BinarySearch

14y1.9kVisual Basic .NET

Following example shows BinarySearch in the numerical array. If the array is string how to do a BinarySearch following the same method. For example string[] zips = { "BF 45633", "GJ 76895", "BK 87946", "NM 38657" };

using System;
public class BinarySearchDemo
{
public static void Main()
{
int[] idNumbers = { 122, 167, 204, 219, 345 };
//idNumbers must be in ascending order to perform a BinarySearch.

int x;
string entryString;
int entryId;

Console.Write("Enter an Employee ID: ");

entryString = Console.ReadLine();
entryId = Convert.ToInt32(entryString);

x = Array.BinarySearch(idNumbers, entryId);

if (x < 0)
Console.WriteLine("ID {0} not found", entryId);
else
Console.WriteLine("ID {0} was found at position {1}", entryId, x);

/*The method returns -1 if the value is not
found in the array; otherwise, it returns
the array position of the sought value */

}
}
/*
Enter an Employee ID: 219
ID 219 was found at position 3

Enter an Employee ID: 122
ID 122 was found at position 0

Enter an Employee ID: 888
ID 888 not found
*/

7 Replies

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Sam Hobbs14y ago
I sure did not understand this question. Apparently it took some discussion before the question was understood. The answer of course is easy once the question is clear.
14y ago
Thank you very much for your help Vulpes.
Vulpes14y ago
Normally, before calling Array.BinarySearch, you'd call Array.Sort.

I've jumbled up the 4 zips we have to show that this still works:

using System;

public class BinarySearchDemo
{
public static void Main()
{
string[] zips = { "GJ 76895", "BK 87946", "NM 38657", "BF 45633" }; // jumbled up
//zips must be in ascending order to perform a BinarySearch so sort them first
Array.Sort(zips);

int x;
string entryZip;

Console.Write("Enter a zip : ");

entryZip = Console.ReadLine();

x = Array.BinarySearch(zips, entryZip);

if (x < 0)
Console.WriteLine("Zip {0} not found", entryZip);
else
Console.WriteLine("Zip {0} was found at position {1}", entryZip, x);

/*The method returns -1 if the value is not
found in the array; otherwise, it returns
the array position of the sought value */
}
}
14y ago
We say suppose there is 1000 zips in the string array manually arranging the items according to ascending order is a tedious task. In such a circumstances how is to write a program using BinarySearch. If you can modify the above example it is highly desirable.
Vulpes14y ago
By default, the Array.BinarySearch method uses the String.CompareTo(String) method when comparing strings:

http://msdn.microsoft.com/en-us/library/35f0x18w.aspx

This method uses what's called a 'word' comparison, which is both case sensitive and culture sensitive, using the current culture.

Broadly, for where you are, I'd expect this to be alphabetic order with lower case letters preceding upper case letters. That's why I moved the string beginning with "BK" to be between those starting with "BF" and "GJ".
14y ago
If array is a string how to decide it is in an ascending order. Because array must be in the ascending order to perform a BinarySearch.
Vulpes14y ago
Here's a version of the same program which now searches for a string.

Notice that I've rearranged the zips array so that the elements are now in ascending order:

using System;

public class BinarySearchDemo
{
public static void Main()
{
string[] zips = { "BF 45633", "BK 87946", "GJ 76895", "NM 38657" };
//zips must be in ascending order to perform a BinarySearch.

int x;
string entryZip;

Console.Write("Enter a zip : ");

entryZip = Console.ReadLine();

x = Array.BinarySearch(zips, entryZip);

if (x < 0)
Console.WriteLine("Zip {0} not found", entryZip);
else
Console.WriteLine("Zip {0} was found at position {1}", entryZip, x);

/*The method returns -1 if the value is not
found in the array; otherwise, it returns
the array position of the sought value */
}
}