nose@nbires.UUCP (Steve Dunn) (04/01/86)
And now for another issue of "Can you explain this" from
whinin' Steve Dunn.
This:
set a = ''
set a = ($a '')
echo $#a
Yields 1
But this:
set a = ('' '')
echo $#a
Yields 2
This curious behavior has not actually caused me any trouble but I
am a bit puzzled
-Steve "Grep-face" Dunnjsdy@hadron.UUCP (04/03/86)
In article <684@nbires.UUCP> nose@nbires.UUCP (Steve Dunn) writes: >set a = '' >set a = ($a '') >echo $#a > >Yields 1 > >set a = ('' '') >echo $#a > >Yields 2 set a = '' set a = ("$a" '') echo $#a 2 Note that $a, where a is empty or containing all manner of white space, gets fed back into the parser just as it appears: as no characters or white space. To be taken seriously, it must appear in double quotes (which does sometimes cause problems). Consider: if ($a == "") ... if ("" == "$a") ... I use the latter form, partly because $a might be -x or something (!). The former form could all too easily turn into if ( == "") if what you r e a l l y wanted to test was true. Testing this, I am enthralled to find that if ( == "") and even if ( == ) both test out true in csh!!! I guess this is because of the parsing that occurs at different times, as I mentioned in another letter. This gives a syntax error in any sh that I've ever used: Thompson, Mashey, Bourne, or Korn. -- Joe Yao hadron!jsdy@seismo.{CSS.GOV,ARPA,UUCP}
jwp@sdchem.UUCP (John Pierce) (04/03/86)
In article <684@nbires.UUCP> nose@nbires.UUCP (Steve Dunn) writes: > set a = '' > set a = ($a '') set a = ('' '') > echo $#a echo $#a > > Yields 1 Yields 2 and asks for an explanation.... In the first case $a is expanded to a null string before the assignment is done, and, therefore, isn't seen as a string at the time the assignment is made; there is no such thing as an unquoted null string (this is implied, though not stated explicitly in exactly those words by csh(1), page 1, "Lexical Structure"). Note that set a = '' set a = ( "$a" '' ) echo $#a yields 2 since the null string that is $a is quoted and is, therefore, a word. John Pierce, Chemistry, UC San Diego jwp%chem@ucsd.edu sdcsvax!sdchem!jwp
karl@cbrma.UUCP (Karl Kleinpaste) (04/03/86)
In article <684@nbires.UUCP> nose@nbires.UUCP (Steve Dunn) writes: >This: >set a = '' >set a = ($a '') >echo $#a >Yields 1 > >But this: > >set a = ('' '') >echo $#a >Yields 2 > >This curious behavior has not actually caused me any trouble but I >am a bit puzzled It's because of the order in which things are being substituted in the input line. That is, what is happening is that set a = ($a '') becomes set a = ( '') when $a's empty value is stuffed into the line. Then the interpretation of the remainder gives `a' the single value ''. As a counterpoint, try set a = '' set a = ("$a" '') echo $#a which yields 2. The reason is that you've stipulated extra quotings, which preserve values for longer than the simple replacement of $a in the input line with its null value. The use of double quotes causes $a to be interpreted within them, subsituting its null string, while leaving that null string quoted, so that two null strings result. -- Karl Kleinpaste
chris@umcp-cs.UUCP (Chris Torek) (04/04/86)
In article <684@nbires.UUCP> nose@nbires.UUCP writes: >set a = '' >set a = ($a '') >echo $#a > >Yields 1 > >set a = ('' '') >echo $#a > >Yields 2 It is all a matter of quoting: set a = ''; set a = ("$a" ''); echo $#a gives `2'. Your first example gives `1' because the C shell does history substitution (none), then variable expansion---now the command is set a = ( '') ---then alias expansion (none), and finally executes it. With the double quotes, variable expansion yeilds set a = ("" '') which is of course two words. It is often important to quote variables being expanded. For instance: set a = '*' set a = ($a) echo $a prints the same thing as `echo *'. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1415) UUCP: seismo!umcp-cs!chris CSNet: chris@umcp-cs ARPA: chris@mimsy.umd.edu