php - Programming In General - Binary Search Algorithms -

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given array $array of n numbers , key $key, write binary search algorithm in plain english. if $array contains $key, return index of $key; otherwise, return -1.

can show me how this?

doesn't seem should give code here, maybe description can help?

  1. sort list.

  2. let i = length / 2

  3. compare term @ index i key.

    a. if equal, return index.

    b. if key greater term, repeat 3 (recurse) on upper half of list i = (i + length) / 2 (or (i + top) / 2 depending how implement)

    c. if key less term, repeat 3 on lower half i = i/2 or (i + bottom)/2

stop recursion if/when new i same old i. means you've exhausted search. return -1

be careful off-by-one errors, can make exclude terms mistake, or cause infinite recursion, general idea. pretty straightforward.

think of playing 'guess number' numbers 1 through 100. take guess, tell higher or lower. 50, lower. 25, higher. 37...


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