Recommend a guide to M language?

[gazpage]

Can anyone point me to a good guide to the M language? I have M is for Data Monkey, which is good, but I need the next level I think.

In particular I would like to understand 'each' and '_' well enough to apply them myself. I can usually read code that includes them well enough, but not write my own.

 

[MarcelBeug]

Actually I'm working on a guide, but it won't be available shortly.

But I can share the draft text about 'each' and '_':

When performing conversions on structured values (lists, records, tables), the conversions are actually applied to each element of the structured value. So when adding a column to a table, each value in the new column is determined using the same conversion function.
An example: add column “b” to a table as 10 * the values from column “a”:

Added_b = Table.AddColumn(MyTable,”b”, each 10 * [a])

The parameter starting with each is actually a function, according to the syntax of Table.AddColumn:

Table.AddColumn(table as table, newColumnName as text, [B]columnGenerator as function[/B], optional columnType as nullable type) as table

So this querystep can be rewritten as:

columnGenerator = each 10 * [a],
Added_b = Table.AddColumn(MyTable, "b", columnGenerator)

In fact, keyword each is equivalent with function parameter _, so the first function can be rewritten as:

Added_b = Table.AddColumn(MyTable,”b”, (_) => 10 * [a])

Which is equivalent with:

Added_b = Table.AddColumn(MyTable,”b”, (_) => 10 * _[a])

It is allowed to omit the _ parameter, but this is restricted to _ only, so this won’t work:

Added_b = Table.AddColumn(MyTable,”b”, (x) => 10 * [a])

The correct code:

Added_b = Table.AddColumn(MyTable,”b”, (x) => 10 * x[a])

 

[gazpage]

Sounds great. Where will the guide be when it is ready? Will it be available to buy?

Thanks for the example, but I am still not sure I follow it. Maybe I can't even use 'each' for the thing I was trying to do anyway. What I was trying to do was remove the space from all of my column names.

Doing it manually I ended up with part of the code line that looked like:

{{"John Smith","JohnSmith"},{"Jane Doe","Jane Dow"}}

So a list of lists. I created a list of the source column names and a list without the space easily enough, but wasn't able to work out how to combine them together. Is this the kind of thing that could be done using each'?

 

[MarcelBeug]

A bit strange that you provided a similar answer here.

Anyhow, this can be done with:

= Table.TransformColumnNames(PreviousStep, each Text.Remove(_," "))

in which "PreviousStep" is the name of yourpevious step.

 

[gazpage]

Sorry for any confusion. That answer is just from when I was googling and doesn't represent any understanding on my part.

I am trying to improve my understanding so that I develop the ability resolve other problems that I have. Another recent problem I had was with an Expand Columns step but I didn't know what the underlying columns are ahead of time. When you Expand Columns using the UI you end up with the actual Column names in your code. I think I eventually found an answer to that as well via Google or a kind stranger but this really shouldn't be beyond me.

I will be reviewing your earlier response in detail and hopefully I can get to grips with it.

Thanks again for your help.

 

[MarcelBeug]

Your topics seem to be in the area of a series of blog posts by Gil Raviv, that you might find useful (I think they are).

 

[gazpage]

Awesome, thanks so much. This looks like just what I am looking for.