Array list: number: Difference between revisions

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'''Variant:''' <math>p</math> is moved one step forward so as to point to the next array list item.
'''Variant:''' <math>p</math> is moved one step forward so as to point to the next array list item.
'''Break condition:''' It is <math>p=</math> void.
'''Break condition:''' It is <math>p=</math> void.



Revision as of 14:15, 16 May 2015

Algorithmic problem: Linear sequence: number

Prerequisites:

Type of algorithm: loop

Auxiliary data:

  1. A pointer [math]\displaystyle{ p }[/math] of type "pointer to array list item of type [math]\displaystyle{ K }[/math]".
  2. A counter [math]\displaystyle{ c \in \mathbb{N}_0 }[/math].

Abstract view

Invariant: After [math]\displaystyle{ i \ge 0 }[/math] iterations:

  1. The pointer [math]\displaystyle{ p }[/math] points to the array list item at position [math]\displaystyle{ i+1 }[/math] (or is void if there is no such item).
  2. The value of is the sum of the values n of all array list items at positions [math]\displaystyle{ 1,...,i }[/math].

Variant: [math]\displaystyle{ p }[/math] is moved one step forward so as to point to the next array list item.

Break condition: It is [math]\displaystyle{ p= }[/math] void.

Induction basis

Abstract view: Initialize [math]\displaystyle{ p }[/math] so as to point to the first array list item.

Implementation:

  1. Set [math]\displaystyle{ p:=first }[/math].
  2. Set [math]\displaystyle{ c:=0 }[/math].

Proof: Nothing to show.

Induction step

Abstract view: Add the number of elements in the current array and then go forward.

Implementation:

  1. If void, terminate the algorithm and return the value of [math]\displaystyle{ c }[/math].
  2. Otherwise:
    1. Set [math]\displaystyle{ c:=c+p }[/math].n.
    2. Set [math]\displaystyle{ p:=p }[/math].next.

Correctness: Nothing to show.

Complexity

Statement: Linear in the length of the sequence in the worst case (for a fixed value of ).

Proof: Obvious.

Further information