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Text File | 1995-07-25 | 17KB | 463 lines

PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) NNNNAAAAMMMMEEEE perltrap - Perl traps for the unwary DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN The biggest trap of all is forgetting to use the ----wwww switch; see the _p_e_r_l_r_u_n manpage. Making your entire program runnable under use strict; can help make your program more bullet-proof, but sometimes it's too annoying for quick throw-away programs. AAAAwwwwkkkk TTTTrrrraaaappppssss Accustomed aaaawwwwkkkk users should take special note of the following: o+ The English module, loaded via use English; allows you to refer to special variables (like $RS) as though they were in aaaawwwwkkkk; see the _p_e_r_l_v_a_r manpage for details. o+ Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter. o+ Curly brackets are required on ifs and whiles. o+ Variables begin with "$" or "@" in Perl. o+ Arrays index from 0. Likewise string positions in _s_u_b_s_t_r() and _i_n_d_e_x(). o+ You have to decide whether your array has numeric or string indices. o+ Associative array values do not spring into existence upon mere reference. o+ You have to decide whether you want to use string or numeric comparisons. o+ Reading an input line does not split it for you. You get to split it yourself to an array. And _s_p_l_i_t() operator has different arguments. o+ The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is Page 1 (printed 6/30/95) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) the name of the program executed.) See the _p_e_r_l_v_a_r manpage. o+ $<_d_i_g_i_t> does not refer to fields--it refers to substrings matched by the last match pattern. o+ The _p_r_i_n_t() statement does not add field and record separators unless you set $, and $.. You can set $OFS and $ORS if you're using the English module. o+ You must open your files before you print to them. o+ The range operator is "..", not comma. The comma operator works as in C. o+ The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.) o+ The exponentiation operator is "**", not "^". "^" is the XOR operator, as in C. (You know, one could get the feeling that aaaawwwwkkkk is basically incompatible with C.) o+ The concatenation operator is ".", not the null string. (Using the null string would render /pat/ /pat/ unparsable, since the third slash would be interpreted as a division operator--the tokener is in fact slightly context sensitive for operators like "/", "?", and ">". And in fact, "." itself can be the beginning of a number.) o+ The next, exit, and continue keywords work differently. o+ The following variables work differently: Awk Perl ARGC $#ARGV or scalar @ARGV ARGV[0] $0 FILENAME $ARGV FNR $. - something FS (whatever you like) NF $#Fld, or some such NR $. OFMT $# OFS $, ORS $\ RLENGTH length($&) RS $/ RSTART length($`) SUBSEP $; Page 2 (printed 6/30/95) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) o+ You cannot set $RS to a pattern, only a string. o+ When in doubt, run the aaaawwwwkkkk construct through aaaa2222pppp and see what it gives you. CCCC TTTTrrrraaaappppssss Cerebral C programmers should take note of the following: o+ Curly brackets are required on if's and while's. o+ You must use elsif rather than else if. o+ The break and continue keywords from C become in Perl last and next, respectively. Unlike in C, these do _N_O_T work within a do { } while construct. o+ There's no switch statement. (But it's easy to build one on the fly.) o+ Variables begin with "$" or "@" in Perl. o+ _p_r_i_n_t_f() does not implement the "*" format for interpolating field widths, but it's trivial to use interpolation of double-quoted strings to achieve the same effect. o+ Comments begin with "#", not "/*". o+ You can't take the address of anything, although a similar operator in Perl 5 is the backslash, which creates a reference. o+ ARGV must be capitalized. o+ System calls such as _l_i_n_k(), _u_n_l_i_n_k(), _r_e_n_a_m_e(), etc. return nonzero for success, not 0. o+ Signal handlers deal with signal names, not numbers. Use kill -l to find their names on your system. SSSSeeeedddd TTTTrrrraaaappppssss Seasoned sssseeeedddd programmers should take note of the following: o+ Backreferences in substitutions use "$" rather than "\". o+ The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front. o+ The range operator is ..., rather than comma. Page 3 (printed 6/30/95) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) SSSShhhheeeellllllll TTTTrrrraaaappppssss Sharp shell programmers should take note of the following: o+ The backtick operator does variable interpretation without regard to the presence of single quotes in the command. o+ The backtick operator does no translation of the return value, unlike ccccsssshhhh. o+ Shells (especially ccccsssshhhh) do several levels of substitution on each command line. Perl