How do I check if a data in table A exists in table B?

[CUCOOPE]

Lets say I have a table in sharepoint like this:

ID	Name
123	Peter
234	John
345	Mary

And a input text box for user to input the ID of the people to generate a list. For example, if the user input: 123, 234 and press the "submit" button, a TrimEnd and Split function was used to generate a list like:

Record

123

234

Then I use a gallary to display the new list and use lookup() to look up the corrsponding data from the sharepoint list. So that it could generate a table like this in powerapps:

ID	Name
123	Peter
234	John

But one thing I couldn't figure out is that how can I prevent users from entering data that does not exist in the sharepoint list? I was thinking to lookup each data from the 2nd list above and warns the user that the record (e.g. 789) does not exist if the size of the returned table is 0 when the user press the "submit' button. But I couldn't figure out how can I use each data correspondingly in Powerapps (like in C++, using for loop on an array like A[i] etc.) Does anyone knows how can I accomplish it? Thanks!

[theapurva · Accepted solution]

instead of using textbox for putting 123, 234  and then splitting etc., 
I think its better to use combobox and put select multiple property as true.
you can give combo box items property as Distinct from the other list and proceed further accordingly