Recall that an expression is a piece of program code that represents or computes a value. The value of an expression can be assigned to a variable, used as the output value in an output routine, or combined with other values into a more complicated expression.
Expressions are an essential part of programming.
So far, these notes have dealt only informally with expressions. Recall that a function is a subroutine that returns a value. Literals, variables, and function calls are simple expressions. More complex expressions can be built up by using operators to combine simpler expressions. When several operators appear in an expression, there is a question of precedence , which determines how the operators are grouped for evaluation.