GAWK: Effective AWK Programming

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124 GAWK: Effective AWK Programmingdelete arrayThis ability is a gawk extension; it is not available in compatibility mode (see Section 11.2[Command-Line Options], page 177).Using this version of the delete statement is about three times more efficient than theequivalent loop that deletes each element one at a time.The following statement provides a portable but nonobvious way to clear out an array: 1split("", array)The split function (see Section 8.1.3 [String-Manipulation Functions], page 132) clearsout the target array first. This call asks it to split apart the null string. Because there isno data to split out, the function simply clears the array and then returns.Caution: Deleting an array does not change its type; you cannot delete an array andthen use the array’s name as a scalar (i.e., a regular variable). For example, the followingdoes not work:a[1] = 3; delete a; a = 37.7 Using Numbers to Subscript ArraysAn important aspect about arrays to remember is that array subscripts are always strings.When a numeric value is used as a subscript, it is converted to a string value before beingused for subscripting (see Section 5.4 [Conversion of Strings and Numbers], page 79). Thismeans that the value of the built-in variable CONVFMT can affect how your program accesseselements of an array. For example:xyz = 12.153data[xyz] = 1CONVFMT = "%2.2f"if (xyz in data)printf "%s is in data\n", xyzelseprintf "%s is not in data\n", xyzThis prints ‘12.15 is not in data’. The first statement gives xyz a numeric value. Assigningto data[xyz] subscripts data with the string value "12.153" (using the defaultconversion value of CONVFMT, "%.6g"). Thus, the array element data["12.153"] is assignedthe value one. The program then changes the value of CONVFMT. The test ‘(xyz indata)’ generates a new string value from xyz—this time "12.15"—because the value ofCONVFMT only allows two significant digits. This test fails, since "12.15" is a different stringfrom "12.153".According to the rules for conversions (see Section 5.4 [Conversion of Strings and Numbers],page 79), integer values are always converted to strings as integers, no matter whatthe value of CONVFMT may happen to be. So the usual case of the following works:for (i = 1; i
Chapter 7: Arrays in awk 125Hexadecimal Numbers], page 75) are converted internally into numbers, and their originalform is forgotten. This means, for example, that array[17], array[021], and array[0x11]all refer to the same element!As with many things in awk, the majority of the time things work as one would expectthem to. But it is useful to have a precise knowledge of the actual rules which sometimescan have a subtle effect on your programs.7.8 Using Uninitialized Variables as SubscriptsSuppose it’s necessary to write a program to print the input data in reverse order.reasonable attempt to do so (with some test data) might look like this:$ echo ’line 1> line 2> line 3’ | awk ’{ l[lines] = $0; ++lines }> END {> for (i = lines-1; i >= 0; --i)> print l[i]> }’⊣ line 3⊣ line 2Unfortunately, the very first line of input data did not come out in the output!At first glance, this program should have worked. The variable lines is uninitialized,and uninitialized variables have the numeric value zero. So, awk should have printed thevalue of l[0].The issue here is that subscripts for awk arrays are always strings. Uninitialized variables,when used as strings, have the value "", not zero. Thus, ‘line 1’ ends up stored in l[""].The following version of the program works correctly:{ l[lines++] = $0 }END {for (i = lines - 1; i >= 0; --i)print l[i]}Here, the ‘++’ forces lines to be numeric, thus making the “old value” numeric zero.This is then converted to "0" as the array subscript.Even though it is somewhat unusual, the null string ("") is a valid array subscript.gawk warns about the use of the null string as a subscript if ‘--lint’ is provided on thecommand line (see Section 11.2 [Command-Line Options], page 177).7.9 Multidimensional ArraysA multidimensional array is an array in which an element is identified by a sequence ofindices instead of a single index. For example, a two-dimensional array requires two indices.The usual way (in most languages, including awk) to refer to an element of a two-dimensionalarray named grid is with grid[x,y].Multidimensional arrays are supported in awk through concatenation of indices into onestring. awk converts the indices into strings (see Section 5.4 [Conversion of Strings andA
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