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Range Operator

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Range Operator

Binary ``..'' is the range operator, which is really two different operators depending on the context. In a list context, it returns an array of values counting (by ones) from the left value to the right value. This is useful for writing for (1..10) loops and for doing slice operations on arrays. Be aware that under the current implementation, a temporary array is created, so you'll burn a lot of memory if you write something like this:

    for (1 .. 1_000_000) {
        # code

In a scalar context, ``..'' returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each ``..'' operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. (It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation (as in sed), use three dots (``...'') instead of two.) The right operand is not evaluated while the operator is in the ``false'' state, and the left operand is not evaluated while the operator is in the ``true'' state. The precedence is a little lower than || and &&. The value returned is either the null string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string ``E0'' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar ``..'' is a numeric literal, that operand is implicitly compared to the $. variable, the current line number. Examples:

As a scalar operator:

    if (101 .. 200) { print; }  # print 2nd hundred lines
    next line if (1 .. /^$/);   # skip header lines
    s/^/> / if (/^$/ .. eof()); # quote body

As a list operator:

    for (101 .. 200) { print; } # print $_ 100 times
    @foo = @foo[0 .. $#foo];    # an expensive no-op
    @foo = @foo[$#foo-4 .. $#foo];      # slice last 5 items

The range operator (in a list context) makes use of the magical auto-increment algorithm if the operands are strings. You can say

    @alphabet = ('A' .. 'Z');

to get all the letters of the alphabet, or

    $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];

to get a hexadecimal digit, or

    @z2 = ('01' .. '31');  print $z2[$mday];

to get dates with leading zeros. If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified.

Source: Perl operators and precedence
Copyright: Larry Wall, et al.
Next: Conditional Operator

Previous: C-style Logical Or

(Corrections, notes, and links courtesy of RocketAware.com)

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