How to apply list of materials in one layer of wall?

[Raha_R]

Hello All,
I have created my wall layers. I want to change material for one layer automatically. I got stocked in these nodes.
That would be great to have your helps. Thank you so much.

[Raha_R]

I have updated my nodes. Could you please help me to figure out my mistakes?
Is this approach correct?
Why this error?

[Kulkul]

Hi Raha,

Your approach is closer but you need to modify your graph little as shown below.Good Luck!