How to create a function with an unknown number of parameters?

Question

I need to create a function to calculate the average of int types. how to do it better?

Example:

   double calculate_average(int num1, ...) {
      .... 
      return sum / count;
   }

   int main() {
     calculate_average(5, 4, 3, 2, 1);
     calculate_average(3, 2); 
   }

The term is "variadic function". But you'd probably be better off just passing a container of values, such as a `std::vector`. — Fred Larson, May 21 '20 at 13:45
https://en.cppreference.com/w/cpp/utility/variadic , https://en.cppreference.com/w/cpp/language/variable_template , see also https://en.cppreference.com/w/cpp/algorithm/accumulate — Jesper Juhl, May 21 '20 at 13:46
*"how to do it better?"* pass a `std::vector const&` for example, this will save many of the headaches that come with dealing with variadic templates. — Cory Kramer, May 21 '20 at 13:46
@CoryKramer, because we will know that the function already takes at least 1 argument - the vector — Weezyg, May 21 '20 at 13:54

cigien · Accepted Answer · 2020-05-21T14:18:36.633

3

You can use fold expressions for this:

template<typename ...Ts>
double calculate_average(Ts... ts)
{
  return (((double)ts / sizeof...(ts)) + ... + 0);
}

Note the sizeof...(ts), is a separate primitive for parameter packs. It's not a pack expansion, but instead gives the number of arguments that are passed into the function.

Here's a demo.

edited May 21 '20 at 14:18

answered May 21 '20 at 13:46

cigien

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it's work! but how fix if call: calculate_average(2147483647, 1) – Weezyg May 21 '20 at 13:58
Try `calculate_average(2147483647l, 1);`. After all, the arguments you passed in are not `int`s anyway, so might as well pass in `long`s. – cigien May 21 '20 at 14:00
@cigien You could also fix your calculation with `return (((double)ts/sizeof...(ts)) + ...);` – NathanOliver May 21 '20 at 14:02
@NathanOliver thanks. edited an option. This should convert all arguments to `double`s at least. – cigien May 21 '20 at 14:04
@NathanOliver but doesn't take care of overflow, so added that option, thanks – cigien May 21 '20 at 14:05
@cigien `sizeof...(ts)`, could you explain that part? Is that another fold expression, a special case or what? – john May 21 '20 at 14:16
@cigien It's OK I've got it, https://en.cppreference.com/w/cpp/language/sizeof... – john May 21 '20 at 14:18
@john added anyway :) It is a special primitive, not a pack expansion. – cigien May 21 '20 at 14:18

How to create a function with an unknown number of parameters?

1 Answers1