C++11 - for_each, auto, constexpr, the begin & the end...

Auto:

It's a new keyword really usefull to code faster but it can be scary to use as it leads to more flexibility and less readability.

srand(1234);

 auto vector_size = 10;

 std::vector<int> a(vector_size);
 auto b = std::vector<int>(vector_size);

 auto lambda = [](int& i){i = abs(i);};

 std::for_each(begin(a), end(a), lambda);

 for each (auto var in a)
 {
  std::cout << var << ",";
 }
 std::cout << std::endl;

and you can also use 'auto' as return type qualifier.

int foo() { return 5; } 
       auto j = foo();

==>  Allows to avoid typos
BUT i recommend an extensive usage of VisualAssistX to change function or variable signature and unit-test improvements.

ConstExpr:

As you may know, C98 doesn't allow to declare an array with an expression.

// Comment
//constexpr int f() //Maybe available in VS'11 Final ...
//{
// return 5;
//}

const int g()
{
 return 5;
}

void ConstExpressionDemo()
{
 const int vector_size = 10;
 int A[vector_size];

 int B[g()];//C++98 cannot compile
 /*int B[f()];*///C++11 should compile if constexpr available.
 
 return;
}

Visual C++ 11: Not available.

But using the compiler static analysis, the following code couldn't compile.

// Comment
constexpr int f() //shouldn't compile ...
{
 return rand();
}

for_each and for-loop improvement:

C# and modern languages provided tools to easily iterate over list, collection or array. Before C++11 programmers who wants to iterate on containers elements wrote loops like:

const auto vector_size = 10;
std::vector<int> a(vector_size);
srand(1234);
  
auto lambda_random_init = [](int& i){i = rand();};

for (std::vector<int>::iterator it = a.begin() ; it != a.end() ;  ++it)
{
  *it = rand();
}

But now we can use auto, lambda, std::for_each and range base for to provide compact, and elegant code.
As example, with the following code we can dump vector, deque or array, in fact any container containing any kind of object supporting "cout".

template<class T> void cout_container(T _Container)
{
    for each (auto var in _Container)
    {
        std::cout << var << ",";
    }
    std::cout << std::endl;
}

cout_container<std::vector<int>>(a);

in the following example i implemented vector initialization using a lambda and several vector accumulation based on different loop-style.

const auto vector_size = 10;
    std::vector<int> a(vector_size);

    srand(1234);

    auto lambda_random_init = [](int& i){i = rand();};

    //init using begin & end 
    std::for_each(begin(a), end(a), lambda_random_init);

    cout_container<std::vector<int>>(a);

    //for_each with lambda capture by ref and return sum
    int sum_of_elems = std::accumulate(begin(a), end(a), 0);
    std::cout << sum_of_elems << std::endl;

    std::for_each(begin(a), end(a), [&sum_of_elems](int& n){ sum_of_elems += n; }); //only 1 elt by ref
    std::cout << sum_of_elems << std::endl;
    std::for_each(begin(a), end(a), [&](int& n){ sum_of_elems += n; });//all elt by ref
    std::cout << sum_of_elems << std::endl;

    int tmp = 0;
    //std::for_each(begin(a), end(a), [&sum_of_elems](int& n){ sum_of_elems += n; tmp = 0; });//cannot compile as tmp is not in the capture list!!!
    std::for_each(begin(a), end(a), [&sum_of_elems, &tmp](int& n){ sum_of_elems += n; tmp += n; });//target several output captured by value !!!
    std::cout << sum_of_elems << " " << tmp << std::endl;

    for each (int& n : A) 
    {
        sum_of_elems += n;
    }
    std::cout << sum_of_elems << std::endl;

    cout_container<std::vector<int>>(a);;

C#: prefer LINQ instead of foreach

In this post, i want to focus in LINQ and its advantage on foreach.

In fact, i meet yesterday the following case: i had a list and i would remove some elements matching a criteria.

As you know C# provide a lots of interface like IQueryable, IEnumerable, etc.... And face to my problem i started by using my list as an IEnumerable through a foreach.... so wrong am i.

In fact if you try something like this:

 var collect = new Collection<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
 foreach (var x in collect)
 {
    if (x % 2 == 0)
    {
       collect.Remove(x);
    }
 }
You will meet an exception: System.InvalidOperationException :  .... Because you change the collection itself during the enumeration.

But some very simple solutions exists.
1) If the collection is a List of something implementing the IList Interface you can use "RemoveAll"

collect.RemoveAll(x => x % 2 == 0);


2) If your collection doesn't implement IList, you can use the following code:

 foreach (var x in collect.Where(x => x % 2 == 0).ToList())
 {
    collect.Remove(x);
 } 

  As ToList doesn't provide a new object of type List containing only the element we want to remove and we get through this new list.

No more problems !