# Lambda Expressions

C++11 lambda expressions make the oneAPI Threading Building Blocks (oneTBB) `parallel_for` much easier to use. A lambda expression lets the compiler do the tedious work of creating a function object.

Below is the example from the previous section, rewritten with a lambda expression. The lambda expression, replaces both the declaration and construction of function object `ApplyFoo` in the example of the previous section.

    #include "oneapi/tbb.h"

    using namespace oneapi::tbb;

    void ParallelApplyFoo( float* a, size_t n ) {
        parallel_for( blocked_range<size_t>(0,n),
            [=](const blocked_range<size_t>& r) {
                for(size_t i=r.begin(); i!=r.end(); ++i)
                    Foo(a[i]);
            }
        );
    }

The [=] introduces the lambda expression. The expression creates a function object very similar to `ApplyFoo`. When local variables like `a` and `n` are declared outside the lambda expression, but used inside it, they are “captured” as fields inside the function object. The [=] specifies that capture is by value. Writing [&] instead would capture the values by reference. After the [=] is the parameter list and definition for the `operator()` of the generated function object. The compiler documentation says more about lambda expressions and other implemented C++11 features. It is worth reading more complete descriptions of lambda expressions than can fit here, because lambda expressions are a powerful feature for using template libraries in general.

C++11 support is off by default in the compiler. The following table shows the option for turning it on.

Environment | Intel® C++ Compiler Classic | Intel® oneAPI DPC++/C++ Compiler
--- | --- | ---
Windows* OS systems | `icl /Qstd=c++11 foo.cpp` | `icx /Qstd=c++11 foo.cpp`
Linux* OS systems | `icc -std=c++11 foo.cpp` | `icx -std=c++11 foo.cpp`

For further compactness, oneTBB has a form of `parallel_for` expressly for parallel looping over a consecutive range of integers. The expression `parallel_for(first,last,step,f)` is like writing `for(auto i=first; i<last; i+=step)f(i)` except that each f(i) can be evaluated in parallel if resources permit. The `step` parameter is optional. Here is the previous example rewritten in the compact form:

    #include "oneapi/tbb.h"

    using namespace oneapi::tbb;

    #pragma warning(disable: 588)

    void ParallelApplyFoo(float a[], size_t n) {
        parallel_for(size_t(0), n, [=](size_t i) {Foo(a[i]);});
    }

The compact form supports only unidimensional iteration spaces of integers and the automatic chunking feature detailed on the following section.