _ })

which would result in:

Column A | Column B
-------------------
Z        | 1
Y        | 2

I know there are ways to do this, but I'm trying to find one with the least amount of steps/transformations. For example, I know I could use Table.AddColumn to add a new Column A based on a function that looks at Column B, but then I have to remove the original Column A and replace it with the new Column A which requires multiple additional steps.

Here is how I ended up doing this:

Table1:
Column A | Column B
-------------------
X        | 1
Y        | 2

= Table.FromRecords(Table.TransformRows(Table1,
    (r) => Record.TransformFields(r,
        {"A", each if r[Column B]="1" then "Z" else _})))

Result:

Column A | Column B
-------------------
Z        | 1
Y        | 2

This way you can transform multiple columns at once by using a nested list in the Record.TransformFields function.

Kudos to LoganTheSnowEater... I love your answer. You mentioned that it could be expanded upon for multiple actions and I wanted to post an example of how I successfully used it to do so:

#"Patch Records" = Table.FromRecords(Table.TransformRows(#"Sorted Rows",
(r) => Record.TransformFields( r, {
    {"Min Commit", each 
        if r[service_id] = "21430" then 81 else
        if r[service_id] = "24000" then 230 else
        if r[service_id] = "24008" then 18 else
        if r[service_id] = "24009" then 46.9 else
        _},
    {"Installed", each 
        if r[service_id] = "21430" then 90 else
        if r[service_id] = "24000" then 230 else
        if r[service_id] = "24008" then 18 else
        if r[service_id] = "24009" then 52 else
        _},
    {"Requested", each 
        if r[service_id] = "21430" then 90 else
        if r[service_id] = "24000" then 230 else
        if r[service_id] = "24008" then 18 else
        if r[service_id] = "24009" then 52 else
        _}})))

My only surprise was that I had to reset all of the data types after I was done, but that makes sense in retrospect.

p.s. I didn't have enough reputation points to post this as a comment to the solution

Another alternative is this:

= Table.ReplaceValue(Table1, each [Column A], each if [Column B]=1 then "Z" else [Column A] , Replacer.ReplaceText, {"Column A"})

The code is a bit more readable, but can only be applied to one column at a time.

Most of it can be generated by using the UI, like shown here: http://www.thebiccountant.com/2017/07/23/transforming-a-column-with-values-from-another-column-in-powerbi-and-powerquery-in-excel/

To build on LoganTheSnowEater's answer, here is one method to retain the table column types by leveraging Table.Schema to construct a table type to use as the second argument in Table.FromRecords as specified in the M Reference:

Table1:
Column A | Column B
-------------------
X        | 1
Y        | 2

= Table.FromRecords(
    Table.TransformRows(
        Table1,
        (r) => Record.TransformFields(r,
            {"Column A", each if r[Column B]="1" then "Z" else _}
        )
    ),
    Expression.Evaluate(
        let s = Table.Schema(Table1) in 
        "type table ["&
        Text.Combine(
            List.Transform(List.Zip({s[Name],s[TypeName]}),each "#"""&_{0}&""" = "&_{1}),
            ", ")
        &"]",
        #shared
    )
)

Table.Schema produces a table with column Name and column TypeName, which are used to construct the text for defining the table type, which in this case is

type table [#"Column A" = Text.Type, #"Column B" = Int64.Type]

Of course, it's possible to manually type in the table type instead, which you would need to do if the transformations changed the types of the values in the column.

You can't transform the existing column with such a step. Use a new column, do the transform, then delete the existing column. What's the big deal? In Excel you would not expect a formula to change the value of a different column, either. In Power Query, the result of a formula is also stored in a column, and that cannot be the column that provides one of the formula input values.

Sorry I was not so good editing the formula, this now works:

= Table.ReplaceValue(   #"Removed Columns", 
                   each [Contract Start Date],
                    each if [Contract Start Date] < #date(2019,01,01) then #date(2019,01,01) else
                          [Contract Start Date]
                    ,Replacer.ReplaceValue,
                    {"Contract Start Date"})

来源：https://stackoverflow.com/questions/31548135/power-query-transform-a-column-based-on-another-column

标签

excel

powerquery