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It seemed like a good idea at the time.
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
.32
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.
It happens that BWK
awk
, gawk
, and mawk
all “get it right,”
but you should not rely on this.
Next: Assignment Ops, Previous: Arithmetic Ops, Up: All Operators [Contents][Index]