Because the == operator returns either true or false, it is ideal for use in conditional expressions, because conditional expressions are expected to evaluate to either true or false. For an example, look at Listing 2.3, which compares two numbers read in from the standard input file.

Listing 2.3. A program that uses the equality-comparison operator to compare two numbers entered at the keyboard.

1:  #!/usr/local/bin/perl

2:  

3:  print ("Enter a number:\n");

4:  $number1 = <STDIN>;

5:  chop ($number1);

6:  print ("Enter another number:\n");

7:  $number2 = <STDIN>;

8:  chop ($number2);

9:  if ($number1 == $number2) {

10:         print ("The two numbers are equal.\n");

11: }

12: print ("This is the last line of the program.\n");

$ program2_3

Enter a number:

17

Enter another number:

17

The two numbers are equal.

This is the last line of the program.

$

Lines 3-5 are again similar to statements you've seen before. They print a message on your screen, read a number into the variable $number1, and chop the newline character from the number

Lines 6-8 repeat the preceding process for a second number, which is stored in $number2.

Lines 9-11 contain the if statement that compares the two numbers. Line 9 contains the conditional expression

$number1 == $number2

If the two numbers are equal, the conditional expression is true, and the print statement in line 10 is executed. If the two numbers are not equal, the conditional expression is false, so the print statement in line 10 is not executed; in this case, the Perl interpreter skips to the first statement after the if statement, which is line 12.

Line 12 is executed regardless of whether or not the conditional expression in line 9 is true. It prints the following message on the screen:

This is the last line of the program.

Make sure that you don't confuse the = and == operators. Because any expression can be used as a conditional expression, Perl is quite happy to accept statements such as if ($number = 5) { print ("The number is five.\n"); } Here, the if statement is evaluated as follows: The number 5 is assigned to $number, and the following expression yields the result 5: $number = 5 The value 5 is nonzero, so the conditional expression is true. Because the conditional expression is true, this statement is executed: print ("The number is five.\n"); Note that the print statement is executed regardless of what the value of $number was before the if statement. This is because the value 5 is assigned to $number by the conditional expression. To repeat: Be careful when you use the == operator

Other Comparison Operators

The == operator is just one of many comparison operators that you can use in conditional expressions. For a complete list, refer to Chapter 4.

Two-Way Branching Using if and else

When you examine Listing 2.3 (shown previously), you might notice a problem. What happens if the two numbers are not equal? In this case, the statement

print ("The two numbers are equal.\n");

is not printed. In fact, nothing is printed.

Suppose you want to modify Listing 2.3 to print one message if the two numbers are equal and another message if the two numbers are not equal. One convenient way of doing this is with the if-else statement.

Listing 2.4 is a modification of the program in Listing 2.3. It uses the if-else statement to print one of two messages, depending on whether the numbers are equal.

Listing 2.4. A program that uses the if-else statement.

1:  #!/usr/local/bin/perl

2:  

3:  print ("Enter a number:\n");

4:  $number1 = <STDIN>;

5:  chop ($number1);

6:  print ("Enter another number:\n");

7:  $number2 = <STDIN>;

8:  chop ($number2);

9:  if ($number1 == $number2) {

10:         print ("The two numbers are equal.\n");

11: } else {

12:         print ("The two numbers are not equal.\n");

13: }

14: print ("This is the last line of the program.\n");

$ program2_4

Enter a number:

17

Enter another number:

18

The two numbers are not equal.

This is the last line of the program.

$

Lines 3-8 are identical to those in Listing 2.3. They read in two numbers, assign them to $number1 and $number2, and chop their newline characters

Line 9 compares the value stored in $number1 to the value stored in $number2. If the two values are equal, line 10 is executed, and the following message is printed:

The two numbers are equal.

The Perl interpreter then jumps to the first statement after the if-else statement-line 14.

If the two values are not equal, line 12 is executed, and the following message is printed:

The two numbers are not equal.

The interpreter then continues with the first statement after the if-else-line 14.

In either case, the Perl interpreter executes line 14, which prints the following message:

This is the last line of the program.

The syntax for the if-else statement is

if (expr) {

        statement_block_1

} else {

        statement_block_2

}

As in the if statement, expr is any expression (it is usually a conditional expression). statement_block_1 is the block of statements that the Perl interpreter executes if expr is true, and statement_block_2 is the block of statements that are executed if expr is false.

Note that the else part of the if-else statement cannot appear by itself; it must always follow an if.

