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C++ Primer Plus

int c;

if (a > b)

c = a;

else

c = b;

Compared to the if else sequence, the conditional operator is more concise but, at first glance, less obvious. One difference between the two approaches is that the conditional operator produces an expression and hence a single value that can be assigned or be incorporated into a larger expression, as the program in Listing 6.9 does when it assigns the value of the conditional expression to the variable c. The conditional operator’s concise form, unusual syntax, and overall weird appearance make it a great favorite among programmers who appreciate those qualities. One favorite trick for the reprehensible goal of concealing the purpose of code is to nest conditional expressions within one another, as the following mild example shows:

const char x[2] [20] = {"Jason ","at your service\n"};

const char * y = "Quillstone ";

for (int i = 0; i < 3; i++)

cout << ((i < 2)? !i ? x [i] : y : x[1]);

This is merely an obscure (but, by no means maximally obscure) way to print the three strings in the following order:

Jason Quillstone at your service

In terms of readability, the conditional operator is best suited for simple relationships and simple expression values:

x = (x > y) ? x : y;

If the code becomes more involved, it can probably be expressed more clearly as an if else statement.

The switch Statement

Suppose you create a screen menu that asks the user to select one of five choices—for example, Cheap, Moderate, Expensive, Extravagant, and Excessive. You can extend an if else if else sequence to handle five alternatives, but the C++ switch statement more easily handles selecting a choice from an extended list. Here’s the general form for a switch statement:

switch (integer-expression)

{

case label1 : statement(s)

case label2 : statement(s)

...

default : statement(s)

}

A C++ switch statement acts as a routing device that tells the computer which line of code to execute next. On reaching a switch statement, a program jumps to the line labeled with the value corresponding to the value of integer-expression. For example, if integer-expression has the value 4, the program goes to the line that has a case 4: label. The value integer-expression, as the name suggests, must be an expression that reduces to an integer value. Also each label must be an integer constant expression. Most often, labels are simple int or char constants, such as 1 or 'q', or enumerators. If integer-expression doesn’t match any of the labels, the program jumps to the line labeled default. The default label is optional. If you omit it and there is no match, the program jumps to the next statement following the switch (see Figure 6.3).

Figure 6.3. The structure of switch statements.

The switch statement is different from similar statements in languages such as Pascal in a very important way. Each C++ case label functions only as a line label, not as a boundary between choices. That is, after a program jumps to a particular line in a switch, it then sequentially executes all the statements following that line in the switch unless you explicitly direct it otherwise. Execution does not automatically stop at the next case. To make execution stop at the end of a particular group of statements, you must use the break statement. This causes execution to jump to the statement following the switch.

Listing 6.10 shows how to use switch and break together to implement a simple menu for executives. The program uses a showmenu() function to display a set of choices. A switch statement then selects an action based on the user’s response.

Note

Some hardware/operating system combinations treat the \a escape sequence (used in case 1 in Listing 6.10) as silent.

Listing 6.10. switch.cpp

// switch.cpp -- using the switch statement

#include

using namespace std;

void showmenu(); // function prototypes

void report();

void comfort();

int main()

{

showmenu();