C語言二分查詢程式

二分查詢是一種快速搜尋演算法，其執行時間複雜度為Ο(log n)。這種搜尋演算法基於分治的原理。為了使該演算法正常工作，資料集合必須按排序形式排列。

C語言實現

#include <stdio.h>

#define MAX 20

// array of items on which linear search will be conducted. 
int intArray[MAX] = {1,2,3,4,6,7,9,11,12,14,15,16,17,19,33,34,43,45,55,66};

void printline(int count) {
   int i;
	
   for(i = 0;i <count-1;i++) {
      printf("=");
   }
	
   printf("=\n");
}

int find(int data) {
   int lowerBound = 0;
   int upperBound = MAX -1;
   int midPoint = -1;
   int comparisons = 0;      
   int index = -1;
	
   while(lowerBound <= upperBound) {
      printf("Comparison %d\n" , (comparisons +1) );
      printf("lowerBound : %d, intArray[%d] = %d\n",lowerBound,lowerBound,
         intArray[lowerBound]);
      printf("upperBound : %d, intArray[%d] = %d\n",upperBound,upperBound,
         intArray[upperBound]);
      comparisons++;
		
      // compute the mid point
      // midPoint = (lowerBound + upperBound) / 2;
      midPoint = lowerBound + (upperBound - lowerBound) / 2;
		
      // data found
      if(intArray[midPoint] == data) {
         index = midPoint;
         break;
      } else {
         // if data is larger 
         if(intArray[midPoint] < data) {
            // data is in upper half
            lowerBound = midPoint + 1;
         }
         // data is smaller 
         else {
            // data is in lower half 
            upperBound = midPoint -1;
         }
      }               
   }
   printf("Total comparisons made: %d" , comparisons);
   return index;
}

void display() {
   int i;
   printf("[");
	
   // navigate through all items 
   for(i = 0;i<MAX;i++) {
      printf("%d ",intArray[i]);
   }
	
   printf("]\n");
}

void main() {
   printf("Input Array: ");
   display();
   printline(50);
	
   //find location of 1
   int location = find(55);

   // if element was found 
   if(location != -1)
      printf("\nElement found at location: %d" ,(location+1));
   else
      printf("\nElement not found.");
}

輸出

如果我們編譯並執行上述程式，則會產生以下結果：

Input Array: [1 2 3 4 6 7 9 11 12 14 15 16 17 19 33 34 43 45 55 66 ]
==================================================
Comparison 1
lowerBound : 0, intArray[0] = 1
upperBound : 19, intArray[19] = 66
Comparison 2
lowerBound : 10, intArray[10] = 15
upperBound : 19, intArray[19] = 66
Comparison 3
lowerBound : 15, intArray[15] = 34
upperBound : 19, intArray[19] = 66
Comparison 4
lowerBound : 18, intArray[18] = 55
upperBound : 19, intArray[19] = 66
Total comparisons made: 4
Element found at location: 19