Multiple Tests and if-else Statements

Often you want to distinguish between more than two distinct cases, but conditions only have two possible results, true or false, so the only direct choice is between two options. As anyone who has played “20 Questions” knows, you can distinguish more cases with further questions. If there are more than two choices, a single test may only reduce the possibilities, but further tests can reduce the possibilities further and further. Since most any kind of statement can be placed in the sub-statements in an if-else statement, one choice is a further if or if-else statement. For instance consider a function to convert a numerical grade to a letter grade, ‘A’, ‘B’, ‘C’, ‘D’ or ‘F’, where the cutoffs for ‘A’, ‘B’, ‘C’, and ‘D’ are 90, 80, 70, and 60 respectively. One way to write the function would be to test for one grade at a time, and resolve all the remaining possibilities inside the next else clause. If we do this consistent with our indentation conventions so far:

static char letterGrade(double score)
{
   char letter;
   if (score >= 90) {
      letter = 'A';
   }
   else {   // grade must be B, C, D or F
      if (score >= 80) {
         letter = 'B';
      }
      else { // grade must be C, D or F
         if (score >= 70) {
            letter = 'C';
         }
         else {   // grade must D or F
            if (score >= 60) {
               letter = 'D';
            }
            else {
               letter = 'F';
            }
         }   //end else D or F
      }      // end of else C, D, or F
   }         // end of else B, C, D or F
   return letter;
}

This repeatedly increasing indentation with an if statement in the else clause can be annoying and distracting. Here is a preferred alternative in this situation, that avoids all this further indentation: Combine each else and following if onto the same line, and note that the if part after each else is just a single (possibly very complicated) statement. This allows the elimination of some of the braces:

/// Return letter grade for score.
static char letterGrade(double score)
{
   char letter;
   if (score >= 90) {
      letter = 'A';
   }
   else if (score >= 80) { // grade must be B, C, D or F
      letter = 'B';
   }
   else if (score >= 70) { // grade must be C, D or F
      letter = 'C';
   }
   else if (score >= 60) { // grade must D or F
      letter = 'D';
   }
   else {
      letter = 'F';
   }
   return letter;
}

A program testing the letterGrade function is in example program grade1/grade1.cs.

See Grade Exercise.

As in a basic if-else statement, in the general format,

if ( condition1 ) {
statement-block-run-if-condition1-is-true;
}
else if ( condition2 ) {
statement-block-run-if-condition2-is-the-first-true;
}
else if ( condition3 ) {
statement-block-run-if-condition3-is-the-first-true;
}
// …
else {    // no condition!
statement-block-run-if-no condition-is-true;
}

exactly one of the statement blocks gets executed: If some condition is true, the first block following a true condition is executed. If no condition is true, the else block is executed.

Here is a variation. Consider this fragment without a final else:

if (weight > 120) {
    Console.WriteLine("Sorry, we can not take a suitcase that heavy.");
}
else if (weight > 50) {
    Console.WriteLine("There is a $25 charge for luggage that heavy.");
}

This statement only prints one of two lines if there is a problem with the weight of the suitcase. Nothing is printed if there is not a problem.

If the final else clause is omitted from the general if … else if … pattern above, at most one block after a condition is executed: That is the block after the first true condition. If all the conditions are false, none of the statement blocks will be executed.

It is also possible to embed if-else statements inside other if or if-else statements in more complicated patterns.

Sign Exercise

Write a program sign.cs to ask the user for a number. Print out which category the number is in: "positive", "negative", or "zero".

Grade Exercise

Copy grade1/grade1.cs to grade2.cs in your own project. Modify grade2.cs so it has an equivalent version of the letterGrade function that tests in the opposite order, first for F, then D, C, …. Hint: How many tests do you need to do? 1

Be sure to run your new version and test with different inputs that test all the different paths through the program.

Be careful for edge cases: Test the grades on the “edge” of a change in the result.

Wages Exercise

Modify the wages1/wages1.cs or the wages2/wages2.cs example to create a program wages3.cs that assumes people are paid double time for hours over 60. Hence they get paid for at most 20 hours overtime at 1.5 times the normal rate. For example, a person working 65 hours with a regular wage of $10 per hour would work at $10 per hour for 40 hours, at 1.5 * $10 for 20 hours of overtime, and 2 * $10 for 5 hours of double time, for a total of

10*40 + 1.5*10*20 + 2*10*5 = $800.

You may find wages2/wages2.cs easier to adapt than wages1/wages1.cs.

Caution: Be sure to thoroughly test your final program version. It is easy to add new features that work by themselves, but break a part that worked before! In particular in a program with decisions, make sure you test with enough different data to check all lines of your program.

1

4 tests to distinguish the 5 cases, as in the previous version