Forum Discussion
Looping through a table in an m function
I have a function that takes in a table name, and I want to loop through the 2 fields in the table, and create a string that has a comma between the 2 fields for a line and a new line for each row. Each of the values would go through some calculations before being added to the string. I need to know how to do a loop that will loop through a table (with no index).
Here is the file, look at the fn_basic_line_chart function.
Edit: Here is a more in depth look at what I am trying to do, note that the calculations will be WAY more complex in the real life situation, but this is the loop I am looking for.
So I want to loop through a table in a function. Let’s call this function fn_my_function and at the moment it looks like this (I am stripping out anything we don’t need):
(lineData as table)=> let
Source = “Something”
in
Source
So at this point, the function brings in a table as the variable lineData, then just returns the text string “Something”. I want to loop through the table and process the data (including some calculations) into a string. Lets say we have this table sent in:
| Year | Price |
| 2000 | 55 |
| 2001 | 57 |
| 2002 | 50 |
Now once I loop through and process it, lets say we subtract 1000 of the year and double the price and return a string with a comma between the year and price, and a new line for each new row. So we would return a string that looks like:
1000,110 1001,114 1002,100
And the function would become something like (though not the correct syntax):
(lineData as table)=>
let
Source = “” // blank text string
Foreach(row in lineData) //loop through each row on the table
{
If row <> 1 //check if it is not the first row
Source = Source & “
“ // add a new line if it is not the first row
Source = Source & row[Year]-1000 & “,” //add the year after subtracting 1000 as well as the comma after
Source = Source & row[Price]*2 //add the price after doubling it
} //end the loop
in
Source
So now we return the string we have created that should look like the one above. However, I am not sure how to loop through a table (the foreach is more how it is done in php, I am not sure how it is done in power query).
So given the edits to your question I would do this in two steps, first doing an add column to produce the row by row operation where you subtract 1000 from the year and multiply the price by 2. Then do a List.Accumulate over that column adding line breaks between each row.
eg
let Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjIwMFDSUTI1VYrVAfMMQTxzGM8IxDNQio0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Year = _t, Price = _t]), #"Changed Type" = Table.TransformColumnTypes(Source,{{"Year", Int64.Type}, {"Price", Int64.Type}}), #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each Number.ToText([Year] - 1000) & "," & Number.ToText( [Price]*2)), Custom = #"Added Custom"[Custom], result = List.Accumulate(Custom, "", (state, current) => state & current & "#(cr)#(lf)" ) in resultPS. The Binary.FromText in the first line is just the pasted data from your question
11 Replies
Table.AddColumn will have the same effect as looping through each row and applying logic to generate a new column. You can either use one of the options in the "Add Column" ribbon in the Query Editor (there is even a button there that will allow you to call an existing custom function). Or you could type in the expression manually.
eg.= Table.AddColumn(#"Changed Type", "Custom", each Number.ToText( [Year]) & "," & Number.ToText([Price]))
But I am not trying to add a column, I am not in any way modifying the source table. I just want the values so I can create a string out of them after doing some calculations (the one string for the entire table). Seems kind of weird that nobody seems to know how to loop through a table, I would have thought that would be one of the most important functions in power query!
Anonymous wrote:But I am not trying to add a column, I am not in any way modifying the source table. I just want the values so I can create a string out of them after doing some calculations (the one string for the entire table). Seems kind of weird that nobody seems to know how to loop through a table, I would have thought that would be one of the most important functions in power query!
It would probably have been easier to understand if you provided a few rows of example data and the output you were after. I though you wanted to produce a new string per row. But if you want to produce a single string after looping through all the rows in a table then you could look at calling Table.ToList then using List.Accumulate
Reza has a great post on List.Accumulate here including a number of examples one of which shows using it to do a concatenate operation.