Difference between math expression in python and C++ in fenics

Question

I posted a question about the difference in the way that an expression was defined in c++ and python in some demos provided by fenics team.

I posted the question here.

https://fenicsproject.org/qa/10088/difference-in-defining-function-expression-in-python-and-c

Dr. Kent has already explained why it is done. In addition to that, it would be great if someone can explain how the two expressions are equal.

Answer 1

I think this is a general problem with many programming languages.
It has nothing to do with FEniCS.
Consider the case where you have an array 'a' and you want to compute
b = cos(a) + sin(a) + a

what will happen here is that an temporal array is constructed
that holds the values of sin(a) + a. This temp array is then later
added to cos(a). If you have a more complicated expression then
many temp arrays may be constructed during the evaluation.
If you don't what this to happen, you need to vectorize
the expression.