二分查找如何工作?
为了使二分查找有效,必须对目标数组进行排序。我们将通过一个图形示例来学习二分查找的过程。以下是我们的排序数组,让我们假设我们需要使用二分查找来搜索值31的位置。
首先,我们将使用此公式确定数组的一半-
mid = low + (high - low) / 2
就是(0 + 9)/ 2 = 4(4.5的整数值)。因此,4是数组的中间。
现在,我们将存储在位置4的值与要搜索的值(即31)进行比较。我们发现位置4的值是27,这不是匹配项。由于该值大于27,并且我们有一个已排序的数组,因此我们也知道目标值必须在数组的右边。
我们将低点更改为中间+1,然后再次找到新的中间值。
low = mid + 1
mid = low + (high - low) / 2
现在我们的新中数是7。我们将存储在位置7的值与目标值31进行比较。
存储在位置7的值不是匹配项,而是大于我们所寻找的值。因此,该值必须位于此位置的左边。
因此,我们再次计算中点。这次是5。
我们将存储在位置5的值与目标值进行比较。我们发现这次匹配了。
我们得出结论,目标值31存储在位置5。二分查找将可搜索的项目减半,从而将进行比较的次数减少到非常少的数目。
伪代码
Procedure binary_search
A ← sorted array
n ← size of array
x ← value to be searched
Set lowerBound = 1
Set upperBound = n
while x not found
if upperBound < lowerBound
EXIT: x does not exists.
set midPoint = lowerBound + ( upperBound - lowerBound ) / 2
if A[midPoint] < x
set lowerBound = midPoint + 1
if A[midPoint] > x
set upperBound = midPoint - 1
if A[midPoint] = x
EXIT: x found at location midPoint
end while
end procedure
用C编程语言实现二分查找-
#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.");
}
尝试一下
如果我们编译并运行上述程序,它将产生以下结果-
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