Class 20: Control Flow: Conditionals (2)

Held: Friday, 25 February 2011

Summary: We consider additional forms of conditionals in C.

Related Pages:

• EBoard.
• Reading: K&R 3.4.

Notes:

• Celebration of MathCounts! success yesterday.
• Sorry for not posting obvious links to the reading last night.
• Next reading: K&R 3.4-3.5.
• Assignment 5 is now ready.
• EC: CS Table today!

Overview:

• Case statements.
• Conditional expressions.
• Lab.

The Switch Statement

Form:

switch (integer-expression):
  {
    case val0:
    case val1:
      statements;
    case val2:
      statements;
      break;
    default:
      statements;
      break;
  } // switch

Meaning:

• Evaluate the expression.
• Step through the cases until you hit the first matching value.
• Execute any subsequent statements until you hit break, return, or the end of the statement.

Useful because

• Less code than the nested if statements.
• Can be compiled to more efficient code than the nested if statements. (It isn't always compiled to more efficient code, but it can be.)
• The fall through option is powerful (although dangerous).

Conditional Expressions

Context:

• Conditional statements (particularly the if) can be used to select between different sequences of instructions.
• What if we want to select between different expressions, as in Scheme's if?
• Let x be the larger of y and z.
• Enter the conditional expression.

Form: test ? consequent : alternate

Meaning:

• Evaluate the test.
• If the test evaluates to a non-zero number, evaluate the consequent and use its value.
• If the test evaluates to zero, evaluate the alternate and use its value.

Examples

• x = y *gt; z ? y : z;

Lab

• Do the lab.
• Be prepared to reflect.