In c++, is it safe to use std::numeric_limits<double>::max() as a special “flag”?

问题

Given

std::vector<double> a;
std::vector<int> ind;

where ind is ¹sorted in ascending order.

I want to do the equivalent of the following:

for (auto it=ind.rbegin();it!=ind.rend();it++) a.erase(a.begin() + *it);

I came up with this:

for (auto it=ind.begin();it!=ind.end();it++) 
   a[*it] = std::numeric_limits<double>::max();
std::erase(std::remove(a.begin(),a.end(),std::numeric_limits<double>::max()),a.end());

This is very fast, but it doesn't feel right to use the std::numeric_limits::max() as a flag in this context.

Of course feelings shouldn't play too much into the equation ... clearly comparing the doubles within the std::remove is safe, and the limit will never occur in practice in this vector in a working application, so it should be ok, no?

Any thoughts on this?

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^{1) Ref comment by the OP. – Alf}

回答1:

Let's look at your "baseline" code, that you say you want to do the "equivalent" of:

std::vector<double> a;
std::vector<int> ind;

for (auto it = ind.rbegin(); it != ind.rend(); it++)
    a.erase(a.begin() + *it);

What we gather from this is that ind is a vector of indexes in a which should be removed, and that they are sorted ascending. Those indexes must be removed from a. I assume your goal is to do this efficiently in terms of space and time.

If you think about it in terms of the minimum number of operations required, you have to shift many/most/all of the elements in a in order to erase the ones indicated by ind. You don't want to erase() several times because this means shifting some elements more than once. One optimal solution (in the general case, not imposing special requirements on the values in a) looks like this:

size_t slow = 0; // where we are currently copying "to"
std::vector<int> inditer = ind.begin();
for (size_t fast = 0; fast != a.size(); ++fast) {
    if (inditer != ind.end() && *inditer == fast) {
        // "remove" this element by ignoring it
        ++inditer;
    } else {
        a[slow] = a[fast];
        ++slow;
    }
}
a.resize(slow);

Now, you could probably reformulate this using STL algorithms and a custom predicate (functor) which remembers its "current" position in ind, but it will not be much less code and it might be harder to understand.

来源：https://stackoverflow.com/questions/28548637/in-c-is-it-safe-to-use-stdnumeric-limitsdoublemax-as-a-special-flag