Records are next to lists the most important way to collect objects together. A record is a collection of components. Each component has a unique name, which is an identifier that distinguishes this component, and a value, which is an object of arbitrary type. We often abbreviate value of a component to element. We also say that a record contains its elements. You can access and change the elements of a record using its name.
Record literals are written by writing down the components in order between
``rec('' and ``)'', and separating them by commas ``,''. Each
component consists of the name, the assignment operator :=, and the value.
The empty record, i.e., the record with no components, is written as
rec().
gap> rec( a := 1, b := "2" ); # a record with two components rec( a := 1, b := "2" ) gap> rec( a := 1, b := rec( c := 2 ) ); # record may contain records rec( a := 1, b := rec( c := 2 ) )
We may use the Display function to illustrate the hierarchy of the record
components.
gap> Display( last );
rec(
a := 1,
b := rec(
c := 2 ) )
Records usually contain elements of various types, i.e., they are usually not homogeneous like lists.
IsRecord( obj ) C
IsRecordCollection( obj ) C
IsRecordCollColl( obj ) C
gap> IsRecord( rec( a := 1, b := 2 ) ); true gap> IsRecord( IsRecord ); false
RecNames( rec ) A
returns a list of strings corresponding to the names of the record components of the record rec.
gap> r := rec( a := 1, b := 2 );; gap> RecNames( r ); [ "a", "b" ]
Note that you cannot use the string result in the ordinary way to access
or change a record component. You must use the rec.(name)
construct (see Accessing Record Elements and Record Assignment).
.name O
The above construct evaluates to the value of the record component with the name name in the record rec. Note that the name is not evaluated, i.e. it is taken literal.
gap> r := rec( a := 1, b := 2 );; gap> r.a; 1 gap> r.b; 2
.(name) O
This construct is similar to the above construct. The difference is that the second operand name is evaluated. It must evaluate to a string or an integer otherwise an error is signalled. The construct then evaluates to the element of the record rec whose name is, as a string, equal to name.
gap> old := rec( a := 1, b := 2 );; gap> new := rec(); rec( ) gap> for i in RecNames( old ) do > new.(i) := old.(i); > od; gap> Display( new ); rec( a := 1, b := 2 )
.name := obj O
The record assignment assigns the object obj, which may be an object of arbitrary type, to the record component with the name name, which must be an identifier, of the record rec. That means that accessing the element with name name of the record rec will return obj after this assignment. If the record rec has no component with the name name, the record is automatically extended to make room for the new component.
gap> r := rec( a := 1, b := 2 );; gap> r.a := 10;; gap> Display( r ); rec( a := 10, b := 2 ) gap> r.c := 3;; gap> Display( r ); rec( a := 10, b := 2, c := 3 )
Note that assigning to a record changes the record.
The function IsBound can be used to test if a record
has a component with a certain name, the function Unbind (see Unbind)
can be used to remove a component with a certain name again.
gap> IsBound(r.a); true gap> IsBound(r.d); false gap> Unbind(r.b); gap> Display( r ); rec( a := 10, c := 3 )
.(name) := obj O
This construct is similar to the above construct. The difference is that the second operand name is evaluated. It must evaluate to a string or an integer otherwise an error is signalled. The construct then assigns obj to the record component of the record rec whose name is, as a string, equal to name.
With the record assignment (see Record Assignment) it is possible to change a record. This section describes the semantic consequences of this fact which are essentially the same as for lists (see Identical Lists).
r := rec( a := 1 ); r := rec( a := 1, b := 2 );
The second assignment does not change the first record, instead it
assigns a new record to the variable r. On the other hand, in the
following example the record is changed by the second assignment.
r := rec( a := 1 ); r.b := 2;
To understand the difference first think of a variable as a name for an
object. The important point is that a record can have several names at
the same time. An assignment var := record means in this
interpretation that var is a name for the object record. At the end
of the following example r2 still has the value rec( a := 1 ) as
this record has not been changed and nothing else has been assigned to
r2.
r1 := rec( a := 1 ); r2 := r1; r1 := rec( a := 1, b := 2 );
But after the following example the record for which r2 is a name has
been changed and thus the value of r2 is now rec( a := 1, b := 2 ).
r1 := rec( a := 1 ); r2 := r1; r1.b := 2;
We shall say that two records are identical if changing one of them by a record assignment also changes the other one. This is slightly incorrect, because if two records are identical, there are actually only two names for one record. However, the correct usage would be very awkward and would only add to the confusion. Note that two identical records must be equal, because there is only one records with two different names. Thus identity is an equivalence relation that is a refinement of equality.
Let us now consider under which circumstances two records are identical.
