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Built-in functions are functions that are always available for
your awk program to call. This chapter defines all the built-in
functions in awk; some of them are mentioned in other sections,
but they are summarized here for your convenience. (You can also define
new functions yourself. See section User-defined Functions.)
To call a built-in function, write the name of the function followed
by arguments in parentheses. For example, atan2(y + z, 1)
is a call to the function atan2, with two arguments.
Whitespace is ignored between the built-in function name and the open-parenthesis, but we recommend that you avoid using whitespace there. User-defined functions do not permit whitespace in this way, and you will find it easier to avoid mistakes by following a simple convention which always works: no whitespace after a function name.
Each built-in function accepts a certain number of arguments. In most cases, any extra arguments given to built-in functions are ignored. The defaults for omitted arguments vary from function to function and are described under the individual functions.
When a function is called, expressions that create the function's actual parameters are evaluated completely before the function call is performed. For example, in the code fragment:
i = 4 j = sqrt(i++)
the variable i is set to 5 before sqrt is called
with a value of 4 for its actual parameter.
Here is a full list of built-in functions that work with numbers:
int(x)
For example, int(3) is 3, int(3.9) is 3, int(-3.9)
is -3, and int(-3) is -3 as well.
sqrt(x)
sqrt(4) is 2.
exp(x)
log(x)
sin(x)
cos(x)
atan2(y, x)
y / x, with the
quotient understood in radians.
rand()
rand are
uniformly-distributed between 0 and 1. The value is never 0 and never
1.
Often you want random integers instead. Here is a user-defined function you can use to obtain a random nonnegative integer less than n:
function randint(n) {
return int(n * rand())
}
The multiplication produces a random real number greater than 0 and less
than n. We then make it an integer (using int) between 0
and n - 1.
Here is an example where a similar function is used to produce random integers between 1 and n:
awk '
# Function to roll a simulated die.
function roll(n) { return 1 + int(rand() * n) }
# Roll 3 six-sided dice and print total number of points.
{
printf("%d points\n", roll(6)+roll(6)+roll(6))
}'
Note: rand starts generating numbers from the same
point, or seed, each time you run awk. This means that
a program will produce the same results each time you run it.
The numbers are random within one awk run, but predictable
from run to run. This is convenient for debugging, but if you want
a program to do different things each time it is used, you must change
the seed to a value that will be different in each run. To do this,
use srand.
srand(x)
srand sets the starting point, or seed,
for generating random numbers to the value x.
Each seed value leads to a particular sequence of "random" numbers. Thus, if you set the seed to the same value a second time, you will get the same sequence of "random" numbers again.
If you omit the argument x, as in srand(), then the current
date and time of day are used for a seed. This is the way to get random
numbers that are truly unpredictable.
The return value of srand is the previous seed. This makes it
easy to keep track of the seeds for use in consistently reproducing
sequences of random numbers.
time()
time (not in all versions of awk) returns the
current time in seconds since January 1, 1970.
ctime(then)
ctime (not in all versions of awk) takes an numeric
argument in seconds and returns a string representing the corresponding date,
suitable for printing or further processing.
The functions in this section look at the text of one or more strings.
index(in, find)
awk 'BEGIN { print index("peanut", "an") }'
prints `3'. If find is not found, index returns 0.
length(string)
length("abcde") is 5. By
contrast, length(15 * 35) works out to 3. How? Well, 15 * 35 =
525, and 525 is then converted to the string `"525"', which has
three characters.
If no argument is supplied, length returns the length of $0.
match(string, regexp)
match function searches the string, string, for the
longest, leftmost substring matched by the regular expression,
regexp. It returns the character position, or index, of
where that substring begins (1, if it starts at the beginning of
string). If no match if found, it returns 0.
The match function sets the built-in variable RSTART to
the index. It also sets the built-in variable RLENGTH to the
length of the matched substring. If no match is found, RSTART
is set to 0, and RLENGTH to -1.
For example:
awk '{
if ($1 == "FIND")
regex = $2
else {
where = match($0, regex)
if (where)
print "Match of", regex, "found at", where, "in", $0
}
}'
This program looks for lines that match the regular expression stored in
the variable regex. This regular expression can be changed. If the
first word on a line is `FIND', regex is changed to be the
second word on that line. Therefore, given:
FIND fo*bar My program was a foobar But none of it would doobar FIND Melvin JF+KM This line is property of The Reality Engineering Co. This file created by Melvin.
awk prints:
Match of fo*bar found at 18 in My program was a foobar Match of Melvin found at 26 in This file created by Melvin.
split(string, array, fieldsep)
array[1], the second piece in array[2], and so
forth. The string value of the third argument, fieldsep, is used
as a regexp to search for to find the places to split string. If
the fieldsep is omitted, the value of FS is used.
split returns the number of elements created.
