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Node Slice

rascal-0.41.2

Synopsis​

Retrieve a slice of a node's argument list.

Syntax​

  • Exp₁ [ Exp₂ .. Exp₄]
  • Exp₁ [ Exp₂ , Exp3 .. Exp₄]

where Exp₂ and Exp₄ are optional.

Types​

Exp₁Exp₂Exp₃Exp₄Exp₁ [ Exp₂ .. Exp₄ ] or Exp₁ [ Exp₂ , Exp₃ .. Exp₄]
nodeintintintlist[value]

Description​

A Node slice is similar to a list Slice and uses the integer values of Exp₂ and Exp₄ to determine the begin (inclusive) and end (exclusive) of a slice from the children of the node value ND of Exp₁. Negative indices count from the end of the list of children backwards. Using the second form, an extra index Exp₃ is given that determines the index of the second element in the slice and establishes the step between successive elements in the slice. The default step is 1. If end is smaller than begin, the slice is constructed backwards.

Let Len be the number of children of ND and let N₂, N₃ and N₄ be the respective values of the expressions Exp₂, Exp₂ and Exp₂ when they are present.

The slice parameters begin, end, and step are determined as follows:

  • Exp₂: If Exp₂ is absent, then begin = 0. Otherwise, if N₂ >= 0 then begin = N₂ else begin = N₂ + Len.
  • Exp₄: If Exp₄ is absent, then end = Len. Otherwise, if N₄ >= 0, then end = N₄ else end = N₄ + Len.
  • Exp₃: If Exp₃ is absent, then if begin < end then step = 1 else step = -1. Otherwise, if begin < end, then step = N₃ - begin else step = begin - N₃.

Now, the constraints 0 <= begin < Len and 0 < end < Len should hold, otherwise the exception IndexOutOfBounds is thrown.

The slice consists of the children ND[begin], ND[begin+step], ND[end - step]. When begin >= end, the elements are listed in reverse order.

Examples​

Consider the list ND = "f"(0, "abc", 20, false, 40, [3,4,5], 60, {"a", "b"}, 80); as running example.

Here is a view on the children of ND that will help to correlate positive and negative indices:

i012345678
ND[i]0"abc"20false40[3,4,5]60{"a", "b"}80
-i-9-8-7-6-5-4-3-2-1

Some common use cases (with begin <= end):

SliceMeans:
ND[begin..end]children with indices begin through end-1
ND[begin..]children with indices begin through the rest of the list of children
ND[..end]children with indices from the beginning through end-1
ND[..]the whole list of children
ND[-1]last child of the list of children
ND[-2..]the last two children of the list of children
ND[..-2]all children except the last two.

Let's put this into practice now.

rascal>ND = "f"(0, "abc", 20, false, 40, [3,4,5], 60, {"a", "b"}, 80);
node: "f"(
  0,
  "abc",
  20,
  false,
  40,
  [3,4,5],
  60,
  {"a","b"},
  80)

Slices with begin < end

rascal>ND[1..3];
list[value]: ["abc",20]
rascal>ND[1..];       // empty end => end of list of children
list[value]: [
  "abc",
  20,
  false,
  40,
  [3,4,5],
  60,
  {"a","b"},
  80
]
rascal>ND[..3];       // empty begin => first child of list
list[value]: [0,"abc",20]
rascal>ND[..];        // both empty => whole list of children
list[value]: [
  0,
  "abc",
  20,
  false,
  40,
  [3,4,5],
  60,
  {"a","b"},
  80
]

Slices with begin >= end

rascal>ND[3..1];      // slice contains children with indices 3 and 2 (in that order)
list[value]: [false,20]
rascal>ND[3..3];      // empty slice when begin == end
list[value]: []

Slices with negative begin or end:

rascal>ND[2..-2];     // equivalent to ND[2..7]
list[value]: [
  20,
  false,
  40,
  [3,4,5],
  60
]
rascal>ND[2..7];
list[value]: [
  20,
  false,
  40,
  [3,4,5],
  60
]
rascal>ND[-4..-2];    // equivalent to ND[5..7]
list[value]: [
  [3,4,5],
  60
]
rascal>ND[5..7];
list[value]: [
  [3,4,5],
  60
]

Slices with an explicit second index:

rascal>ND[1,3..6];
list[value]: [
  "abc",
  false,
  [3,4,5]
]
rascal>ND[5,3..];
list[value]: [
  [3,4,5],
  false,
  "abc"
]

Slicing does not go "out of bounds". When we go beyond the end or the beginning, it stops there:

rascal>ND[..10];
list[value]: [
  0,
  "abc",
  20,
  false,
  40,
  [3,4,5],
  60,
  {"a","b"},
  80
]
rascal>ND[..-11];
list[value]: [0]