Expressions vs. Statements

Expressions

• Expressions evaluate to something. They have types.
• Examples:
  • Constants: 42,   "Hello"
  • Variables: i,   celsiusValue
  • Simple operations: i + 3,   a * b,   answer == 42
  • Complex expressions: ((i + 3) * a * b) <= 42
  • Function/Method calls (if they have return values): fish1.atWall()

Statements

• Statements change state (usually the state of memory, but could be external state, by producing output for example).
• Examples:
  • Assignment statements: i = 0;
  • If statements and Loops (whose bodies are made up of statements)
  • Procedure calls (if they change the values of parameter, change the state of an object, produce output, etc.)
• Statements often contain expressions (right-hand side of assignment, conditional expressions, parameters)
• NOTE: in i = 0;, 0 is an expression, but i is not. We're not evaluating i, we're using its address as the location where we should put the result of evaluating the expression on the right-hand side.

So what?

• Most languages make a clear distinction between expressions and statements.
• Some languages distinguish between functions (expressions that return a value but have no "side effects") and procedures (statements that change state and have no return value).
• Purely functional languages have only expressions, no statements!
• Example: Conditional Statements and Conditional Expressions

  if ( a < b ) max = b; else max = a;

  max(a, b) : if a < b then b else a

  (define (max a b) (if (< a b) b a))

What about C?

• C frequently blurs the line between expressions and statements. It has expressions that change state ("side effects") and statements that evaluate to values.
• Examples:
  • j = i++;   i++ evaluates to the value of i before the increment and increments i.
  • i = j = 0;   evaluates right-to-left; j = 0 assigns 0 to j and evaluates to 0 (the new value of j), giving us i = 0;.
  • functions can return values and have side effects

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  Alyce Brady, Kalamazoo College