Better way than if else if else... for linear interpolation

question is easy. Lets say you have function

double interpolate (double x);

and you have a table that has map of known x-> y
for example
5 15
7 18
10 22
note: real tables are bigger ofc, this is just example.

so for 8 you would return 18+((8-7)/(10-7))*(22-18)=19.3333333

One cool way I found is http://www.bnikolic.co.uk/blog/cpp-map-interp.html (long story short it uses std::map, key= x, value = y for x->y data pairs).

If somebody asks what is the if else if else way in title it is basically:

if ((x>=5) && (x<=7))
{
//interpolate
}
else 
     if((x>=7) && x<=10)
     {
      //interpolate
     }

So is there a more clever way to do it or map way is the state of the art? :)

Btw I prefer soutions in C++ but obviously any language solution that has 1:1 mapping to C++ is nice.