## Posts Tagged ‘recursion’

## Introduciton to Erlang : Recursion (2/2)

### Accumulators

In several cases, as with the `mlists:length/1`

example, the non-tail recursive function can be easily turned to a tail recursive one by using the notion of *accumulator*. An accumulator is an extra argument introduced to a function in order to aggregate the partial results of the function. It turns the “bottom-up” collection of the final result to “top-down”.

In order to add and initialize the accumulator argument one has to introduce an extra function definition.

tlr(...) -> tlr(..., Accumulator_initial_value). % the clause that "breaks" the recursion and % returns the result tlr(..., Accumulator) -> Accumulator; tlr(..., Accumulator) -> ..., Accumulator_new_value = ..., ..., trl(..., Accumulator_new_value). |

Notice that typically you would only export the `tlr/1`

function and the `tlr/2`

would remain for inner-module use and not visible to the module’s users.

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## Introduction to Erlang : Recursion (1/2)

### Recursion

The definition of the word *recursion* is “*(mathematics) an expression such that each term is generated by repeating a particular mathematical operation”, according to the WordNet*. Recursion is one of the most powerful “tools” in a functional programming language and so it is for Erlang. Recursion can be used to apply *divide and conquer* techniques to problem solving, where a problem is broken to smaller subproblems, the subproblems are solved, and the results are “merged” to generate the final result.

Recursion happens when a function’s body definition includes a call to the function itself.

functionA(...) -> Body_before_recursion, % optional functionA(...), Body_after_recursion. % optional |

Recursion is used instead of the conventional loop statements of other programming languages, such as `while`

and `for`

in C.

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