In Mathematica's standard notebook interface, you are directly giving input and getting output every time you press Shift+Enter. Although much more rarely needed than in more ...

Mathematica normally takes any expression it is given, and evaluates it as far as possible. But built into the Mathematica language is a collection of flexible primitives ...

AbsoluteTiming[expr] evaluates expr, returning a list of the absolute number of seconds in real time that have elapsed, together with the result obtained.

Compiled is an option for various numerical and plotting functions which specifies whether the expressions they work with should automatically be compiled.

Gradient is an option for FindMinimum and related functions that specifies the gradient vector to assume for the function being extremized.

Head[expr] gives the head of expr.

Most[expr] gives expr with the last element removed.

NotebookSelection[] represents the current selection in the current evaluation notebook in the front end.NotebookSelection[nb] represents the current selection associated ...

Rest[expr] gives expr with the first element removed.

RowBox[{box_1, box_2, ...}] is a low-level box construct that represents a row of boxes or strings in a notebook expression.