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For more information about the W3C website, see the Webmaster FAQ. Normal arrays are ordered lists of scalars indexed by number, starting with 0. Values are usually referred to by name, or through a named reference. The first character of the name tells you to what sort of data structure it refers. The rest of the name tells you the particular value to which it refers. Usually this name is a single identifier, that is, a string beginning with a letter or underscore, and containing letters, underscores, and digits.
For a more in-depth discussion on identifiers, see Identifier parsing. It’s possible to substitute for a simple identifier, an expression that produces a reference to the value at runtime. Perl also has its own built-in variables whose names don’t follow these rules. They have strange names so they don’t accidentally collide with one of your normal variables. In addition, several special variables that provide windows into the inner working of Perl have names containing punctuation characters. English word “the” in that it indicates a single value is expected.
English, in that it indicates multiple values are expected. Every variable type has its own namespace, as do several non-variable identifiers. This means that you can, without fear of conflict, use the same name for a scalar variable, an array, or a hash–or, for that matter, for a filehandle, a directory handle, a subroutine name, a format name, or a label. This may seem a bit weird, but that’s okay, because it is weird. They are reserved with respect to labels and filehandles, however, which don’t have an initial special character. You can’t have a filehandle named “log”, for instance.
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It is possible to replace such an alphanumeric name with an expression that returns a reference to the appropriate type. For a description of this, see perlref. Names that start with a digit may contain only more digits. Names that do not start with a letter, underscore, digit or a caret are limited to one character, e. Most of these one character names have a predefined significance to Perl. And all such names are reserved for Perl’s possible use.
18, the actual rules of what a valid identifier was were a bit fuzzy. However, in general, anything defined here should work on previous versions of Perl, while the opposite — edge cases that work in previous versions, but aren’t defined here — probably won’t work on newer versions. That is, a “start” character followed by any number of “continue” characters. Perl additionally accepts identfier names beginning with an underscore. That is, any word character in the ASCII range, as long as the first character is not a digit. Normal identifiers can start or end with a double colon, and can contain several parts delimited by double colons.
Additionally, if the identifier is preceded by a sigil — that is, if the identifier is part of a variable name — it may optionally be enclosed in braces. For the most part, all of the identifiers in this category have a special meaning given by Perl. Because they have special parsing rules, these generally can’t be fully-qualified. Similar to the above, a sigil, followed by bareword text in braces, where the first character is a caret. The use of the other characters is unwise, as these are all reserved to have special meaning to Perl, and none of them currently do have special meaning, though this could change without notice. This is a combination of the previous two forms.
ASCII word characters up to the trailing brace. The same caveats as the previous form apply: The non-graphic characters are no longer allowed with “use utf8”, it is unwise to use this form at all, and utf8ness makes a big difference. There are two major contexts: list and scalar. Certain operations return list values in contexts wanting a list, and scalar values otherwise. If this is true of an operation it will be mentioned in the documentation for that operation.
In a reciprocal fashion, an operation provides either a scalar or a list context to each of its arguments. STDIN and passing it back to the integer operation, which will then find the integer value of that line and return that. Assignment is a little bit special in that it uses its left argument to determine the context for the right argument. Assignment to a scalar evaluates the right-hand side in scalar context, while assignment to an array or hash evaluates the righthand side in list context. Perl’s -w command-line option, you may see warnings about useless uses of constants or functions in “void context”. It still counts as scalar context for functions that care whether or not they’re being called in list context. User-defined subroutines may choose to care whether they are being called in a void, scalar, or list context.
Most subroutines do not need to bother, though. That’s because both scalars and lists are automatically interpolated into lists. See wantarray for how you would dynamically discern your function’s calling context. A scalar may contain one single value in any of three different flavors: a number, a string, or a reference.
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In general, conversion from one form to another is transparent. Although a scalar may not directly hold multiple values, it may contain a reference to an array or hash which in turn contains multiple values. Scalars aren’t necessarily one thing or another. There’s no place to declare a scalar variable to be of type “string”, type “number”, type “reference”, or anything else. TRUE if it is anything else. The Boolean context is just a special kind of scalar context where no conversion to a string or a number is ever performed. Although in early versions of Perl, an undefined scalar could become defined when first used in a place expecting a defined value, this no longer happens except for rare cases of autovivification as explained in perlref.
