Is it possible to update the multiple values in a map with a single call

[David Simmons]

Hi.

I have a simple map m = %{name: "Fred", age 50}

I can update name using m = %{m | name: "Dave"}

but is it possible to update both name and age at the same time?

many thanks.

dave

[Chris McGrath]

You can update both using the update syntax:

iex(2)> m = %{m | name: "Dave", age: 25 }

%{age: 25, name: "Dave”}

The update fields must already exist in the map though:

iex(3)> m = %{m | name: "Dave", age: 25, foo: :bar }

** (ArgumentError) argument error

    (stdlib) :maps.update(:foo, :bar, %{age: 25, name: "Dave"})

    (stdlib) erl_eval.erl:255: anonymous fn/2 in :erl_eval.expr/5

    (stdlib) lists.erl:1261: :lists.foldl/3

HTH,

Chris

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[David Simmons]

Cheers Chris - I'd tried the option you suggested but must have had a type so failed for me.  Re-tried and it worked.

thanks for your quick response.

Dave 

[David Simmons]

Sorry Chris.

One more question if I may. I have a second map x = %{name: "Bart", age: 20}. I thought I might be able to update my first map with:

%{ m | x} but this results in an error. Any idea how I pass in a map as a 2nd argument?

cheers

Dave

[Chris McGrath]

Map.merge(m, x) should do what you want here. I think it’s not working because %{} is a special form and so it doesn’t dereference the variable. That’s a total guess though!

Chris

[David Simmons]

Perfect - many thanks.

[Ben Wilson]

Do keep in mind that unlike %{ | } Map.merge CAN introduce new keys in your map if there are keys in the second map not currently present in the first.

On Saturday, April 11, 2015 at 5:37:17 AM UTC-4, David Simmons wrote:

Perfect - many thanks.

[David Simmons]

@Ben - did think about that but feel it is a small price to pay :-).