[erlang-questions] Calling a Function with Same argument twice

sapan shah <>
Thu Oct 15 06:13:26 CEST 2009


>
> sapan shah wrote:
>
>> Hi,
>>
>> see the code below.
>> ...
>>
>> We call temp:start(1), the output is 4. The output should have been 1.
>>
>
> No, 4 is right.
>
>  Here, when start(1) is called, it intern calls
>> {element(1, test(1,1), element(2, test(1,1)}
>>
>
> test(2,2) results in two calls to test(1,1),
> which each result in two calls to test(0,0), each
> incrementing the 'test' object by 1.
>
>
>> The first call of test(1,1) should recursively evaluate test(1,1) while
>> the
>> second call should already use the value returned by the first call.. Is
>> this a right thinking???
>>
>
> I assume you're thinking that it would work like:
>
> test(0,0) ->
>    put(test, get(test) +1),
>    {0,0};
> test(X,Y) ->
>  Z = test(X-1, Y-1),
>  {element(1, Z), element(2, Z)}.
>
> Yes, I solved the original problem that I am working in this manner only...


but here you make only one call and reuse the result.
> If the test/2 function werer purely functional (i.e. no side-
> effects), the two would be equivalent, but here you use get and
> put, so it really makes a difference how many times you explicitly
> call the function.
>
>
So, the basic question is: If the test/2 function is side effect free (no
put, no get or no io:format..), would erlang evaluater still call the test/2
function again, if it had called the same fuction with same arguments
erlier???

Btw, since test/2 returns a two-tuple, the above is of course
> equivalent to
>
> test(0,0) ->
>    put(test, get(test) +1),
>    {0,0};
> test(X,Y) ->
>  test(X-1, Y-1).
>
> ...which is almost equivalent to
>
> test(X,X) when is_integer(X) -> put(test, get(test) +1), {0,0}.
>
> (It differs only slightly in how it fails on erroneous input).
>
> Think twice before using get/put. Also, why wrap the return values
> inside a 1-tuple? Remove the tuple, and you can remove some of the
> calls to element/2.
>
> But of course it was a contrived example, so I assume the real
> problem you're trying to solve is somewhat different?
>
> BR,
> Ulf W
> --
> Ulf Wiger
> CTO, Erlang Training & Consulting Ltd
> http://www.erlang-consulting.com
>



-- 
Regards,
Sapan Shah


More information about the erlang-questions mailing list