How does Power Query evaluate expressions?

[Anonymous]

PSB piece of PQ code 

let
MyList = { 5, 10, 15, 20 },
Index = -1,
Found = (Index = List.PositionOf(MyList, 15)) <> -1
in
[Found in Array = Found, Array Location = Index]

This results in the following output

[TRUE, -1]

I am trying to wrap my head around how Found can be TRUE and still Index is -1. 

[Mariusz]

Hi @Anonymous 

This might be useful as well.

https://docs.microsoft.com/en-us/powerquery-m/evaluation-model

Best Regards,
Mariusz

If this post helps, then please consider Accepting it as the solution.

[Anonymous]

Actually I want the ability to be able to get the index of an element in a list (say list A) in 1 condition and use the returned index to access a diferent list (list B) all within an each statement. Without repeating the List.PositionOf in the second condition again, it does not seem possible. 

[artemus]

Just use List.Transform e.g.

= each List.Transform([MyListColumn], (elm) => [MyOtherListColumn]{elm})

The thing to note is that "each" is just shorthand for "(_) =>" and "_" is the default variable, so that where "[Column]" is, it just is shorthand for "_[Column]"

[Mariusz]

Hi @Anonymous 

The original index was not overridden because it's been changed only within Found, so technically you referred to the original state within your output.

The below will make more sense

let
    MyList = { 5, 10, 15, 20 },
    Index = List.PositionOf( MyList, 15 ),
    Found = Index <> -1
in
    [Found in Array = Found, Array Location = Index]

Best Regards,
Mariusz

If this post helps, then please consider Accepting it as the solution.