GAWK: Effective AWK Programming

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126 GAWK: Effective AWK ProgrammingNumbers], page 79) and concatenates them together, with a separator between them. Thiscreates a single string that describes the values of the separate indices. The combined stringis used as a single index into an ordinary, one-dimensional array. The separator used is thevalue of the built-in variable SUBSEP.For example, suppose we evaluate the expression ‘foo[5,12] = "value"’ when the valueof SUBSEP is "@". The numbers 5 and 12 are converted to strings and concatenated with an‘@’ between them, yielding "5@12"; thus, the array element foo["5@12"] is set to "value".Once the element’s value is stored, awk has no record of whether it was stored with a singleindex or a sequence of indices. The two expressions ‘foo[5,12]’ and ‘foo[5 SUBSEP 12]’are always equivalent.The default value of SUBSEP is the string "\034", which contains a nonprinting characterthat is unlikely to appear in an awk program or in most input data. The usefulness ofchoosing an unlikely character comes from the fact that index values that contain a stringmatching SUBSEP can lead to combined strings that are ambiguous. Suppose that SUBSEPis "@"; then ‘foo["a@b", "c"]’ and ‘foo["a", "b@c"]’ are indistinguishable because bothare actually stored as ‘foo["a@b@c"]’.To test whether a particular index sequence exists in a multidimensional array, use thesame operator (‘in’) that is used for single dimensional arrays. Write the whole sequenceof indices in parentheses, separated by commas, as the left operand:(subscript1, subscript2, ...) in arrayThe following example treats its input as a two-dimensional array of fields; it rotatesthis array 90 degrees clockwise and prints the result. It assumes that all lines have the samenumber of elements:{}if (max_nf < NF)max_nf = NFmax_nr = NRfor (x = 1; x
Chapter 7: Arrays in awk 1274 3 2 15 4 3 26 5 4 31 6 5 42 1 6 53 2 1 67.10 Scanning Multidimensional ArraysThere is no special for statement for scanning a “multidimensional” array. There cannotbe one, because, in truth, there are no multidimensional arrays or elements—there is onlya multidimensional way of accessing an array.However, if your program has an array that is always accessed as multidimensional, youcan get the effect of scanning it by combining the scanning for statement (see Section 7.5[Scanning All Elements of an Array], page 122) with the built-in split function (seeSection 8.1.3 [String-Manipulation Functions], page 132). It works in the following manner:for (combined in array) {split(combined, separate, SUBSEP)...}This sets the variable combined to each concatenated combined index in the array, andsplits it into the individual indices by breaking it apart where the value of SUBSEP appears.The individual indices then become the elements of the array separate.Thus, if a value is previously stored in array[1, "foo"]; then an element with index"1\034foo" exists in array. (Recall that the default value of SUBSEP is the character withcode 034.) Sooner or later, the for statement finds that index and does an iteration withthe variable combined set to "1\034foo". Then the split function is called as follows:split("1\034foo", separate, "\034")The result is to set separate[1] to "1" and separate[2] to "foo". Presto! The originalsequence of separate indices is recovered.7.11 Sorting Array Values and Indices with gawkThe order in which an array is scanned with a ‘for (i in array)’ loop is essentially arbitrary.In most awk implementations, sorting an array requires writing a sort function.While this can be educational for exploring different sorting algorithms, usually that’s notthe point of the program. gawk provides the built-in asort and asorti functions (seeSection 8.1.3 [String-Manipulation Functions], page 132) for sorting arrays. For example:populate the array datan = asort(data)for (i = 1; i
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GAWK: Effective AWK Programming

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