Arrays Summary (The GNU Awk User’s Guide)

8.7 Summary

Standard awk provides one-dimensional associative arrays (arrays indexed by string values). All arrays are associative; numeric indices are converted automatically to strings.
Array elements are referenced as array[indx]. Referencing an element creates it if it did not exist previously.
The proper way to see if an array has an element with a given index is to use the in operator: ‘indx in array’.
Use ‘for (indx in array) …’ to scan through all the individual elements of an array. In the body of the loop, indx takes on the value of each element’s index in turn.
The order in which a ‘for (indx in array)’ loop traverses an array is undefined in POSIX awk and varies among implementations. gawk lets you control the order by assigning special predefined values to PROCINFO["sorted_in"].
Use ‘delete array[indx]’ to delete an individual element. To delete all of the elements in an array, use ‘delete array’. This latter feature has been a common extension for many years and is now standard, but may not be supported by all commercial versions of awk.
Standard awk simulates multidimensional arrays by separating subscript values with commas. The values are concatenated into a single string, separated by the value of SUBSEP. The fact that such a subscript was created in this way is not retained; thus, changing SUBSEP may have unexpected consequences. You can use ‘(sub1, sub2, …) in array’ to see if such a multidimensional subscript exists in array.
gawk provides true arrays of arrays. You use a separate set of square brackets for each dimension in such an array: data[row][col], for example. Array elements may thus be either scalar values (number or string) or other arrays.
Use the isarray() built-in function to determine if an array element is itself a subarray.