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Conditionals are used in if and while expressions. This section covers some additional important features for conditionals. Note that the examples below are code fragments; you will need to put them inside your main() function.

• In the case where there is only one statement in a block following an if, the braces are optional and can be omitted.

if (a < b)
	printf("a is smaller!\n");

When an if's condition is true, the computer will execute the (one) statement immediately following the if. In order to execute multiple statements, they are combined into a block (multiple statements in braces); the block is considered a single statement. This applies to all conditionals, e.g. else, else if (below), while, and for (explained in the next section).
• What happens if you forget and put a semicolon after an if or while condition? The computer identifies the semicolon as an empty statement. Consider the following examples -- the erroneous semicolon is marked in red.

  if (a < b);
  	printf("boo!");
  
  while (a < b);
  {
  	// some code here...
  	a = a + 1;
  }
  

  • In the first example, when the if's condition is true, the statement following the if is executed -- this is the semicolon / empty statement! So, "boo!" will always be printed. (Note that code indentation is ignored by the compiler.)
  • The second example is an excellent illustration of an infinite loop. If the condition (a < b) is true, execution will move to the statement following the while condition, in this case the empty statement specified by the semicolon. After executing that statement (doing nothing), execution returns to the condition. But, since nothing is changing a or b, the condition will never become false, and execution will never leave the loop!
• To check another condition in the else case following an if, you can use the "else if" clause. When the condition of the if is false, the condition of the else if will be checked. If it is true, the block (or statement) following it will be executes; if the else if's condition is false, the block will be skipped, and execution will continue after it.

  if (score >= 90)
  	printf("you got an A\n");
  else if (score >= 80)
  	printf("you got a B\n");
  else if (score >= 70)
  	printf("you got a C\n");
  else
  	printf("better luck next time!\n");
  printf("done...\n");
  

  Some examples using the above code:
  • score = 95: the if's condition is true, so its statement is executed (the "A" message is printed). The remaining else if and else statements are then skipped, and execution continues with the statement after the if-else group (the "done" message).
  • score = 85: the if's condition is false, so execution moves to the (first) else if. That condition is true, so its statement is executed (the "B" message is printed). The remaining else if and else statements are then skipped, and execution continues with the statement after the if-else group (the "done" message).
  • score = 65: the if's condition is false, so execution moves to the (first) else if. Its condition is false, so execution moves to the second else if (comparing score to 70). That is also false, so execution moves to the else at the end; this is the block that catches conditions not handled by any of the if or else if conditions. It prints the "better luck" message, then execution continues with the "done" message.
• You can use a compound Boolean expression in a conditional clause to check multiple conditions. Use && for a logical AND, and || for a logical OR. Note that both operands must be complete Boolean expressions.

if (age >= 13 && age <= 19)
	printf("you are a teenager\n");
if (age < 10 || age >= 65)
	printf("you get a discount\n");

• The operator NOT (!) will negate an expression, turning a true expression false, or a false expression true.
• It's useful to know the opposite of conditional expressions. This is what happens if you want the logical NOT of an expression.

Expression		Opposite		Opposite Simplified
a < b		!(a < b)		a >= b
a <= b		!(a <= b)		a > b
a == b		!(a == b)		a != b

• Sometimes it's easier to express a conditional that's the opposite of what you want to check in code. For example, if you expect input between 1 and 10 (inclusive), and want to show an error when you don't get something in that range. In this case, it's fairly straightforward to express the desired situation:
  a >= 1 && a <= 10
  Since we want to check the opposite, we need to negate the expression:
  !(a >= 1 && a <= 10)
  While we could use the expression above, it's better to resolve the NOT and use a simpler expression. Here are the rules for negating expressions that use logical AND and logical OR:

  Expression		Opposite		Opposite Simplified
  a && b		!(a && b)		!a || !b
  a || b		!(a || b)		!a && !b

  So, our expression becomes:
  !(a >= 1) || !(a <= 10)
  Recall from above how to find the opposite of an inequality, which simplifies our expression to:
  a < 1 || a > 10

  if (a < 1 || a > 10)
  	printf("Bad value!");
  

• If you want to compare a variable to several values, you could use a series of else if statements (starting with if and possibly ending with else). But, a better way is to use a switch statement. One common example is a menu, where the program displays a list of options, and the user enters a number to select what the program should do.

  switch (a)
  {
  case 1:
  	printf("option 1...");
  	break;
  case 2:
  	printf("option 2...");
  	break;
  case 3:
  	printf("option 3...");
  	break;
  case 0:
  case 99:
  	printf("exiting...");
  	break;
  default:
  	printf("unknown option!");
  	break;
  }
  

  • In this example, the printf statements would be replaced with code to actually perform the specified option.
  • The contents of the parentheses following the switch must be an integer (or char; see next section), and is typically a variable, as shown here, but could also be a mathematical expression.
  • The case keyword must be followed by a constant value (not a variable).
  • Each case statement must list a single value; it cannot specify a range.
  • When the program runs, it looks at the value of variable a, jumps to the correct case statement, and begins executing code there. (Note that values 0 and 99 both result in printing the exit message.)
  • The break statement at the end of each case causes execution to "break" out of the switch, continuing execution at the statement immediately after the switch statement's closing brace. If a break statement is eliminated, execution will "fall through" to the next case statement.
  • The default statement is optional, but is a convenient way to handle all values that aren't specifically listed with a case statement. It is like the final else after a series of else if statements.

Summary

• Braces around a block following an if or else are optional if there is only one statement in the block.
• Use else if to make multiple comparisons in a row.
• Use logical AND (&&) and logical OR (||) operators to combine Boolean expressions in a conditional expression.
• Use switch to conveniently and efficiently choose among several values for a variable.

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