How to get a value by percentage in a range of two numbers?

Question

Programming

For example:
A value of 10% in the range between 0.0 and 20.0 would be 2.0
A value of 50% in the range between -5.0 and 5.0 would be 0
The difficulty is that this is a vector calculation, and - on - does not give here +
The range can be, for example, ( -5.0, -10.0), (5.0, 10.0), (-5.0, 10.0), in general, go both up and down.
This task arose in the development of the game. Example:

(float StartValue, float EndValue, float Percent) {
  const float OnePercent = (EndValue - StartValue) / 100.0f;
  return StartValue + OnePercent * Percent;
}

But this calculation only works if both numbers are positive and if StartValue is always lower than EndValue.

Answer 1

Read about interpolation. Your example is quite correct both with negative numbers and with the reverse order. Think better.
UPD : moreover, this formula is valid for vectors of arbitrary dimension, and for angles, and for many other quantities.

Answer 2

Формула универсальная для любых чисел, шаг в сторону endvalue от startvalue
Чем не устраивает формула , покажите на примере конкретных вычислений.

Answer 3

EndValue-((EndValue-StartValue)*percent/100)