6.1.7  Finding a subsequence or a sublist

The bracket notation used to find elements of sequences and lists can also be used to extract a range of elements. If S is a sequence or list of size n, then S[n₁..n₂] returns the subsequence or sublist of S consisting of the elements with indices from n₁ to n₂, where 0 ≤ n₁≤ n₂ < s (in Xcas syntax mode) or 0 < n₁≤ n₂ ≤ s in other syntax modes.

For lists, the at command can also be used to get a sublist.

• at takes two arguments:
  • L, a list.
  • n₁..n₂, a range of integers.
• at(L,n₁..n₂) returns the sublist of L consisting of the elements with indices from n₁ to n₂.

Again, at cannot be used for sequences.

An alternative to using at for finding a sublist is the mid command, which again cannot be used for sequences.

• mid takes two mandatory and one optional argument:
  • L, a list.
  • n, the index of the beginning of the sublist.
  • Optionally, l, the length of the sublist.
• mid(L,n  ⟨,l⟩) returns the sublist of L with index beginning at n. With the option l, the length of the sublist will be l, otherwise it will go to the end of the list L.

Examples