《The C Programming Language》第三章例程(二分查找)中的错误

先看程序：

#include <stdio.h> #include <assert.h> int binary_search(int x, int v[], int n) { int high, low, mid; low = 0; high = n - 1; mid = 0; while(low <= high){ mid = (low + high) / 2; if(x < v[mid]) /*searvh the left half*/ high = mid + 1; else if(x > v[mid]) /*search the right half*/ low = mid + 1; else return mid; } return -1; /*not match*/ } int main() { int i; int array[10]; int val; int result = 11; for(i = 0; i < 10; ++i){ array[i] = i * 2 + 1; //printf("%d ", array[i]); } printf("Input value you want to compare to the array:"); scanf("%d", &val); result = binary_search(val, array, 10); for(i = 0; i < 10; ++i){ //array[i] = i * 2 + 1; printf("%d ", array[i]); } if(-1 == result || result >= 10) printf("/nnot found!/n"); else printf("/nThe position in the array is %d/n", result); return 0; }

当读入的val的值为偶数如4时，程序进入死循环，关键点在

if(x < v[mid]) /*searvh the left half*/ high = mid + 1;

应该是high = mid -1 ;

#include <stdio.h> #include <assert.h> int binary_search(int x, int v[], int n) { int high, low, mid; low = 0; high = n - 1; mid = 0; while(low <= high){ mid = (low + high) / 2; if(x < v[mid]) /*searvh the left half*/ high = mid - 1; else if(x > v[mid]) /*search the right half*/ low = mid + 1; else return mid; } return -1; /*not match*/ } int main() { int i; int array[10]; int val; int result = 11; for(i = 0; i < 10; ++i){ array[i] = i * 2 + 1; //printf("%d ", array[i]); } printf("Input value you want to compare to the array:"); scanf("%d", &val); result = binary_search(val, array, 10); for(i = 0; i < 10; ++i){ //array[i] = i * 2 + 1; printf("%d ", array[i]); } if(-1 == result || result >= 10) printf("/nnot found!/n"); else printf("/nThe position in the array is %d/n", result); return 0; }

但是

mid =(low + high) / 2

还是个bug，因为low+high超出int表示的正数范围时就会变成负数，这时mid也跟着错了，而后面用mid取数组元素自然也不对了。可改成：

int mid = low + ((high - low) / 2);