If you enter a record literal then the record denoted by this literal is
a new record that is not identical to any other record. Thus in the
following example r1 and r2 are not identical, though they are equal
of course.
r1 := rec( a := 1 ); r2 := rec( a := 1 );
Also in the following example, no records in the list l are identical.
l := []; for i in [1..10] do l[i] := rec( a := 1 ); od;
If you assign a record to a variable no new record is created. Thus the
record value of the variable on the left hand side and the record on the
right hand side of the assignment are identical. So in the following
example r1 and r2 are identical records.
r1 := rec( a := 1 ); r2 := r1;
If you pass a record as argument, the old record and the argument of the
function are identical. Also if you return a record from a function, the
old record and the value of the function call are identical. So in the
following example r1 and r2 are identical record
r1 := rec( a := 1 ); f := function ( r ) return r; end; r2 := f( r1 );
The functions StructuralCopy and ShallowCopy (see StructuralCopy and
ShallowCopy) accept a record and return a new record that is equal to the
old record but that is not identical to the old record. The difference
between StructuralCopy and ShallowCopy is that in the case of
ShallowCopy the corresponding components of the new and the old records
will be identical, whereas in the case of StructuralCopy they will only be
equal. So in the following example r1 and r2 are not identical records.
r1 := rec( a := 1 ); r2 := Copy( r1 );
If you change a record it keeps its identity. Thus if two records are
identical and you change one of them, you also change the other, and they
are still identical afterwards. On the other hand, two records that are
not identical will never become identical if you change one of them. So
in the following example both r1 and r2 are changed, and are still
identical.
r1 := rec( a := 1 ); r2 := r1; r1.b := 2;
= rec2 O
<> rec2 O
Two records are considered equal, if for each component of one record the other record has a component of the same name with an equal value and vice versa.
gap> rec( a := 1, b := 2 ) = rec( b := 2, a := 1 ); true gap> rec( a := 1, b := 2 ) = rec( a := 2, b := 1 ); false gap> rec( a := 1 ) = rec( a := 1, b := 2 ); false gap> rec( a := 1 ) = 1; false
< rec2 O
<= rec2 O
To compare records we imagine that the components of both records are
sorted according to their names. Then the records are compared
lexicographically with unbound elements considered smaller than anything
else. Precisely one record rec1 is considered less than another record
rec2 if rec2 has a component with name name2 and either rec1 has
no component with this name or rec1.name2 < rec2.name2 and for
each component of rec1 with name name1 < name2 rec2 has a
component with this name and rec1.name1 = rec2.name1.
gap> rec( a := 1, b := 2 ) < rec( b := 2, a := 1 ); false # they are equal gap> rec( a := 1, b := 2 ) < rec( a := 2, b := 0 ); true # the `a' elements are compared first and 1 is less than 2 gap> rec( a := 1 ) < rec( a := 1, b := 2 ); true # unbound is less than 2 gap> rec( a := 1 ) < rec( a := 0, b := 2 ); false # the `a' elements are compared first and 0 is less than 1 gap> rec( b := 1 ) < rec( b := 0, a := 2 ); true # the `a'-s are compared first and unbound is less than 2
27.5 IsBound and Unbind for Records
.name )' O
IsBound returns true if the record rec has a
component with the name name (which must be an identifier) and false
otherwise. rec must evaluate to a record, otherwise an error is
signalled.
gap> r := rec( a := 1, b := 2 );; gap> IsBound( r.a ); true gap> IsBound( r.c ); false
.name )' O
Unbind deletes the component with the name name in
the record rec. That is, after execution of Unbind, rec no longer
has a record component with this name. Note that it is not an error to
unbind a nonexisting record component. rec must evaluate to a record,
otherwise an error is signalled.
gap> r := rec( a := 1, b := 2 );; gap> Unbind( r.a ); r; rec( b := 2 ) gap> Unbind( r.c ); r; rec( b := 2 )
Note that IsBound and Unbind are special in that they do not evaluate
their argument, otherwise IsBound would always signal an error when it is
supposed to return false and there would be no way to tell Unbind which
component to remove.
Internally, record accesses are done using the operations listed in this section. For the records implemented in the kernel, kernel methods are provided for all these operations but otherwise it is possible to install methods for these operations for any object. This permits objects to simulate record behavior.
To save memory, records do not store a list of all component names, but only numbers identifying the components. There numbers are called RNams. GAP keeps a list of all RNams that are used and provides functions to translate RNams to strings that give the component names and vice versa.
NameRNam(nr) F
returns a string representing the component name corresponding to the RNam nr.
RNamObj(str) F
RNamObj(int) F
returns a number (the RNam) corresponding to the string str. It is also possible to pass a positive integer int in which case the decimal expansion of int is used as a string.
gap> NameRNam(798);
"BravaisSupergroups"
gap> RNamObj("blubberflutsch");
2075
gap> NameRNam(last);
"blubberflutsch"
The correspondence between Strings and RNams is not predetermined ab initio, but RNams are assigned to component names dynamically on a ``first come, first serve'' basis. Therefore, depending on the version of the library you are using and on the assignments done so far, the same component name may be represented by different RNams in different runs of GAP.
The following operations are called for record accesses to arbitrary objects. If applicable methods are installed, they are called when the object is accessed as a record.
\.(obj,rnam) O
IsBound\.(obj,rnam) O
\.\:\=(obj,rnam) O
Unbind\.(obj,rnam) O
These operations implement component access, test for element boundness, component assignment and removal of the component represented by the RNam rnam.
The component identifier rnam is always declared as IsPosInt.
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GAP 4 manual
May 2002