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First of all, Pike calculates the values of all the arguments. In the call

average(19.0 + 11.0, 10.0)

the two arguments are calculated, giving the values 30.0 and 10.0. Then the argument values are sent to the method. If we look at the method head,

float average(float x1, float x2)

we see that it has two formal parameters. The argument values will be put in the two parameter variables x1 and x2, which work as local variables but with the argument values as initial values.

Execution will then continue with the body of the method. In this case, the body is

{
  return (x1 + x2) / 2;
}

The value of (x1 + x2) / 2 will be calculated, giving 20.0. This value is then returned to the point where the method was called, and is used as the value of the method-call expression.

Note that Pike uses "call by value". This means that Pike always calculates the value of the arguments before calling a method, and then it sends those values (or, more precisely: copies of those values) to the called method. This means that in the example

average(1.0, average(2.0, 3.0))

Pike will first call average with the two values 2.0 and 3.0, and when average has returned the value 2.5, it will send the two values 1.0 and 2.5 to average. This second call of average will return 1.75, and this is the value of the entire expression.