Expressions

• expression
  • A non-empty sequence of literals, variables, operators, and function calls that calculates a value
• The process of executing an expression is called evaluation, and the resulting value produced is called the result of the expression (also sometimes called the return value)

# different expressions
2               // 2 is a literal that evaluates to value 2
"Hello world!"  // "Hello world!" is a literal that evaluates to text "Hello world!"
x               // x is a variable that evaluates to the value held by variable x
2 + 3           // operator+ uses operands 2 and 3 to evaluate to value 5
five()          // evaluates to the return value of function five()

• expressions evaluating to different things
  • literals evaluate to their own values
  • variables evaluate to the value of the variable
  • operators (such as operator+) use their operands to evaluate to some other value
  • function calls evaluate to whatever value the function returns
• expressions are in statements
• type identifier { expression };

// 2 + 3 is an expression that has no semicolon -- the semicolon is at the end of the statement containing the expression
int x{ 2 + 3 }; 

Expression statements

• a statement that consists of an expression followed by a semicolon
• Thus, we can take any expression (such as x = 5), and turn it into an expression statement (x = 5;) that will compile.
• When an expression is used in an expression statement, any result generated by the expression is discarded (because it is not used). For example, when the expression x = 5 evaluates, the return value of operator= is discarded. And that’s fine, because we just wanted to assign 5 to x anyway.

Subexpressions, full expressions, and compound expressions

2               // 2 is a literal that evaluates to value 2
2 + 3           // 2 + 3 uses operator + to evaluate to value 5
x = 4 + 5       // 4 + 5 evaluates to value 9, which is then assigned to variable x

• subexpression is an expression used as an operand.
  • the subexpressions of x = 4 + 5 are x and 4 + 5. The subexpressions of 4 + 5 are 4 and 5.
• full expression is an expression that is not a subexpression.
  • All three expressions above (2, 2 + 3, and x = 4 + 5) are full expressions.
• In casual language, a compound expression is an expression that contains two or more uses of operators. 
  • x = 4 + 5 is a compound expression because it contains two uses of operators (operator= and operator+). 2 and 2 + 3 are not compound expressions.