Editing Topics:SICP in other languages:Erlang:Chapter 1 (section)

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==% 1.2.4 Procedures and the Processes They Generate - Exponentiation==
<pre><nowiki>
section_1_2_4() ->
   print (even(3)),
   print (multiply(3, 4)).

% Linear recursion
expt(_, 0) -> 1;
expt(B, N) -> B * expt(B, (N - 1)).

% Linear iteration
expt_iter(_, 0, Product) -> Product;
expt_iter(B, Counter, Product) ->
   expt_iter(B, Counter - 1, B * Product).

expt_1(B, N) ->
   expt_iter(B, N, 1).

% Logarithmic iteration
even(N) -> ((N rem 2) =:= 0).

fast_expt(_, 0) -> 1;
fast_expt(B, N) ->
  case (even(N)) of
     true  -> square(fast_expt(B, N div 2));
     false -> B * fast_expt(B, N - 1)
   end.

% Exercise 1.17
multiply(_, 0) -> 0;
multiply(A, B) ->
  plus(A, multiply(A, dec(B))).

% Exercise 1.19
fib_iter_2(_, B, _, _, 0) -> B;
fib_iter_2(A, B, P, Q, Count) ->
   case (even(Count)) of
      true  -> fib_iter_2(A, B, (P*P + Q*Q), (2*P*Q + Q*Q), Count div 2);
      false -> fib_iter_2((B * Q) + (A * Q) + (A * P), (B * P) + (A * Q), P, Q, Count - 1)
   end.

fib_2(N) ->
   fib_iter_2(1, 0, 0, 1, N).

</nowiki></pre>

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