does substitution only in certain constructs such as double quotes, backticks, angle brackets, and search patterns. o+ Shells interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for BEGIN blocks, which execute at compile time). o+ The arguments are available via @ARGV, not $1, $2, etc. o+ The environment is not automatically made available as separate scalar variables. PPPPeeeerrrrllll TTTTrrrraaaappppssss Practicing Perl Programmers should take note of the following: o+ Remember that many operations behave differently in a list context than they do in a scalar one. See the _p_e_r_l_d_a_t_a manpage for details. o+ Avoid barewords if you can, especially all lower-case ones. You can't tell just by looking at it whether a bareword is a function or a string. By using quotes on strings and parens on function calls, you won't ever get them confused. o+ You cannot discern from mere inspection which built-ins are unary operators (like _c_h_o_p() and _c_h_d_i_r()) and which are list operators (like _p_r_i_n_t() and _u_n_l_i_n_k()). (User- defined subroutines can oooonnnnllllyyyy be list operators, never unary ones.) See the _p_e_r_l_o_p manpage. o+ People have a hard type remembering that some functions default to $_, or @ARGV, or whatever, but that others which you might expect to do not. o+ Remember not to use "=" when you need "=~"; these two constructs are quite different: Page 4 (printed 6/30/95) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) $x = /foo/; $x =~ /foo/; o+ The do {} construct isn't a real loop that you can use loop control on. o+ Use _m_y() for local variables whenever you can get away with it (but see the _p_e_r_l_f_o_r_m manpage for where you can't). Using _l_o_c_a_l() actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping. PPPPeeeerrrrllll4444 TTTTrrrraaaappppssss Penitent Perl 4 Programmers should take note of the following incompatible changes that occurred between release 4 and release 5: o+ @ now always interpolates an array in double-quotish strings. Some programs may now need to use backslash to protect any @ that shouldn't interpolate. like subroutine calls if a subroutine by that name is defined before the compiler sees them. For example: o+ Barewords that used to look like strings to Perl will now look sub SeeYa { die "Hasta la vista, baby!" } $SIG{QUIT} = SeeYa; In Perl 4, that set the signal handler; in Perl 5, it actually calls the function! You may use the ----wwww switch to find such places. o+ Symbols starting with _ are no longer forced into package main, except for $_ itself (and @_, etc.). o+ s'$lhs'$rhs' now does no interpolation on either side. It used to interpolate $lhs but not $rhs. o+ The second and third arguments of _s_p_l_i_c_e() are now evaluated in scalar context (as the book says) rather than list context. o+ These are now semantic errors because of precedence: shift @list + 20; $n = keys %map + 20; Because if that were to work, then this couldn't: sleep $dormancy + 20; Page 5 (printed 6/30/95) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) o+ open FOO || die is now incorrect. You need parens around the filehandle. While temporarily supported, using such a construct will generate a non-fatal (but non-suppressible) warning. o+ The elements of argument lists for formats are now evaluated in list context. This means you can interpolate list values now. o+ You can't do a goto into a block that is optimized away. Darn. o+ It is no longer syntactically legal to use whitespace as the name of a variable, or as a delimiter for any kind of quote construct. Double darn. o+ The _c_a_l_l_e_r() function now returns a false value in a scalar context if there is no caller. This lets library files determine if they're being required. o+ m//g now attaches its state to the searched string rather than the regular expression. o+ reverse is no longer allowed as the name of a sort subroutine. o+ ttttaaaaiiiinnnnttttppppeeeerrrrllll is no longer a separate executable. There is now a ----TTTT switch to turn on tainting when it isn't turned on automatically. o+ Double-quoted strings may no longer end with an unescaped $ or @. o+ The archaic while/if BLOCK BLOCK syntax is no longer supported. o+ Negative array subscripts now count from the end of the array. o+ The comma operator in a scalar context is now guaranteed to give a scalar context to its arguments. o+ The ** operator now binds more tightly than unary minus. It was documented to work this way before, but didn't. o+ Setting $#array lower now discards array elements. o+ _d_e_l_e_t_e() is not guaranteed to return the old value for _t_i_e()d arrays, since this capability may be onerous for some modules to implement. Page 6 (printed 6/30/95) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLTTTTRRRRAAAAPPPP((((1111)))) o+ Some error messages will be different. o+ Some bugs may have been inadvertently removed. Page 7 (printed 6/30/95)