Using Double-Quoted Strings

Perl supports scalar variable substitution in character strings enclosed by double quotation-mark characters. For example, consider the following assignments:

$number = 11;

$text = "This text contains the number $number.";

When the Perl interpreter sees $number inside the string in the second statement, it replaces $number with its current value. This means that the string assigned to $text is actually

This text contains the number 11.

The most immediate practical application of this is in the print statement. So far, many of the print statements you have seen contain several arguments, as in the following:

print ("The final result is ", $result, "\n");

Because Perl supports scalar variable substitution, you can combine the three arguments to print into a single argument, as in the following:

print ("The final result is $result\n");

Escape Sequences

Character strings that are enclosed in double quotes accept escape sequences for special characters. These escape sequences consist of a backslash (\) followed by one or more characters. The most common escape sequence is \n, which represents the newline character as shown in this example:

$text = "This is a string terminated by a newline\n";

Table 3.1 lists the escape sequences recognized in double-quoted strings.

The \Q escape sequence is useful only when the string is used as a pattern. Patterns are described on Chapter 7, "Pattern Matching."

The escape sequences \L, \U, and \Q can be turned off by \E, as follows:

$a = "T\LHIS IS A \ESTRING"; # same as "This is a STRING"

To include a backslash or double quote in a double-quoted string, precede the backslash or quote with another backslash:

$result = "A quote \" in a string";

$result = "A backslash \\ in a string";

A backslash also enables you to include a $ character in a string. For example, the statements

$result = 14;

print("The value of \$result is $result.\n");

print the following on your screen:

The value of $result is 14.

You can specify the ASCII value for a character in base 8 or octal notation using \nnn, where each n is an octal digit; for example:

$result = "\377";        # this is the character 255, or EOF

You can also use hexadecimal notation to specify the ASCII value for a character. To do this, use the sequence \xnn, where each n is a hexadecimal digit.

$result = "\xff";        # this is also 255

Listing 3.4 is an example of a program that uses escape sequences. This program takes a line of input and converts it to a variety of cases.

Listing 3.4. A case-conversion program.

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

2:  

3:  print ("Enter a line of input:\n");

4:  $inputline = <STDIN>;

5:  print ("uppercase: \U$inputline\E\n");

6:  print ("lowercase: \L$inputline\E\n");

7:  print ("as a sentence: \L\u$inputline\E\n");

$ program3_4

Enter a line of input:

tHis Is My INpUT LiNE.

uppercase: THIS IS MY INPUT LINE.

lowercase: this is my input line.

as a sentence: This is my input line.

$

Line 3 of this program reads a line of input and stores it in the scalar variable $inputline

Line 5 replaces the string $inputline with the current value of the scalar variable $inputline. The escape character \U tells the Perl interpreter to convert everything in the string into uppercase until it sees a \E character; as a result, line 4 writes the contents of $inputline in uppercase.

Similarly, line 6 writes the input line in all lowercase characters by specifying the escape character \L in the string.

Line 7 combines the escape characters \L and \u. The \L specifies that everything in the string is to be in lowercase; however, the \u special character temporarily overrides this and tells the Perl interpreter that the next character is to be in uppercase. When this character-the first character in the line-is printed, the \L escape character remains in force, and the rest of the line is printed in lowercase. The result is as if the input line is a single sentence in English. The first character is capitalized, and the remainder is in lowercase.

Single-Quoted Strings

Perl also enables you to enclose strings using the ' (single quotation mark) character:

$text = 'This is a string in single quotes';

There are two differences between double-quoted strings and single-quoted strings. The first difference is that scalar variables are replaced by their values in double-quoted strings but not in single-quoted strings. The following is an example:

$string = "a string";

$text = "This is $string";  # becomes "This is a string"

$text = 'This is $string';  # remains 'This is $string'

The second difference is that the backslash character, \, does not have a special meaning in single-quoted strings. This means that the statement

$text = 'This is a string.\n';

assigns the following string to $text:

This is a string.\n

The \ character is special in only two instances for single-quoted strings. The first is when you want to include a single-quote character ' in a string.

$text = 'This string contains \', a quote character';

The preceding line of code assigns the following string to $text:

This string contains ', a quote character

The second instance is to escape the backslash itself.

$text = 'This string ends with a backslash \\';

The preceding code line assigns the following string to $text:

This string ends with a backslash \

As you can see, the double backslash makes it possible for the backslash character (\) to be the last character in a string.

