31 Ranges

A range is a dense list of integers, such that the difference between consecutive elements is a nonzero constant. Ranges can be abbreviated with the syntactic construct [ first, second .. last ] or, if the difference between consecutive elements is 1, as [ first .. last ].

If first last, [first,second..last] is the empty list, which by definition is also a range. If first = last, [first,second..last] is a singleton list, which is a range too. Note that last - first must be divisible by the increment second - first, otherwise an error is signalled.

Note that a range is just a special case of a list. So everything that is possible for lists (see Lists) is also possible for ranges. Thus you can access elements in such a range (see List Elements), test for membership (see In), etc. You can even assign to such a range (see List Assignment). Of course, unless you assign last + second-first to the entry range[Length(range)+1], the resulting list will no longer be a range.

Most often ranges are used in connection with the for-loop (see For). Here the construct
for var in [first..last] do statements od replaces the
for var from first to last do statements od, which is more usual in other programming languages.

Note that a range is at the same time also a set (see Sets), because it contains no holes or duplicates and is sorted, and also a vector (see Vectors), because it contains no holes and all elements are integers.

    gap> r := [10..20];
    [ 10 .. 20 ]
    gap> Length( r );
    11
    gap> r[3];
    12
    gap> 17 in r;
    true
    gap> r[12] := 25;; r;
    [ 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25 ]
    gap> r := [1,3..17];
    [ 1, 3 .. 17 ]
    gap> Length( r );
    9
    gap> r[4];
    7
    gap> r := [0,-1..-9];
    [ 0, -1 .. -9 ]
    gap> r[5];
    -4
    gap> r := [ 1, 4 .. 32 ];
    Error, Range: <high>-<low> must be divisible by <inc>
    gap> s := [];;  for i  in [10..20]  do Add( s, i^2 );  od;  s;
    [ 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400 ] 

The first section in this chapter describes the function that tests if a list is a range (see IsRange).

The other section tells you more about the internal representation of ranges (see More about Ranges).

Subsections

1. IsRange
2. More about Ranges