Editing ALGOL 68 (section)

===struct, union & {{code|[:]}}: Structures, unions and arrays===
ALGOL 68 supports [[Array data structure|arrays]] with any number of dimensions, and it allows for the ''slicing'' of whole or partial rows or columns.
  '''MODE''' '''VECTOR''' = [1:3]    '''REAL''';   ''# vector '''MODE''' declaration (typedef)  #''
  '''MODE''' '''MATRIX''' = [1:3,1:3]'''REAL''';   ''# matrix '''MODE''' declaration (typedef)  #''
  '''VECTOR''' v1  := (1,2,3);         ''# array variable initially (1,2,3)   #''
  []'''REAL''' v2   = (4,5,6);         ''# constant array, type equivalent to '''VECTOR''', bounds are implied  #''
  '''OP''' + = ('''VECTOR''' a,b) '''VECTOR''':    ''# binary '''OP'''erator definition         #''
    ('''VECTOR''' out; '''FOR''' i '''FROM''' ⌊a '''TO''' ⌈a '''DO''' out[i] := a[i]+b[i] '''OD'''; out);
  '''MATRIX''' m := (v1, v2, v1+v2);
  print ((m[,2:]));              ''# a slice of the 2nd and 3rd columns #''

Matrices can be sliced either way, e.g.:

  '''REF''' '''VECTOR''' row = m[2,];  ''# define a '''REF''' (pointer) to the 2nd row #''
  '''REF''' '''VECTOR''' col = m[,2];  ''# define a '''REF''' (pointer) to the 2nd column #''

ALGOL 68 supports multiple field structures ('''STRUCT''') and '''[[united modes]]'''. Reference variables may point to any '''MODE''' including array slices and structure fields.

For an example of all this, here is the traditional linked list declaration:

  '''MODE''' '''NODE''' = '''UNION''' ('''VOID''', '''REAL''', '''INT''', '''COMPL''', '''STRING'''),
       '''LIST''' = '''STRUCT''' ('''NODE''' val, '''REF''' '''LIST''' next);

Usage example for '''UNION''' '''CASE''' of '''''NODE''''':

{| border="1"  style="border-collapse:collapse; border:5px double gray; float:left;" class="wikitable"
! Algol68<sup>[[#The language of the unrevised report|r0]]</sup> as in the 1968 Final Report
! Algol68<sup>[[#Revisions|r1]]</sup> as in the 1973 Revised Report
|-
|
  '''NODE''' n := "1234";
  '''REAL''' r; '''INT''' i; '''COMPL''' c; '''STRING''' s
  '''CASE''' r,i,c,s::=n '''IN'''
    print(("real:", r)),
    print(("int:", i)),
    print(("compl:", c)),
    print(("string:", s))
    '''OUT''' print(("?:", n))
  '''ESAC'''
|
  '''NODE''' n := "1234";
  # or n := EMPTY; #
   '''CASE''' n '''IN'''
    ('''VOID'''):     print(("void:", "EMPTY")),
    ('''REAL''' r):   print(("real:", r)),
    ('''INT''' i):    print(("int:", i)),
    ('''COMPL''' c):  print(("compl:", c)),
    ('''STRING''' s): print(("string:", s))
    '''OUT'''         print(("?:", n))
  '''ESAC'''
|}
{{clear}}