Awk array iteration for multi-dimensional arrays

AWK fakes multidimensional arrays by concatenating the indices with the character held in the SUBSEP variable (0x1c). You can iterate through a two-dimensional array using split like this (based on an example in the info gawk file):

awk 'BEGIN { OFS=","; array[1,2]=3; array[2,3]=5; array[3,4]=8;   for (comb in array) {split(comb,sep,SUBSEP);    print sep[1], sep[2], array[sep[1],sep[2]]}}'

Output:

2,3,53,4,81,2,3

You can, however, iterate over a numerically indexed array using nested for loops:

for (i = 1; i <= width; i++)    for (j = 1; j < = height; j++)        print array[i, j]

Another noteworthy bit of information from the GAWK manual:

To test whether a particular index sequence exists in a multidimensional array, use the same operator (in) that is used for single dimensional arrays. Write the whole sequence of indices in parentheses, separated by commas, as the left operand:

     (subscript1, subscript2, ...) in array

Gawk 4 adds arrays of arrays. From that link:

for (i in array) {    if (isarray(array[i])) {        for (j in array[i]) {            print array[i][j]        }    }    else        print array[i]}

Also see Traversing Arrays of Arrays for information about the following function which walks an arbitrarily dimensioned array of arrays, including jagged ones:

function walk_array(arr, name,      i){    for (i in arr) {        if (isarray(arr[i]))            walk_array(arr[i], (name "[" i "]"))        else            printf("%s[%s] = %s\n", name, i, arr[i])    }} 

No, the syntax

for(index1 in arr2) for(index2 in arr2) {    print arr2[index1][index2];}

won't work. Awk doesn't truly support multi-dimensional arrays. What it does, if you do something like

x[1,2] = 5;

is to concatenate the two indexes (1 & 2) to make a string, separated by the value of the SUBSEP variable. If this is equal to "*", then you'd have the same effect as

x["1*2"] = 5;

The default value of SUBSEP is a non-printing character, corresponding to Ctrl+\. You can see this with the following script:

BEGIN {    x[1,2]=5;    x[2,4]=7;    for (ix in x) {        print ix;    }}

Running this gives:

% awk -f scriptfile | cat -v1^\22^\4

So, in answer to your question - how to iterate a multi-dimensional array - just use a single for(a in b) loop, but you may need some extra work to split up a into its x and y parts.

The current versions of gawk (the gnu awk, default inlinux, and possible to install everywhere you want), has real multidimensional arrays.

for(b in a)   for(c in a[b])      print a[b][c], c , b

See also function isarray()