C++ for Scientists - Technische Universität Dresden

38 CHAPTER 2. C++ BASICS
for (...; ...; ...) {
...
if (dx == 0.0) continue;
x+= dx;
...
if (r < eps) break;
...
}
In the example above we assumed that the remainder of the iteration is not needed when
dx == 0.0. In some iterative computations it might be clear in the middle of an iteration (here
when r < eps) that work is already done.
Understanding the program behavior becomes more difficult the more breaks and continues
are used. One should always aim for moving as much loop control as possible into the loop
head. However, avoiding breaks and continues by excessive if-then-else branches is even less
comprehensible.
Sometimes, one might prefer performing some surplus operations inside a loop (if it has no
perceivable impact on the overall performance) and keep the program simpler. Simpler programs
on the other hand have a better chance to get optimized by the compiler. There is certainly
no golden rule but as practical approach one should implement software first for maximal
clarity and simplicity (but using efficient algorithms as early as possible). Once the software is
working correctly one can try variations to investigate the impact of implementation details on
performance.
2.5.6 Switch Statement
A switch is like a special kind of if. It provides a concise notation when different computations
for different cases of a given integral value are performed:
switch(op code) {
case 0: z= x + y; break;
case 1: z= x − y; cout ≪ ”compute diff\n”; break;
case 2:
case 3: z= x ∗ y; break;
default: z= x / y;
}
When people see the switch statement for the first time, they are usually surprised that one
needs to say at end of each case that the statement is terminated. Otherwise the statements of
the next case are executed as well. This can be used to perform the same operation for different
cases, e.g. for 2 and 3 in the example above.
The continuation allows us also to implement short loops without the termination test after
each iteration. Say we have vectors with dimension ≤ 5. Then we could implement a vector
addition without a loop:
assert(size(v)