Introduction
The bracket operator is the use of '[' and ']' like accessing
arrays in C.
In YCP, this operator is used to ease handling with (possibly
nested) lists and maps.
The bracket operator can be applied to any list or map variable
and should be used in favour of (deeply) nested lookup() and select()
cascades.
Access variant
The access variant of the bracket operator is used for accessing
elements of a list or a map. It effectively replaces select
for lists and lookup for maps.
Accessing lists
General syntax:
for simple lists:
<list-var>[<index>]:<default-value>
for nested lists:
<list-var>[<index>,<index> <, ...>]:<default-value>
index must be an integer and counts from 0 up to the number of elements-1.
It will return the default-value if you try to access an out-of-bounds element.
Note that there must be no space between the closing bracket and the colon.
Examples:
{
list l = [1, 2, 3];
integer three = l[2]:0; // == 3
integer zero = l[42]:0; // default value
list ll = [[1,2], [3,4], [5,6]];
return (ll[1,0]:0 == three); // returns true
}
Accessing maps
General syntax:
for simple maps:
<map-var>[<key>]:<default-value>
for nested lists:
<map-var>[<key>,<key> <, ...>]:<default-value>
key must have an allowed type for maps, integer, string, or symbol.
It will return default-value if you try to access an non existing key.
Note that there must be no space between the closing bracket and the colon.
Examples:
{
map m = $["a":1, "b":2, "c":3];
integer three = m["c"]:0; // == 3
integer zero = m["notthere"]:0; // default value
map mm = $["a":$[1:2], "b":$[3:4], "c":$[5:6]];
return (mm["b",0]:0 == three); // returns true
}
Mixed map/list access
Since the bracket operator applies to list and maps, you can use it
to access nested lists and maps. But be careful not to mix up the
index/key types.
Examples:
{
map map_of_lists = $["a":[1, 2, 3], "b":[4,5,6], "c":[7,8,9]];
integer three = map_of_lists["a",2]:0; // == 3
list list_of_maps = [$[1:2], $[3:4], $[5:6]];
return (list_of_maps[1,0]:0 == three); // returns true
}
Assign variant
The bracket operator can also be used on the left side of an assignment (lvalue).
This changes the list or map element in place (!!) and must be
used with care. See the alias trap for details.
If the map or list element does not exist, it will be created. The bracket
operator can therefore replace add and change.
Creating a new list element will extend the size of the list. Holes will
be filled with nil. See the examples below.
If used as an lvalue, the default value is not allowed.
Examples:
{
list l = [1,2,3];
// change the second element
l[1] = 25; // l = [1,25,3] now !
// change the "c" element
map m = $["a":1, "b":2, "c":3];
m["c"] = 42; // m = $["a":1, "b":2, "c":42] now
// extend the list to 7 elements (0-6)
l[6] = 6; // l = [1,25,3,nil,nil,nil,6] now !
// add a new element to m
m["zz"] = 13;
return (mm); // $["a":1, "b":2, "c":42, "zz":13]
}
Again: Be careful with the assignment variant of the bracket operator
and don't fall into the alias trap.
Klaus Kämpf
Last modified: Wed Nov 28 15:21:42 MEST 2001