Wolfram: List. Sort Reverse Ordering

• Sort[list]
• Sort[list, p]

sort a list or any expression.

p is the ordering function that takes 2 args.

in p[a,b], it must return one of:

• 1 → args order is correct.
• 0 → args order is considered same.
• -1 → args order is reversed.

or

• True → args order is correct.
• False → args order is reversed.

• Sort

Sort[ {3, 5, 2} ]
(* {2, 3, 5} *)

Sort[ {"cat", "dog", "bird"} ]
(* {bird, cat, dog} *)

(* sort by sum *)
Sort[
 {{9, 3}, {1, 0}, {2, 1}},
 Function[{a,b}, Total[ a ] < Total[ b ] ]
]
(* {{1, 0}, {2, 1}, {9, 3}} *)

• SortBy[list, f]
• SortBy[list, {f1, f2, etc}]
• SortBy[list, f, p]

• Sort by applying f to items and compare the result.
• f takes one arg returns a order ranking index, e.g. an integer.
• When there is a list of f, each successive one is used to break the tie.

• Predicate p (the ordering function) can be used to break the tie of f
• The spec for p is the same used for Sort.

• SortBy

(* sort by string length *)
SortBy[ {"bird", "dog", "cat"}, StringLength ]
(* {cat, dog, bird} *)

(* note, it also swapped cat and dog, by apply normal sort behavior to string *)

(* sort sum *)
Sort[
{{9, 9, 2}, {2, 3}, {2, 9}},
Function[{x,y}, Total @ x < Total @ y ]
]
(* {{2, 3}, {2, 9}, {9, 9, 2}} *)

return a list of indexes that represent how to make the list into order.

• Ordering

Ordering[ {55, 88, 44, 99} ]
(* {3, 1, 2, 4} *)

(* this means,
Index 3 of input has order rank of 1.
Index 1 of input has order rank of 2.
Index 2 of input has order rank of 3.
Index 4 of input has order rank of 4.
*)

(* when the list is ordered, the result is 1, 2, 3, etc. *)
Ordering[ {1, 2, 3} ]
{1, 2, 3}

(* to get each element's corresponding order rank, call Ordering twice. *)
Ordering @ Ordering @ {55, 88, 44, 99}
(* {2, 3, 1, 4} *)

(* this means,
55 has a order rank of 2.
88 has a order rank of 3.
44 has a order rank of 1.
99 has a order rank of 4.
*)

reverse a list.

• Reverse

Reverse[ {1,2,3,4} ]
(* {4, 3, 2, 1} *)

reverse first n items.

(* reverse first 3 *)
Reverse[ {1,2,3,4}, 3 ]
(* {1, 2, 3, 4} *)

reverse items at Level n.

(* reverse order of specific level *)
Reverse[ {{1, 2, {a, b, c}}, 4, {6, 5}}, {2} ]
(* {{{a, b, c}, 2, 1}, 4, {5, 6}} *)

reverse items at several levels.

(* reverse order of several levels *)
Reverse[ {{1, 2, {a, b, c}}, 4, {6, 5}}, {2, 3} ]
(* {{{c, b, a}, 2, 1}, 4, {5, 6}} *)