Awk array iteration for multi-dimensional arrays

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 -v
1^\2
2^\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.

I'll provide an example of how I use this in my work processing query data. Suppose you have an extract file full of transactions by product category and customer id:

customer_id  category  sales
1111         parts     100.01
1212         parts       5.20
2211         screws      1.33
...etc...

Its easy to use awk to count total distinct customers with a purchase:

awk 'NR>1 {a[$1]++} END {for (i in a) total++; print "customers: " total}' \ 
datafile.txt

However, computing the number of distinct customers with a purchase in each category suggests a two dimensional array:

awk 'NR>1 {a[$2,$1]++} 
      END {for (i in a) {split(i,arr,SUBSEP); custs[arr[1]]++}
           for (k in custs) printf "category: %s customers:%d\n", k, custs[k]}' \
datafile.txt

The increment of custs[arr[1]]++ works because each category/customer_id pair is unique as an index to the associative array used by awk.

In truth, I use gnu awk which is faster and can do array[i][j] as D. Williamson mentioned. But I wanted to be sure I could do this in standard awk.

The current versions of gawk (the gnu awk, default in linux, 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()

awk(1) was originally designed -- in part -- to be teaching tool for the C language, and multi-dimensional arrays have been in both C and awk(1) pretty much forever. as such POSIX IEEE 1003.2 standardized them.

To explore the syntax and semantics, if you create the following file called "test.awk":

BEGIN {
  KEY["a"]="a";
  KEY["b"]="b";
  KEY["c"]="c";
  MULTI["a"]["test_a"]="date a";
  MULTI["b"]["test_b"]="dbte b";
  MULTI["c"]["test_c"]="dcte c";
}
END {
  for(k in KEY) {
    kk="test_" k ;
    print MULTI[k][kk]
  }
  for(q in MULTI) {
    print q
  }
  for(p in MULTI) {
    for( pp in MULTI[p] ) {
      print MULTI[p][pp]
    }
  }
}

and run it with this command:

awk -f test.awk /dev/null

you will get the following output:

date a
dbte b
dcte c
a
b
c
date a
dbte b
dcte c

at least on Linux Mint 18 Cinnamon 64-bit 4.4.0-21-generic #37-Ubuntu SMP