Switch Statements with Numeric Values

Suppose a program asks a user for a length and needs every length in the same unit before it can do any calculation. A user might type a distance in feet, another in yards, and another in miles. The program cannot simply add those numbers together, because a value of 3 in feet and a value of 3 in yards describe very different distances. The program must first translate each measurement into one standard unit. In this article, that standard unit is the inch.

There is more than one way to ask the user for a unit. One design gives the user a menu of numbered options and reads a choice number. A second design asks the user to type the unit's name, such as inch or feet, and then uses that text directly to decide what to do. The second design is the focus here, because it shows how a switch statement can work with strings rather than only with numbers.

Before reading the rules, follow the program's path for a single input. Watch which lines run when the user types a unit name, which branch of the switch is selected, and what value is stored in the variable for inches. Notice where the path changes when the unit name changes. Your goal is to be able to say, at each step, what the program is doing and what it has stored so far.

A switch statement checks the value of a variable and chooses one case to run. In the string version, the variable holds text rather than a number. The variable named units holds whatever unit name the user typed, and the switch compares that text against the labels in each case. The program runs the first case whose label matches, and skips the rest.

A single case can list several labels separated by commas. This is useful because one unit usually has a full name, a plural form, and an abbreviation. For inches, the source accepts the labels inch, inches, and in, and all three lead to the same action. Writing the labels together keeps the three spellings in one place, so the same action is not repeated three times.

Each unit other than the inch needs a conversion factor, which is the number of inches in one of that unit. The source uses three of them. One foot is 12 inches, so a measurement in feet is multiplied by 12. One yard is 36 inches, so a measurement in yards is multiplied by 36. A mile is 5280 feet and each foot is 12 inches, so the factor for miles is 12 times 5280, which is 63360 inches per mile.

The switch has a default case, which runs when no other label matches. The source uses it to catch any unit name the program does not recognize. Typing a unit such as furlong, or misspelling feet as fete, matches no label, so the default branch runs. That branch prints a message saying the unit is illegal and then ends the program.

Trace the path for the input fete. The program checks the inch labels, then the feet label, then the yards and miles labels. None matches, so control reaches the default case. The default case reports the illegal unit and stops the program, so no conversion step runs and the variable for inches is never given a converted value. Stopping is the right behavior here, because a result computed from an unknown unit would be meaningless.

Most errors with this kind of program come from the text the user types not matching the labels exactly. Each mistake below shows where a trace would reveal the problem.