CODEWITHSUNDEEP
crecursiontail-recursion
Роман Полоз
Роман Полоз

Reputation: 13

Will this be a tail call or not?

Is the recursive call in my code a tail call or is it just ordinary recursion?

void sum(const int values[], size_t count,int* result){
  if(count <= 0) return;
  *result += values[0];
  sum(values + 1,count - 1,result);
}

Because as far as I know, a tail call is recursion that occurs as the last call in a function.

Upvotes: 1

Views: 113

Answers (2)

Chris
Chris

Reputation: 36680

Given that C does not require tail-call elimination/optimization, your safe strategy is to avoid recursion where it doesn't naturally fit (think of things like binary search trees).

In your specific case, there's no reason not to write:

void sum(const int *values, size_t count, int *result){
    for (size_t i = 0; i < count; i++) {
        *result += values[i];
    }
}

As an aside: it's interesting that you're using recursion but then modifying a pointer, which is very much not functional. This has the effect that for a sum to be accurate the value result points to must be initialized to 0 or the result will be inaccurate.

Upvotes: 2

ikegami
ikegami

Reputation: 386676

If no code follows a call, it's a tail call.

In C, this would like one of these:

void g( … );

void f( … ) {
   …
   g( … );  // Tail call.
}

void g( … );

void f( … ) {
   …
   g( … );  // Tail call.
   return;
   …
}

T g( … );

T f( … ) {
   …
   return g( … );  // Tail call.
   …
}

It has nothing to do with recursion, but it does include recursive calls.

Tail calls are interesting since they can be eliminated. C neither mandates nor forbids tail-call elimination. That's up to the compiler.

Upvotes: 1

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