Algorithm:
//initialization
//Assume our data is stored in an  array named  list of size N.
list[0..N-1] = {Assume a sorted list}
first=1
last=N-1
result=-1 //used to store the index of a successful search. If result is -1 , the search has failed
middle = (first-last)/2
while (first<=last)
	if list[middle]==key then
		result= middle
break
	else if key<list[middle] then
		last=middle-1;
	        else
		first=middle+1;
	        endif
	middle=(first-last)/2 + first
endif
endwhile
return result.

/*
Demo: Searching a sorted list using 
binary search
*/

#include <stdio.h>
#include <stdlib.h>

int binarySearch(int key, int  list[], int listSize);

int main()
{
    int list[10];
    list[0]=5;
    list[1]=10;
    list[2]=15;
    list[3]=20;
    list[4]=25;
    list[5]=30;
    list[6]=35;
    list[7]=40;
    list[8]=45;
    list[9]=50;

    int key;

    printf("\nenter a search key\n");
    scanf("%d",&key);
    while (key!=-1)
    {
        int result = binarySearch(key,list,10);
        printf("\nSearch result %d\n",result);
        printf("\nenter a search key\n");
        scanf("%d",&key);
    }

    return 0;
}

int binarySearch(int key, int  list[], int listSize)
{
    /*we use first and last to represent a 
    portion of the array. Initially, first and last
    points to the start of the array and the end of 
    the array respectively. Middle indexes the location
     to the middle of the array.
    */
    int result=-1;//-1 represents a failed search
    int first=0;
    int last=listSize-1;
    int middle = (last-first)/2;

    while (first<=last)//while first and last have not crossed...
    {
        if(key==list[middle])//if the key is found....
        {
            result=middle;
            return result;//return the current index and exit function
        }
        else//the key was not found
        {
            if(key<list[middle])//if the key would be found 
                                //from first to middle
                last=middle-1;//we now search the lower half
            else //key would be found from middle to last
                first=middle+1;//we now search the upper half
            middle=((last-first)/2)+first;//find the new midpoint.
        }
    }
    return result;
}

/*
Demo: Searching a sorted list using
binary search implemented using recursion
*/

#include <stdio.h>
#include <stdlib.h>

int binarySearch(int key, int  list[], int first, int last);

int main()
{
    int list[10];
    list[0]=5;
    list[1]=10;
    list[2]=15;
    list[3]=20;
    list[4]=25;
    list[5]=30;
    list[6]=35;
    list[7]=40;
    list[8]=45;
    list[9]=50;

    int key;

    printf("\nenter a search key\n");
    scanf("%d",&key);
    while (key!=-1)
    {
        int result = binarySearch(key,list,0,9);
        printf("\nSearch result %d\n",result);
        printf("\nenter a search key\n");
        scanf("%d",&key);
    }

    return 0;
}

int binarySearch(int key, int  list[], int first, int last)
{
    /*we use first and last to represent a
    portion of the array. Initially, first and last
    points to the start of the array and the end of
    the array respectively. Middle indexes the location
     to the middle of the array.
    */
    int result=-1;//-1 represents a failed search
    int middle = (last-first)/2+first;

    if (first<=last)//if first and last have not crossed...
    {
        if(key==list[middle])//if the key is found....
        {
            result=middle;
            return result;//return the current index and exit function
        }
        else//the key was not found
        {
            if(key<list[middle])//if the key would be found
            {                   //from first to middle

               //we now search the lower half
                result= binarySearch(key,list,first,middle-1);
                return result;
            }

            else //key would be found from middle to last
            {
                result= binarySearch(key,list,middle+1,last);//we now search the upper half
                return result;
            }

        }
    }
    return result;
}

else if key<list[middle] then
		last=middle;
	        else
		first=middle;
	        endif
	middle=(first-last)/2 + first

else if key<list[middle] then
		last=middle-1;
	        else
		first=middle+1;
	        endif
	middle=(first-last)/2 + first