1 1 8.5 Multidimensional Arrays 1 =========================== 1
1
· Multiscanning Scanning multidimensional arrays.
1
1 A "multidimensional array" is an array in which an element is identified
1 by a sequence of indices instead of a single index. For example, a
1 two-dimensional array requires two indices. The usual way (in many
1 languages, including 'awk') to refer to an element of a two-dimensional
1 array named 'grid' is with 'grid[X,Y]'.
1
1 Multidimensional arrays are supported in 'awk' through concatenation
1 of indices into one string. 'awk' converts the indices into strings
1 (⇒Conversion) and concatenates them together, with a separator
1 between them. This creates a single string that describes the values of
1 the separate indices. The combined string is used as a single index
1 into an ordinary, one-dimensional array. The separator used is the
1 value of the built-in variable 'SUBSEP'.
1
1 For example, suppose we evaluate the expression 'foo[5,12] = "value"'
1 when the value of 'SUBSEP' is '"@"'. The numbers 5 and 12 are converted
1 to strings and concatenated with an '@' between them, yielding '"5@12"';
1 thus, the array element 'foo["5@12"]' is set to '"value"'.
1
1 Once the element's value is stored, 'awk' has no record of whether it
1 was stored with a single index or a sequence of indices. The two
1 expressions 'foo[5,12]' and 'foo[5 SUBSEP 12]' are always equivalent.
1
1 The default value of 'SUBSEP' is the string '"\034"', which contains
1 a nonprinting character that is unlikely to appear in an 'awk' program
1 or in most input data. The usefulness of choosing an unlikely character
1 comes from the fact that index values that contain a string matching
1 'SUBSEP' can lead to combined strings that are ambiguous. Suppose that
1 'SUBSEP' is '"@"'; then 'foo["a@b", "c"]' and 'foo["a", "b@c"]' are
1 indistinguishable because both are actually stored as 'foo["a@b@c"]'.
1
1 To test whether a particular index sequence exists in a
1 multidimensional array, use the same operator ('in') that is used for
1 single-dimensional arrays. Write the whole sequence of indices in
1 parentheses, separated by commas, as the left operand:
1
1 if ((SUBSCRIPT1, SUBSCRIPT2, ...) in ARRAY)
1 ...
1
1 Here is an example that treats its input as a two-dimensional array
1 of fields; it rotates this array 90 degrees clockwise and prints the
1 result. It assumes that all lines have the same number of elements:
1
1 {
1 if (max_nf < NF)
1 max_nf = NF
1 max_nr = NR
1 for (x = 1; x <= NF; x++)
1 vector[x, NR] = $x
1 }
1
1 END {
1 for (x = 1; x <= max_nf; x++) {
1 for (y = max_nr; y >= 1; --y)
1 printf("%s ", vector[x, y])
1 printf("\n")
1 }
1 }
1
1 When given the input:
1
1 1 2 3 4 5 6
1 2 3 4 5 6 1
1 3 4 5 6 1 2
1 4 5 6 1 2 3
1
1 the program produces the following output:
1
1 4 3 2 1
1 5 4 3 2
1 6 5 4 3
1 1 6 5 4
1 2 1 6 5
1 3 2 1 6
1