The split function, then, splits strings into pieces in a
manner similar to the way input lines are split into fields. For example:
split("auto-da-fe", a, "-")
splits the string `auto-da-fe' into three fields using `-' as the
separator. It sets the contents of the array a as follows:
a[1] = "auto" a[2] = "da" a[3] = "fe"
The value returned by this call to split is 3.
sprintf(format, expression1,...)
printf would
have printed out with the same arguments (see section Using printf Statements For Fancier Printing). For
example:
sprintf("pi = %.2f (approx.)", 22/7)
returns the string "pi = 3.14 (approx.)".
sub(regexp, replacement, target)
sub function alters the value of target.
It searches this value, which should be a string, for the
leftmost substring matched by the regular expression, regexp,
extending this match as far as possible. Then the entire string is
changed by replacing the matched text with replacement.
The modified string becomes the new value of target.
This function is peculiar because target is not simply
used to compute a value, and not just any expression will do: it
must be a variable, field or array reference, so that sub can
store a modified value there. If this argument is omitted, then the
default is to use and alter $0.
For example:
str = "water, water, everywhere" sub(/at/, "ith", str)
sets str to "wither, water, everywhere", by replacing the
leftmost, longest occurrence of `at' with `ith'.
The sub function returns the number of substitutions made (either
one or zero).
If the special character `&' appears in replacement, it stands for the precise substring that was matched by regexp. (If the regexp can match more than one string, then this precise substring may vary.) For example:
awk '{ sub(/candidate/, "& and his wife"); print }'
changes the first occurrence of `candidate' to `candidate and his wife' on each input line.
The effect of this special character can be turned off by putting a backslash before it in the string. As usual, to insert one backslash in the string, you must write two backslashes. Therefore, write `\\&' in a string constant to include a literal `&' in the replacement. For example, here is how to replace the first `|' on each line with an `&':
awk '{ sub(/\|/, "\\&"); print }'
Note: as mentioned above, the third argument to sub must
be an lvalue. Some versions of awk allow the third argument to
be an expression which is not an lvalue. In such a case, sub
would still search for the pattern and return 0 or 1, but the result of
the substitution (if any) would be thrown away because there is no place
to put it. Such versions of awk accept expressions like
this:
sub(/USA/, "United States", "the USA and Canada")
But that is considered erroneous in gawk.
gsub(regexp, replacement, target)
sub function, except gsub replaces
all of the longest, leftmost, nonoverlapping matching
substrings it can find. The `g' in gsub stands for
"global", which means replace everywhere. For example:
awk '{ gsub(/Britain/, "United Kingdom"); print }'
replaces all occurrences of the string `Britain' with `United Kingdom' for all input records.
The gsub function returns the number of substitutions made. If
the variable to be searched and altered, target, is
omitted, then the entire input record, $0, is used.
As in sub, the characters `&' and `\' are special, and
the third argument must be an lvalue.
substr(string, start, length)
substr("washington", 5, 3) returns "ing".
If length is not present, this function returns the whole suffix of
string that begins at character number start. For example,
substr("washington", 5) returns "ington".
tolower(string)
tolower("MiXeD cAsE 123") returns "mixed case 123".
toupper(string)
toupper("MiXeD cAsE 123") returns "MIXED CASE 123".
close(filename)
See section Closing Input Files and Pipes, regarding closing input files and pipes. See section Closing Output Files and Pipes, regarding closing output files and pipes.
system(command)
awk program. The system function
executes the command given by the string command. It returns, as
its value, the status returned by the command that was executed.
For example, if the following fragment of code is put in your awk
program:
END {
system("mail -s 'awk run done' operator < /dev/null")
}
the system operator will be sent mail when the awk program
finishes processing input and begins its end-of-input processing.
Note that much the same result can be obtained by redirecting
print or printf into a pipe. However, if your awk
program is interactive, system is useful for cranking up large
self-contained programs, such as a shell or an editor.
Some operating systems cannot implement the system function.
system causes a fatal error if it is not supported.
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