10 Arrays in awk

An array is a table of values, called elements. The elements of an array are distinguished by their indices. Indices may be either numbers or strings. Each array has a name, which looks like a variable name, but must not be in use as a variable name in the same awk program.

10.1 Introduction to Arrays

The awk language has one-dimensional arrays for storing groups of related strings or numbers.

Every awk array must have a name. Array names have the same syntax as variable names; any valid variable name would also be a valid array name. But you cannot use one name in both ways (as an array and as a variable) in one awk program.

Arrays in awk super cially resemble arrays in other programming languages; but there are fundamental di erences. In awk, you don't need to specify the size of an array before you start to use it. Additionally, any number or string in awk may be used as an array index.

In most other languages, you have to declare an array and specify how many elements or components it contains. In such languages, the declaration causes a contiguous block of memory to be allocated for that many elements. An index in the array must be a positive integer; for example, the index 0 speci es the rst element in the array, which is actually stored at the beginning of the block of memory. Index 1 speci es the second element, which is stored in memory right after therst element, and so on. It is impossible to add more elements to the array, because it has room for only as many elements as you declared.

A contiguous array of four elements might look like this, conceptually, if the element values are

8, "foo", "" and 30:

Only the values are stored; the indices are implicit from the order of the values. 8 is the value at index 0, because 8 appears in the position with 0 elements before it.

Arrays in awk are di erent: they are associative. This means that each array is a collection of pairs: an index, and its corresponding array element value:

We have shown the pairs in jumbled order because their order is irrelevant.

One advantage of an associative array is that new pairs can be added at any time. For example, suppose we add to the above array a tenth element whose value is "number ten". The result is this:

Now the array is sparse (i.e., some indices are missing): it has elements 1{4 and 10, but doesn't have elements 5, 6, 7, 8, or 9.

Another consequence of associative arrays is that the indices don't have to be positive integers. Any number, or even a string, can be an index. For example, here is an array which translates words from English into French:

Here we decided to translate the number 1 in both spelled-out and numeric form|thus illustrating that a single array can have both numbers and strings as indices.

When awk creates an array for you, e.g., with the split built-in function, that array's indices are consecutive integers starting at 1. (See Section 11.3 [Built-in Functions for String Manipulation], page 90.)

10.2 Referring to an Array Element

The principal way of using an array is to refer to one of its elements. An array reference is an expression which looks like this:

array[index]

Here, array is the name of an array. The expression index is the index of the element of the array that you want.

The value of the array reference is the current value of that array element. For example, foo[4.3] is an expression for the element of array foo at index 4.3.

If you refer to an array element that has no recorded value, the value of the reference is "", the null string. This includes elements to which you have not assigned any value, and elements that have been deleted (see Section 10.6 [The delete Statement], page 85). Such a reference automatically creates that array element, with the null string as its value. (In some cases, this is unfortunate, because it might waste memory inside awk).

You can nd out if an element exists in an array at a certain index with the expression: