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Programming-Idioms

  • Java
  • Pascal
  • Dart

Idiom #124 Binary search for a value in sorted array

Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists.

import 'package:collection/collection.dart';
a.binarySearch(x);

this function is already part of the library.
import java.util.arrays;
static int binarySearch(final int[] arr, final int key) {
    final int index = Arrays.binarySearch(arr, key);
    return index < 0 ? - 1 : index;
}
import static java.util.Arrays.binarySearch;
int i = binarySearch(a, x);
function binarySearch (x: integer; a: array of integer): integer;
var  L, R, M: integer;  // left, right, middle
begin
  if Length(a)=0 then Exit(-1);
  L := Low (a);
  R := High(a);
  while (L <= R) do begin
    M := (L + R) div 2;
    if (x = a[M]) then Exit(M);  // found x in a
    if (x > a[M]) 
    then L := Succ(M)
    else R := Pred(M);
  end;
  Exit(-1) // did not found x in a
end;
function BinarySearch(X: Integer; A: Array of Integer): Integer;
var
  L, R, I, Cur, answer: Integer;
  isIt :boolean;
begin
  isIt := false;
  answer := -1;
  if Length(A) = 0 then Exit;
  L := Low(A);
  R := High(A);
  while ((L <= R) AND (isIt = false)) do
  begin
    I := L + ((R - L) div 2); 
    Cur := A[I];
    if (X = Cur) then begin
	answer := i;  {cur;}
	isIt := true;
    end;
    if (X > Cur) then
       L := I + 1
    else
      R := I - 1
  end;
  BinarySearch := answer;
end;

the index of the array is i;
Cur is the value of A[i]; but we
search the index i
#include <vector>
template<typename T>
int binarySearch(const std::vector<T> &a, const T &x)
{
    if(a.size() == 0) return -1;

    size_t lower = 0;
    size_t upper = a.size() - 1;

    while(lower <= upper)
    {
        auto mid = lower + (upper-lower) / 2;

        if(x == a[mid])
        {
            return (int)mid;
        }
        else if(x > a[mid])
        {
            lower = mid + 1;
        }
        else
        {
            upper = mid - 1;
        }
    }

    return -1;
}

Check for an empty vector up front, otherwise size() will return an unsigned 0, and subtracting 1 will be a big number!

New implementation...
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