Single-quoted strings can be spread over multiple lines. The statement $text = 'This is two lines of text '; is equivalent to the statement $text = "This is two\nlines of text\n"; This means that if you forget the closing ' for a string, the Perl interpreter is likely to get quite confused because it won't detect an error until after it starts processing the next line

Interchangeability of Strings and Numeric Values

As you've seen, you can use a scalar variable to store a character string, an integer, or a floating-point value. In scalar variables, a value that was assigned as a string can be used as an integer whenever it makes sense to do so, and vice versa. In the following example:

$string = "43";

$number = 28;

$result = $string + $number;

the value of $string is converted to an integer and added to the value of $number. The result of the addition, 71, is assigned to $result.

Another instance in which strings are converted to integers is when you are reading a number from the standard input file. The following is some code similar to code you've seen before:

$number = <STDIN>;

chop ($number);

$result = $number + 1;

This is what is happening: When $number is assigned a line of standard input, it really is being assigned a string. For instance, if you enter 22, $number is assigned the string 22\n (the \n represents the newline character). The chop function removes the \n, leaving the string 22, and this string is converted to the number 22 in the arithmetic expression.

If a string contains characters that are not digits, the string is converted to 0 when used in an integer context. For example: $result = "hello" * 5; # this assigns 0 to $result, since "hello" becomes 0 This is true even if the string is a valid hexadecimal integer if the quotes are removed, as in the following: $result = "0xff" + 1; In cases like this, Perl does not tell you that anything has gone wrong, and your results might not be what you expect. Also, strings containing misprints might not contain what you expect. For example: $result = "12O34"; # the letter O, not the number 0 When converting from a string to an integer, Perl starts at the left and continues until it sees a letter that is not a digit. In the preceding instance, 12O34 is converted to the integer 12, not 12034

Initial Values of Scalar Variables

In Perl, all scalar variables have an initial value of the null string, "". This means that you do not need to define a value for a scalar variable.

#!/usr/local/bin/perl

$result = $undefined + 2;   # $undefined is not defined

print ("The value of \$result is $result.\n");

This short program is perfectly legal Perl. The output is

The value of $result is 2.

Because $undefined is not defined, the Perl interpreter assumes that its value is the null string. This null string is then converted to 0, because it is being used in an addition operation. The result of the addition, 2, is assigned to $result.

Summary

Perl supports three kinds of scalar values: integers, floating-point numbers, and character strings.

Integers can be in three notations: standard (decimal) notation, octal notation, and hexadecimal notation. Octal notation is indicated by a leading 0, and hexadecimal notation is indicated by a leading 0x. Integers are stored as floating-point values and can be as long as the machine's floating-point precision (usually 16 digits or so).

Floating-point numbers can consist of a string of digits that contain a decimal point and an optional exponent. The exponent's range can be anywhere from about e-309 to e+308. (This value might be different on some machines.) When possible, floating-point numbers are displayed without the exponent; failing that, they are displayed in scientific notation (one digit before the decimal point).

When you use floating-point arithmetic, be alert for round-off errors. Performing arithmetic operations in the proper order-operating on large numbers first-might yield better results.

You can enclose character strings in either double quotes (") or single quotes ('). If a scalar variable name appears in a character string enclosed in double quotes, the value of the variable is substituted for its name. Escape characters are recognized in strings enclosed in double quotes; these characters are indicated by a backslash (\).

Character strings in single quotes do not support escape characters, with the exception of \\ and \'. Scalar variable names are not replaced by their values.

Strings and integers are freely interchangeable in Perl whenever it is logically possible to do so.

Q&A

Workshop

The Workshop provides quiz questions to help you solidify your understanding of the material covered and exercises to give you experience in using what you've learned. Try and understand the quiz and exercise answers before you go on to tomorrow's lesson.

Quiz

1. Define the following terms:
   a    round-off error
   b    octal notation
   c    precision
   d    scientific notation
2. Convert the following numbers from octal notation to decimal:
   a    0377
   b    06
   c    01131
3. Convert the following numbers from hexadecimal notation to decimal notation:
   a    0xff
   b    0x11
   c    0xbead
4. What does the following line print?
   print ("I am bored\b\b\b\b\bhappy!\n");
5. Suppose the value of $num is 21. What string is assigned to $text in each of the following cases?
   a    $text = "This string contains $num.";
   b    $text = "\\$num is my favorite number.";
   c    $text = 'Assign \$num to this string.';
6. Convert the following numbers to scientific notation:
   a    43.71
   b    0.000006e-02
   c    3
   d    -1.04