Concept:
Accumulator parameter
Headline
An extra paramter for an intermediate result in recursive functions
Illustration
Consider the following recursive formulation of the Factorial function in Language:Java.
public static long factorial(int n) {
if (n<=1)
return 1;
else
return n*factorial(n-1);
}
While perfectly valid as a function definiton, the particular form of recursion, i.e., primitive recursion rather than tail recursion may be a source of ineffeciency. That is, the call stack grows with each recursive call, even though this would be easily avoidable, for example, with a non-recursive forumulation. There is also a more efficient, tail-recursive formulation, which uses an extra parameter to pass on the product computed by the function.
public static long factorial(int n) {
return factorial(1, n);
}
public static long factorial(int r, int n) {
if (n<=1)
return r;
else
return factorial(n*r, n-1);
}
More generally, the idea is to move the computations for post-processing results into the extra argument position.
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