Perl Puzzles Key(standard)

Perl Puzzles Key

This is the key for the Perl puzzles. Corrections welcome.

Answers

1. credit for noting that the two foo's are unrelated.

2. a b

3. c and 3

4. $a is set to the eight character string consisting of foo='\' followed by a newline. $b is set to the eight character string $c='\'\n.

5. credit for noting that empty line will be printed for empty lines and those consisting solely of the character 0 (zero).

6. credit for noting that one outcome is that the Perl interpreter may crash if foo doesn't give consistent answers.

   The crash possibility was the documented behavior prior to 5.005, but apparently has been at least partially addressed in 5.005. The man page perl5005delta(1) describes sort as now "resistant to inconsistent comparison functions". Strangely, it doesn't describe it as "immune", so this bug may still exist in some form. (Thanks to AP for pointing this out.)

7. It sets the initial index of arrays.

8. $x is set to the number 6. $y is set to the string sort.

9. prints 36

10. nothing, it's a syntax error. extra credit for noting that a fix is $mt = (stat($file))[8];

11. credit for noting that it depends on whether @foo exists, and if it does, depends on Perl's complex algorithm or "guess". major extra credit if you actually know the algorithm.

12. credit for noting that it depends on whether somefunc() is evaluated in a list or scalar context.

13. credit for noting that it might call handler immediately, depending on how handler is currently defined.

14. credit for noting that it depends on whether x is currently defined as a sub, whether it was defined with a prototype and which prototype was used.

15. It reads all lines of the standard input and replaces the second element of @foo with the first line read, discarding the rest of the lines read.

16. credit for noting that the last doesn't exit the current do/while loop, but rather will exit an enclosing loop (if present) or cause an error.

17. credit for noting that "exists" will be printed if the key merely exists, whereas the key must have a defined value in the hash in order for "defined" to be printed.

18. It'll read the first ten lines and never print anything (unless $_ has been defined elsewhere).

19. It will assign to $f string fooa if a file with that name exists in the current directory, or else it will assign string foob if a file with that name exists in the current directory, or else it will assign string foor if a file with that name exists in the current directory, or else it will assign the empty string.

20. foo-11-1 or 1-1foo-1

21. a floating point value corresponding to zero minus the square root of two.

22. This was supposed to show you how the left "0" would get interpreted as a string and the right "0" interpreted as a number, but perl 5.8.0 (at least) seems to interpret both as numbers, in apparent contradiction to the perlop man page. Major extra credit if you knew that the documentation and implementation of this feature is out of sync.

    [The answer was supposed to be "credit for noting that the second expression is a number whose binary representation is all ones (the one's complement of zero, etc.), and for noting that the first expression is a one-character string where the ASCII value for the character is the ASCII value for the zero character with its bits flipped."]

    (Thanks to Bob Miller for help with this.)

    (The code given in the question doesn't actually do what I thought it did. I actually don't understand what it's doing. If you know, I'd love to hear an explanation.)

23. 55 5 5

24. credit for noting that length & $print_blanks fails and is not equivalent to, say, length() & $print_blanks.

25. It prints the first line only.

26. It sets $sos to the empty string.

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© 2001 Mike Coleman
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