FAQ CS1 Week 4

Muddiest Points from Lecture Material

1. How do you know what type of control structure to use?

   It really depends on what you need to do. If you need to do one thing after another, you will use a sequential structure. If you need to do different things based on a decision, you will use a selection or conditional structure. Next week we will add another structure as well that helps us to repeat things. Implementation "patterns" can help us understand what's required in a particular situation.

2. Are these if statements equivalent?

   if ( y == 8 )
   if ( x == 5 )
   System.out.println( "1" );
   else
   System.out.println( "2" );
   System.out.println( "3" );
   System.out.println( "4" );
   

   and

   if ( y == 8 && x == 5 )
   System.out.println( "1" );
   else
   System.out.println( "2" );
   System.out.println( "3" );
   System.out.println( "4" );
   

   No, try tracing what happens if y is 7 and x is 4. (For the first if, the output is 3 followed by 4 on the next line. In the second, it is 2 and 3 and 4 all on separate lines!) This is a good example of why both the use of indentation and the use of {}'s are helpful to those of us humans trying to read the code. If written according to our style standards, the two pieces of code would be written as follows:

   if ( y == 8 ) {
      if ( x == 5 ) {
         System.out.println( "1" );
      } else {
         System.out.println( "2" );
      }
   System.out.println( "3" );
   System.out.println( "4" );
   

   and

   if ( y == 8 && x == 5 ) {
      System.out.println( "1" );
   } else {
      System.out.println( "2" );
   }
   System.out.println( "3" );
   System.out.println( "4" );
   

3. I understand that you have to use to "==" in an if statement, but what exactly does "=" in an if statement compare?

   It doesn't compare anything, it's an assignment. Unlike C or C++, Java requires that the expression in an "if" statement be a boolean_expression. It will complain if we try to compile an if statement that incorrectly uses a = instead of ==, UNLESS the assignment is to a boolean variable. For example, see code/CompareWrong.java. If you download it and compile it, you will receive the following error message:

   Compare.java:34: incompatible types
   found : int
   required: boolean
   if ( x = y ) {
         ^
   1 error
   

4. Are switch and if the same thing?

   No, but they are different constructs for making selections of the path your program is to follow. And, you can do many of the same things with each. They differ substantially though in syntax and symantics!

5. I am having trouble understanding switch statements. Can the switch statement can work on multiple cases? If so, for example if we want the same thing to happen for two cases, does the switch statement finish when it first executes a break statement or does it go past the first break statement and continue executing until it reaches the second break statement?

   Yes, the switch statement can work on multiple cases. In ALL cases it stops executing when it reaches the first break statement after the matching case.

6. I don't understand how to convert between if and switch statements. Please help!
   • if to switch conversion
     First of all, not all if statements can be directly converted to a switch. The variables or expressions that decisions are based on are limited to the primitive types of char, byte, short, or int. So, if instead, what you have are any other type, unless you come up with a cleaver way to convert them to one of these types, it's not possible. In addition, not all conditionals lend themselves well to conversion. The best type are those where the variable or expression is being compared for equality against a limited number of possibilities. See the example below.

   • switch to if conversion
     A switch statement can ALWAYS be converted to an if/else statement. The if conditions are constructed by comparing the switch expression to the case value for equality. If there are more than one case values for which the same action is required, then connect those by || (ORs). So, if one of those conditions is met, the statements for that case are executed with the exception of the break. Each successive case becomes a new else if. What's in the default case becomes the else clause.

     For example,

     char grade;

     switch ( grade ) {
         case 'A':
            System.out.println( "A" );
            break;
         case 'B':
            System.out.println( "B" );
            break;
         case 'C':
         case 'D':
            System.out.println( "Other passing grade" );
            break;
         case 'F':
            System.out.println( "Failing" );
            break;
         default:
            System.out.println( "Invalid grade" );
            break;
     }
     

     becomes

     char grade;

     if ( grade == 'A' ) {
         System.out.println( "A" );
     } else if ( grade == 'B') {
         System.out.println( "B" );
     } else if ( grade == 'C' || grade == 'D' ) {
         System.out.println( "Other passing grade" );
     } else if ( grade == 'F') {
         System.out.println( "Failing" );
     } else {
         System.out.println( "Invalid grade" );
     }