GAWK: Effective AWK Programming

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132 GAWK: Effective AWK Programmingsrand([x])a program generates the same results each time you run it. The numbers arerandom within one awk run but predictable from run to run. This is convenientfor debugging, but if you want a program to do different things each time it isused, you must change the seed to a value that is different in each run. To dothis, use srand.The function srand sets the starting point, or seed, for generating randomnumbers to the value x.Each seed value leads to a particular sequence of random numbers. 2 Thus, ifthe seed is set to the same value a second time, the same sequence of randomnumbers is produced again.Different awk implementations use different random-number generators internally.Don’t expect the same awk program to produce the same series of randomnumbers when executed by different versions of awk.If the argument x is omitted, as in ‘srand()’, then the current date and timeof day are used for a seed. This is the way to get random numbers that aretruly unpredictable.The return value of srand is the previous seed. This makes it easy to keep trackof the seeds in case you need to consistently reproduce sequences of randomnumbers.8.1.3 String-Manipulation FunctionsThe functions in this section look at or change the text of one or more strings. Optionalparameters are enclosed in square brackets ([ ]). Those functions that are specific to gawkare marked with a pound sign (‘#’):asort(source [, dest]) #asort is a gawk-specific extension, returning the number of elements in thearray source. The contents of source are sorted using gawk’s normal rules forcomparing values (in particular, IGNORECASE affects the sorting) and the indicesof the sorted values of source are replaced with sequential integers starting withone. If the optional array dest is specified, then source is duplicated into dest.dest is then sorted, leaving the indices of source unchanged. For example, ifthe contents of a are as follows:a["last"] = "de"a["first"] = "sac"a["middle"] = "cul"A call to asort:asort(a)results in the following contents of a:a[1] = "cul"2 Computer-generated random numbers really are not truly random. They are technically known as“pseudorandom.” This means that while the numbers in a sequence appear to be random, you can infact generate the same sequence of random numbers over and over again.
Chapter 8: Functions 133a[2] = "de"a[3] = "sac"The asort function is described in more detail in Section 7.11 [Sorting ArrayValues and Indices with gawk], page 127. asort is a gawk extension; it is notavailable in compatibility mode (see Section 11.2 [Command-Line Options],page 177).asorti(source [, dest]) #asorti is a gawk-specific extension, returning the number of elements in thearray source. It works similarly to asort, however, the indices are sorted,instead of the values. As array indices are always strings, the comparisonperformed is always a string comparison. (Here too, IGNORECASE affects thesorting.)The asorti function is described in more detail in Section 7.11 [Sorting ArrayValues and Indices with gawk], page 127. It was added in gawk 3.1.2. asortiis a gawk extension; it is not available in compatibility mode (see Section 11.2[Command-Line Options], page 177).index(in, find)This searches the string in for the first occurrence of the string find, and returnsthe position in characters where that occurrence begins in the string in.Consider the following example:$ awk ’BEGIN { print index("peanut", "an") }’⊣ 3If find is not found, index returns zero. (Remember that string indices in awkstart at one.)length([string])This returns the number of characters in string. If string is a number, thelength of the digit string representing that number is returned. For example,length("abcde") is 5. By contrast, length(15 * 35) works out to 3. In thisexample, 15 * 35 = 525, and 525 is then converted to the string "525", whichhas three characters.If no argument is supplied, length returns the length of $0.NOTE: In older versions of awk, the length function could be calledwithout any parentheses. Doing so is marked as “deprecated” inthe POSIX standard. This means that while a program can dothis, it is a feature that can eventually be removed from a futureversion of the standard. Therefore, for programs to be maximallyportable, always supply the parentheses.If length is called with a variable that has not been used, gawk forces thevariable to be a scalar. Other implementations of awk leave the variable withouta type. Consider:$ gawk ’BEGIN { print length(x) ; x[1] = 1 }’⊣ 0error gawk: fatal: attempt to use scalar ‘x’ as array
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