CODEWITHSUNDEEP
erlangpattern-matching
Bercovici Adrian
Bercovici Adrian

Reputation: 9360

Pattern matching on groups in list

I am learning Erlang using Learn you some erlang and I am grouping a list in tuples of 3.I do not understand why the book implementation is the following :

group([], Acc) ->Acc
group([A,B,X|Rest], Acc) -> group(Rest, [{A,B,X} | Acc]).

Input

  group([],[1,2]).

since it renders the following exception:

exception error: no function clause matching hth:group([],[1,2]) (d:/Erlang/AeRlang/hth.erl, line 15)

Shouldn't it be:

group(Acc,[X,Y,Z|T])->group([{X,Y,Z}|Acc],T);
group(Acc,_)->Acc.

Upvotes: 1

Views: 144

Answers (1)

7stud
7stud

Reputation: 48599

it renders the following exception:

exception error: no function clause matching hth:group([],[1,2]) (d:/Erlang/AeRlang/hth.erl, line 15)

Really? First, let's fix the syntax error:

group([], Acc) ->Acc;
group([A,B,X|Rest], Acc) -> group(Rest, [{A,B,X} | Acc]).

In the shell:

~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3  (abort with ^G)

1> c(a).
a.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,a}

2> a:group([], [1, 2]).
[1,2]

Shouldn't it be:

group(Acc,[X,Y,Z|T])->group([{X,Y,Z}|Acc],T);
group(Acc,_)->Acc.

Let's try it:

11> a:group([], [1, 2, 3, 4, 5, 6, 7, 8]).   
[{4,5,6},{1,2,3}]

The first definition will throw an error when the number of elements in the list isn't divisible by 3--presumably to alert the user that something is wrong. Your version "fails" silently. It looks like all you did was reverse the arguments in the first version, then in the base case your version matches anything rather than the empty list.

Upvotes: 3

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