Addition is not commutative in Pascal or Java. The problem is that a function call, which may be one or both of the operands to the addition operator, could have a side-effect. In that case, the functions must be called in order. If no order is specified within expression evaluation then you can’t even reliably write code with side-effects within an expression. Here’s another example of the problem with side-effects within code. In the code below, it was observed that when the code was compiled with one C++ compiler it printed 1,2 while with another compiler it printed 1,1. In this case, the language definition is the problem. The C++ language definition doesn’t say what should happen in this case. The decision is left to the compiler writer.

The practice problem writes 17 as written. If the expression were b+a() then 15 would be written.