manpagez: man pages & more
info gawk
Home | html | info | man

gawk: Concatenation

 
 6.2.2 String Concatenation
 --------------------------
 
      It seemed like a good idea at the time.
                          -- _Brian Kernighan_
 
    There is only one string operation: concatenation.  It does not have
 a specific operator to represent it.  Instead, concatenation is
 performed by writing expressions next to one another, with no operator.
 For example:
 
      $ awk '{ print "Field number one: " $1 }' mail-list
      -| Field number one: Amelia
      -| Field number one: Anthony
      ...
 
    Without the space in the string constant after the ':', the line runs
 together.  For example:
 
      $ awk '{ print "Field number one:" $1 }' mail-list
      -| Field number one:Amelia
      -| Field number one:Anthony
      ...
 
    Because string concatenation does not have an explicit operator, it
 is often necessary to ensure that it happens at the right time by using
 parentheses to enclose the items to concatenate.  For example, you might
 expect that the following code fragment concatenates 'file' and 'name':
 
      file = "file"
      name = "name"
      print "something meaningful" > file name
 
 This produces a syntax error with some versions of Unix 'awk'.(1)  It is
 necessary to use the following:
 
      print "something meaningful" > (file name)
 
    Parentheses should be used around concatenation in all but the most
 common contexts, such as on the righthand side of '='.  Be careful about
 the kinds of expressions used in string concatenation.  In particular,
 the order of evaluation of expressions used for concatenation is
 undefined in the 'awk' language.  Consider this example:
 
      BEGIN {
          a = "don't"
          print (a " " (a = "panic"))
      }
 
 It is not defined whether the second assignment to 'a' happens before or
 after the value of 'a' is retrieved for producing the concatenated
 value.  The result could be either 'don't panic', or 'panic panic'.
 
    The precedence of concatenation, when mixed with other operators, is
 often counter-intuitive.  Consider this example:
 
      $ awk 'BEGIN { print -12 " " -24 }'
      -| -12-24
 
    This "obviously" is concatenating -12, a space, and -24.  But where
 did the space disappear to?  The answer lies in the combination of
 operator precedences and 'awk''s automatic conversion rules.  To get the
 desired result, write the program this way:
 
      $ awk 'BEGIN { print -12 " " (-24) }'
      -| -12 -24
 
    This forces 'awk' to treat the '-' on the '-24' as unary.  Otherwise,
 it's parsed as follows:
 
          -12 ('" "' - 24)
      => -12 (0 - 24)
      => -12 (-24)
      => -12-24
 
    As mentioned earlier, when mixing concatenation with other operators,
 _parenthesize_.  Otherwise, you're never quite sure what you'll get.
 
    ---------- Footnotes ----------
 
    (1) It happens that BWK 'awk', 'gawk', and 'mawk' all "get it right,"
 but you should not rely on this.
 
© manpagez.com 2000-2018
Individual documents may contain additional copyright information.