Multi-Way Branching Using elsif

Listing 2.4 (which you've just seen) shows how to write a program that chooses between two alternatives. Perl also provides a conditional statement, the if-elsif-else statement, which selects one of more than two alternatives. Listing 2.5 illustrates the use of elsif.

Listing 2.5. A program that uses the if-elsif-else statement.

1:  #!/usr/local/bin/perl

2:  

3:  print ("Enter a number:\n");

4:  $number1 = <STDIN>;

5:  chop ($number1);

6:  print ("Enter another number:\n");

7:  $number2 = <STDIN>;

8:  chop ($number2);

9:  if ($number1 == $number2) {

10:         print ("The two numbers are equal.\n");

11: } elsif ($number1 == $number2 + 1) {

12:         print ("The first number is greater by one.\n");

13: } elsif ($number1 + 1 == $number2) {

14:         print ("The second number is greater by one.\n");

15: } else {

16:         print ("The two numbers are not equal.\n");

17: }

18: print ("This is the last line of the program.\n");

$ program2_5

Enter a number:

17

Enter another number:

18

The second number is greater by one.

This is the last line of the program.

$

You already are familiar with lines 3-8. They obtain two numbers from the standard input file and assign them to $number1 and $number2, chopping the terminating newline character in the process

Line 9 checks whether the two numbers are equal. If the numbers are equal, line 10 is executed, and the following message is printed:

The two numbers are equal.

The Perl interpreter then jumps to the first statement after the if-elsif-else statement, which is line 18.

If the two numbers are not equal, the Perl interpreter goes to line 11. Line 11 performs another comparison. It adds 1 to the value of $number2 and compares it with the value of $number1. If the two values are equal, the Perl interpreter executes line 12, printing the message

The first number is greater by one.

The interpreter then jumps to line 18-the statement following the if-elsif-else statement.

If the conditional expression in line 11 is false, the interpreter jumps to line 13. Line 13 adds 1 to the value of $number1 and compares it with the value of $number2. If these two values are equal, the Perl interpreter executes line 14, which prints

The second number is greater by one.

on the screen. The interpreter then jumps to line 18.

If the conditional expression in line 13 is false, the Perl interpreter jumps to line 15 and executes line 16, which prints

The two numbers are not equal.

on the screen. The Perl interpreter continues with the next statement, which is line 18.

If you have followed the program logic to this point, you've realized that the Perl interpreter eventually reaches line 18 in every case. Line 18 prints this statement:

This is the last line of the program.

The syntax of the if-elsif-else statement is as follows:

if (expr_1) {

        statement_block_1

} elsif (expr_2) {

        statement_block_2

} elsif (expr_3) {

        statement_block_3

...

} else {

        default_statement_block

}

Here, expr_1, expr_2, and expr_3 are conditional expressions. statement_block_1, statement_block_2, statement_block_3, and default_statement_block are blocks of statements.

The ... indicates that you can have as many elsif statements as you like. Each elsif statement has the same form:

} elsif (expr) {

        statement_block

}

Syntactically, an if-else statement is just an if-elsif-else statement with no elsif parts.

If you want, you can leave out the else part of the if-elsif-else statement, as follows:

if (expr_1) {

        statement_block_1

} elsif (expr_2) {

        statement_block_2

} elsif (expr_3) {

        statement_block_3

...

}

Here, if none of the expressions-expr_1, expr_2, expr_3, and so on-are true, the Perl interpreter just skips to the first statement following the if-elsif-else statement.

Writing Loops Using the while Statement

The conditional statements you've seen so far enable the Perl interpreter to decide between alternatives. However, each statement in the Perl programs that you have seen is either not executed or is executed only once.

Perl also enables you to write conditional statements that tell the Perl interpreter to repeat a block of statements a specified number of times. A block of statements that can be repeated is known as a loop.

The simplest way to write a loop in Perl is with the while statement. Here is a simple example of a while statement:

while ($number == 5) {

        print ("The number is still 5!\n");

}

The while statement is structurally similar to the if statement, but it works in a slightly different way. Here's how:

• First, the conditional expression located between the parentheses is tested.
• If the conditional expression is true, the statement block between the { and } is executed. If the expression is false, the statement block is skipped, and the Perl interpreter jumps to the statement following the while statement. (This is called exiting the loop.)
• If the statement block is executed, the Perl interpreter jumps back to the start of the while statement and tests the conditional expression over again. (This is the looping part of the while statement, because at this point the Perl interpreter is executing a statement it has executed before.)