A multidimensional array is an array in which an element is identified
by a sequence of indices instead of a single index. For example, a
two-dimensional array requires two indices. The usual way (in many
languages, including awk) to refer to an element of a
two-dimensional array named grid is with
grid[x,y].
Multidimensional arrays are supported in awk through
concatenation of indices into one string.
awk converts the indices into strings
(see Conversion of Strings and Numbers) and
concatenates them together, with a separator between them. This creates
a single string that describes the values of the separate indices. The
combined string is used as a single index into an ordinary,
one-dimensional array. The separator used is the value of the built-in
variable SUBSEP.
For example, suppose we evaluate the expression ‘foo[5,12] = "value"’
when the value of SUBSEP is "@". The numbers 5 and 12 are
converted to strings and
concatenated with an ‘@’ between them, yielding "5@12"; thus,
the array element foo["5@12"] is set to "value".
Once the element’s value is stored, awk has no record of whether
it was stored with a single index or a sequence of indices. The two
expressions ‘foo[5,12]’ and ‘foo[5 SUBSEP 12]’ are always
equivalent.
The default value of SUBSEP is the string "\034",
which contains a nonprinting character that is unlikely to appear in an
awk program or in most input data.
The usefulness of choosing an unlikely character comes from the fact
that index values that contain a string matching SUBSEP can lead to
combined strings that are ambiguous. Suppose that SUBSEP is
"@"; then ‘foo["a@b", "c"]’ and ‘foo["a", "b@c"]’ are indistinguishable because both are actually
stored as ‘foo["a@b@c"]’.
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:
if ((subscript1, subscript2, ...) in array)
...
Here is an example that treats its input as a two-dimensional array of fields; it rotates this array 90 degrees clockwise and prints the result. It assumes that all lines have the same number of elements:
{
if (max_nf < NF)
max_nf = NF
max_nr = NR
for (x = 1; x <= NF; x++)
vector[x, NR] = $x
}
END {
for (x = 1; x <= max_nf; x++) {
for (y = max_nr; y >= 1; --y)
printf("%s ", vector[x, y])
printf("\n")
}
}
When given the input:
1 2 3 4 5 6 2 3 4 5 6 1 3 4 5 6 1 2 4 5 6 1 2 3
the program produces the following output:
4 3 2 1 5 4 3 2 6 5 4 3 1 6 5 4 2 1 6 5 3 2 1 6