Value assigned by double default constructor in c++

Question

I have this variable: std::map<std::string, double> courses;

And I'd like to print it if it has a value assigned for some key. So my keys are strings, so I want something like:

std::cin >> mystring;
if (!(courses[mystring] == x)) std::cout << courses[mystring] << std::endl;

but I don't know what to make x. Any ideas?

Answer 1

You can simply do courses.find(mystring) != courses.end().

Answer 2

Use the following approach

auto it = courses.find( mystring );

if ( it != courses.end() ) std::cout << it->second << std::endl; 

An alternative is to use member function count. It is even simpler than using method find because there is no need to define the iterator.

if ( courses.count( mystring ) ) std::cout << courses[mystring]  << std::endl; 

Or even it would be better to write

if ( courses.count( mystring ) != 0 ) std::cout << courses[mystring]  << std::endl; 

Answer 3

You can use map::find to check if there is an item in the map corresponding to a key.

std::map<std::string, double>::iterator iter = courses.find(mystring);
if ( iter != courses.end() )
{
  std::cout << iter->second << std::endl;
}

Using find and using the returned iterator makes more sense to me since you have to search for the item only once.

Using something like:

if( courses.count( mystring ) )
{
    cout << courses[mystring]  << endl;
}

has the advantage that the code is simpler. However, it potentially comes at a cost. You may have to search for the item twice.

Answer 4

if( courses.count( mystring ) )
{
    cout << courses[mystring]  << endl;
}

Don't optimize prematurely, just write what's simplest and most clear.

I.e. using count, ^[1]as above.

In this case an optimization using more convoluted and verbose code with find would be extremely ^[2]marginal, since reuse of an iterator can't in general compare to the time involved in i/o, so I advice against that.

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^{1) You can add a comparision with 0 or e.g. a !! conversion to bool, if the old C convention of using a number directly as a boolean looks cryptic or weird.
2) One might think that using count would force a redundant second search for the item, but a map implementation is free to store the last found iterator value as hint for the next search, reducing that to O(1) in this case. E.g. see the complexity of std::map::emplace_hint. That alone classifies this optimization consideration as premature, and additionally there is the order-of-magnitude difference between i/o and searching in a small collection, not to mention the complexity (additional variable) and verbosity of the iterator based code.}