[erlang-questions] Updating elements of a record

Donald Steven <>
Wed Jan 23 00:04:13 CET 2019


I should have said:

test(Rec, [{a, Val},{c, Val}])

On 1/22/2019 5.48 PM, Donald Steven wrote:
> Thanks Roger.  I see how this works, but the issue is as much how to 
> get it to do:
>
> test([{a, Val},{c, Val}]), leaving, for example, field b untouched.
>
> On 1/22/2019 3.57 PM, Roger Lipscombe wrote:
>> On Tue, Jan 22, 2019, at 3:10 PM, Donald Steven wrote:
>>> I would like to pass a list of some fields of a record to a function to
>>> update those fields, leaving the other fields as they were. The list
>>> will be different for each function call.  For example, one call might
>>> want to change fields 1, 2 and 7, another call only field 8, another
>>> call fields 3 and 4.  Of course, each field will have a unique name.
>>> Each call to updateStatus (let's call it) would look like
>>> updateStatus(Status, List_of_Fields_to_Update) -> code.
>>>
>> Records are just tuples. Use setelement with #rec.field:
>>
>> -module(rec).
>> -export([new/0, update/3, test/0]).
>>
>> -record(rec, {a, b, c}).
>>
>> new() ->
>>      #rec{}.
>>
>> update(R, Val, [F | Fields]) ->
>>      R2 = setelement(F, R, Val),
>>      update(R2, Val, Fields);
>> update(R, _Val, []) ->
>>      R.
>>
>> test() ->
>>      R = rec:new(),
>>      R2 = rec:update(R, 2, [#rec.a, #rec.c]),
>>      R3 = rec:update(R2, 1, [#rec.b]),
>>      R3.
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>



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