The length of an array is a scalar value. Shortening an array this way destroys intervening values. Lengthening an array that was previously shortened does not recover values that were in those elements. You can also gain some minuscule measure of efficiency by pre-extending an array that is going to get big. You can also extend an array by assigning to an element that is off the end of the array. If you evaluate an array in scalar context, it returns the length of the array. Note that this is not true of lists, which return the last value, like the C comma operator, nor of built-in functions, which return whatever they feel like returning.
If you evaluate a hash in scalar context, it returns false if the hash is empty. A more precise definition is version dependent. 25 the value returned was a string consisting of the number of used buckets and the number of allocated buckets, separated by a slash. This is pretty much useful only to find out whether Perl’s internal hashing algorithm is performing poorly on your data set. 25 the return was changed to be the count of keys in the hash. String literals are usually delimited by either single or double quotes. The usual C-style backslash rules apply for making characters such as newline, tab, etc.
See hex and oct for more details. The format is useful for accurately presenting floating point values, avoiding conversions to or from decimal floating point, and therefore avoiding possible loss in precision. Notice that while most current platforms use the 64-bit IEEE 754 floating point, not all do. You can also embed newlines directly in your strings, i.
Variable substitution inside strings is limited to scalar variables, arrays, and array or hash slices. In fact, a simple identifier within such curlies is forced to be a string, and likewise within a hash subscript. But anything more complicated in the subscript will be interpreted as an expression. The infinity can be also negative. The not-a-number is the result when the result is undefined or unrepresentable. The infinity and not-a-number have their own special arithmetic rules.
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Case is ignored, and the Win32-specific forms like 1. This is useful for representing Unicode strings, and for comparing version “numbers” using the string comparison operators, cmp , gt , lt etc. Such literals are accepted by both require and use for doing a version check. They were v-strings from Perl 5. 0, but that caused more confusion and breakage than good.
SUB__ gives a reference to the current subroutine. Z, and the tokens __END__ and __DATA__ may be used to indicate the logical end of the script before the actual end of file. The filehandle is left open pointing to the line after __DATA__. Leaving it open leaks filehandles if the module is reloaded for any reason, so it’s a safer practice to close it. Perl will warn you about any such words.
Future versions of Perl are likely to eliminate these arbitrary limitations. Some people may wish to outlaw barewords entirely. The restriction lasts to the end of the enclosing block. If it does guess wrong, or if you’re just plain paranoid, you can force the correct interpretation with curly braces as above. If you’re looking for the information on how to use here-documents, which used to be here, that’s been moved to Quote and Quote-like Operators in perlop.
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In a context not requiring a list value, the value of what appears to be a list literal is simply the value of the final element, as with the C comma operator. LISTs do automatic interpolation of sublists. That is, when a LIST is evaluated, each element of the list is evaluated in list context, and the resulting list value is interpolated into LIST just as if each individual element were a member of LIST. To make a list reference that does NOT interpolate, see perlref. Interpolating it in a list has no effect. Similarly, interpolating an array with no elements is the same as if no array had been interpolated at that point. Not that we’d advise you to use this obfuscation.
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A list value may also be subscripted like a normal array. You must put the list in parentheses to avoid ambiguity. An exception to this is that you may assign to undef in a list. This is handy when you want to do a list assignment in a Boolean context, because most list functions return a null list when finished, which when assigned produces a 0, which is interpreted as FALSE.
It’s also the source of a useful idiom for executing a function or performing an operation in list context and then counting the number of return values, by assigning to an empty list and then using that assignment in scalar context. You can actually put an array or hash anywhere in the list, but the first one in the list will soak up all the values, and anything after it will become undefined. While literal lists and named arrays are often interchangeable, that’s not the case for hashes. Just because you can subscript a list value like a normal array does not mean that you can subscript a list value as a hash. That’s why it’s good to use references sometimes.
Note that just because a hash is initialized in that order doesn’t mean that it comes out in that order. See sort for examples of how to arrange for an output ordering. The array indices start with 0. A negative subscript retrieves its value from the end.
Hash subscripts are similar, only instead of square brackets curly brackets are used. Multi-dimensional array emulation Multidimensional arrays may be emulated by subscripting a hash with a list. 034″, the same as SUBSEP in awk. It’s more convenient than writing out the individual elements as a list of separate scalar values. Since you can assign to a list of variables, you can also assign to an array or hash slice. Since changing a slice changes the original array or hash that it’s slicing, a foreach construct will alter some–or even all–of the values of the array or hash.