(data structure)

Definition: An unordered collection of values that may have duplicates.

Formal Definition: A bag has a single query function, numberIn(v, B), which tells how many copies of an element are in the bag, and two modifier functions, add(v, B) and remove(v, B). These may be defined with axiomatic semantics as follows.

  1. new() returns a bag
  2. numberIn(v, new()) = 0
  3. numberIn(v, add(v, B)) = 1 + numberIn(v, B)
  4. numberIn(v, add(u, B)) = numberIn(v, B) if v ≠ u
  5. remove(v, new()) = new()
  6. remove(v, add(v, B)) = B
  7. remove(v, add(u, B)) = add(u, remove(v, B)) if v ≠ u
where B is a bag and u and v are elements.

The predicate isEmpty(B) may be defined with the following additional axioms.

  1. isEmpty(new()) = true
  2. isEmpty(add(v, B)) = false

Also known as multi-set.

Generalization (I am a kind of ...)
abstract data type.

See also set, list.

Note: A bag, or multi-set, is a set where values may be repeated. Inserting 2, 1, 2 into an empty set gives the set {1, 2}. Inserting those values into an empty bag gives {1, 2, 2}. From another point of view, a set is unordered, too, but has each value at most once.

Author: PEB

Go to the Dictionary of Algorithms and Data Structures home page.

If you have suggestions, corrections, or comments, please get in touch with Paul Black.

Entry modified 11 January 2005.
HTML page formatted Fri Feb 23 10:06:07 2018.

Cite this as:
Paul E. Black, "bag", in Dictionary of Algorithms and Data Structures [online], Vreda Pieterse and Paul E. Black, eds. 11 January 2005. (accessed TODAY) Available from: