【编程珠玑】第五章 编程小事

一，概述

主要讲解如何保证编程的正确性。在程序中加入断言（assert(断言内容) //如果错误，则终止程序。否则正常执行）。

typdef //声明自定义类型

typedef int size; //声明int 型整数的别名

size array[4];

typedef struct tagNode
{
char *pItem;
　pNode  *pNext;
} *pNode;

测试结构题大小的程序

#include "stdio.h"

typedef struct tagNode
{
char *pItem; //32位
struct tagNode  *pNext;//32位
} *pNode;

int main()
{
printf("%d\n",sizeof(struct tagNode));

return 0;
}

二，习题

5）测试、断言优化过程

for (i = 0; i < n - 1; ++i)
assert(a[i] < a[i+1]);

如何利用二分的性质来进行处理还是一个问题。

一种办法是：

int bs(int *a, int b, int e, int v)
{
int *begin = a + b, *end = a + e, *mid;
if (!a || b >= e) return -1;
while (begin < end)
{
mid = begin + ((end - begin) >> 1);
assert(*begin <= *mid && *mid <= *end);
if (*mid > v) end = mid;
else if (*mid < v) begin = mid + 1;
else return mid - a;
}
return -1;
}

但是这个方法需要多次使用才能检测出来，也就是可能在查找某两个数的时候不会报错，可能在查找别的几个数的时候就报错了。很多时候，这不是一种好选择。

根据第五题的题意，可以写如下代码来减少每次检测的量。

int bs(int *a, int b, int e, int v)
{
int *begin = a + b, *end = a + e, *mid, i = b;
static int *record = 0;
if (!a || b >= e) return -1;
if (!record || record != a)
{
while (i < e && a[i] < a[i+1])       ++i;
assert(i == e);
}

while (begin < end)
{
mid = begin + ((end - begin) >> 1);
assert(*begin <= *mid && *mid <= *end);
if (*mid > v) end = mid;
else if (*mid < v) begin = mid + 1;
else return mid - a;
}
return -1;
}

加一个static变量来记录这个数组是否经过检测，来决定以后的检测还需要进行。

当然对于多个数组的检测，可以利用hash来处理，因为一般来说，数组的下标地址都是不一样的。

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plaincopy

for (i = 0; i < n - 1; ++i)

assert(a[i] < a[i+1]);

如何利用二分的性质来进行处理还是一个问题。

一种办法是：

[cpp] view
plaincopy

int bs(int *a, int b, int e, int v)

{

int *begin = a + b, *end = a + e, *mid;

if (!a || b >= e) return -1;

while (begin < end)

{

mid = begin + ((end - begin) >> 1);

assert(*begin <= *mid && *mid <= *end);

if (*mid > v) end = mid;

else if (*mid < v) begin = mid + 1;

else return mid - a;

}

return -1;

}

但是这个方法需要多次使用才能检测出来，也就是可能在查找某两个数的时候不会报错，可能在查找别的几个数的时候就报错了。很多时候，这不是一种好选择。

根据第五题的题意，可以写如下代码来减少每次检测的量。

[cpp] view
plaincopy

int bs(int *a, int b, int e, int v)

{

int *begin = a + b, *end = a + e, *mid, i = b;

static int *record = 0;

if (!a || b >= e) return -1;

if (!record || record != a)

{

while (i < e && a[i] < a[i+1]) ++i;

assert(i == e);

}

while (begin < end)

{

mid = begin + ((end - begin) >> 1);

assert(*begin <= *mid && *mid <= *end);

if (*mid > v) end = mid;

else if (*mid < v) begin = mid + 1;

else return mid - a;

}

return -1;

}

加一个static变量来记录这个数组是否经过检测，来决定以后的检测还需要进行。

当然对于多个数组的检测，可以利用hash来处理，因为一般来说，数组的下标地址都是不一样的。

9,，serch.c 源程序

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

#define MAXN 1000000

typedef int DataType;

DataType x[MAXN];
int n;

/* Scaffolding */

int i = -999999;
#define assert(v) { if ((v) == 0) printf("  binarysearch bug %d %d\n", i, n); }

/* Alg 1: From Programming Pearls, Column 4: raw transliteration */

int binarysearch1(DataType t)
{	int l, u, m;
l = 0;
u = n-1;
for (;;) {
if (l > u)
return -1;
m = (l + u) / 2;
if (x[m] < t)
l = m+1;
else if (x[m] == t)
return m;
else /* x[m] > t */
u = m-1;
}
}

/* Alg 2: Make binarysearch1 more c-ish */

int binarysearch2(DataType t)
{	int l, u, m;
l = 0;
u = n-1;
while (l <= u) {
m = (l + u) / 2;
if (x[m] < t)
l = m+1;
else if (x[m] == t)
return m;
else /* x[m] > t */
u = m-1;
}
return -1;
}

/* Alg 3: From PP, Col 8 */

int binarysearch3(DataType t)
{	int l, u, m;
l = -1;
u = n;
while (l+1 != u) {
m = (l + u) / 2;
if (x[m] < t)
l = m;
else
u = m;
}
if (u >= n || x[u] != t)
return -1;
return u;
}

/* Alg 4: From PP, Col 9 */

int binarysearch4(DataType t)
{	int l, p;
if (n != 1000)
return binarysearch3(t);
l = -1;
if (x[511]   < t) l = 1000 - 512;
if (x[l+256] < t) l += 256;
if (x[l+128] < t) l += 128;
if (x[l+64 ] < t) l += 64;
if (x[l+32 ] < t) l += 32;
if (x[l+16 ] < t) l += 16;
if (x[l+8  ] < t) l += 8;
if (x[l+4  ] < t) l += 4;
if (x[l+2  ] < t) l += 2;
if (x[l+1  ] < t) l += 1;
p = l+1;
if (p >= n || x[p] != t)
return -1;
return p;
}

/* Alg 9: Buggy, from Programming Pearls, Column 5 */

int sorted()
{   int i;
for (i = 0; i < n-1; i++)
if (x[i] > x[i+1])
return 0;
return 1;
}

int binarysearch9(DataType t)
{	int l, u, m;
/* int oldsize, size = n+1; */
l = 0;
u = n-1;
while (l <= u) {
/* oldsize = size;
size = u - l +1;
assert(size < oldsize); */
m = (l + u) / 2;
/* printf("  %d %d %d\n", l, m, u); */
if (x[m] < t)
l = m;
else if (x[m] > t)
u = m;
else {
/* assert(x[m] == t); */
return m;
}
}
/* assert(x[l] > t && x[u] < t); */
return -1;
}

/* Alg 21: Simple sequential search */

int seqsearch1(DataType t)
{	int i;
for (i = 0; i < n; i++)
if (x[i] == t)
return i;
return -1;
}

/* Alg 22: Faster sequential search: Sentinel */

int seqsearch2(DataType t)
{	int i;
DataType hold = x
;
x
= t;
for (i = 0; ; i++)
if (x[i] == t)
break;
x
= hold;
if (i == n)
return -1;
else
return i;
}

/* Alg 23: Faster sequential search: loop unrolling */

int seqsearch3(DataType t)
{	int i;
DataType hold = x
;
x
= t;
for (i = 0; ; i+=8) {
if (x[i] == t)   {          break; }
if (x[i+1] == t) { i += 1; break; }
if (x[i+2] == t) { i += 2; break; }
if (x[i+3] == t) { i += 3; break; }
if (x[i+4] == t) { i += 4; break; }
if (x[i+5] == t) { i += 5; break; }
if (x[i+6] == t) { i += 6; break; }
if (x[i+7] == t) { i += 7; break; }
}
x
= hold;
if (i == n)
return -1;
else
return i;
}

/* Scaffolding to probe one algorithm */

void probe1()
{	int i;
DataType t;
while (scanf("%d %d", &n, &t) != EOF) {
for (i = 0; i < n; i++)
x[i] = 10*i;
printf(" %d\n", binarysearch9(t));
}
}

/* Torture test one algorithm */

#define s seqsearch3
void test(int maxn)
{	int i;
for (n = 0; n <= maxn; n++) {
printf("n=%d\n", n);
/* distinct elements (plus one at top) */
for (i = 0; i <= n; i++)
x[i] = 10*i;
for (i = 0; i < n; i++) {
assert(s(10*i)     ==  i);
assert(s(10*i - 5) == -1);
}
assert(s(10*n - 5) == -1);
assert(s(10*n)     == -1);
/* equal elements */
for (i = 0; i < n; i++)
x[i] = 10;
if (n == 0) {
assert(s(10) == -1);
} else {
assert(0 <= s(10) && s(10) < n);
}
assert(s(5) == -1);
assert(s(15) == -1);
}
}

/* Timing */

int p[MAXN];

void scramble(int n)
{	int i, j;
DataType t;
for (i = n-1; i > 0; i--) {
j = (RAND_MAX*rand() + rand()) % (i + 1);
t = p[i]; p[i] = p[j]; p[j] = t;
}
}

void timedriver()
{	int i, algnum, numtests, test, start, clicks;
while (scanf("%d %d %d", &algnum, &n, &numtests) != EOF) {
for (i = 0; i < n; i++)
x[i] = i;
for (i = 0; i < n; i++)
p[i] = i;
scramble(n);
start = clock();
for (test = 0; test < numtests; test++) {
for (i = 0; i < n; i++) {
switch (algnum) {
case 1:  assert(binarysearch1(p[i]) == p[i]); break;
case 2:  assert(binarysearch2(p[i]) == p[i]); break;
case 3:  assert(binarysearch3(p[i]) == p[i]); break;
case 4:  assert(binarysearch4(p[i]) == p[i]); break;
case 9:  assert(binarysearch9(p[i]) == p[i]); break;
case 21: assert(seqsearch1(p[i]) == p[i]); break;
case 22: assert(seqsearch2(p[i]) == p[i]); break;
case 23: assert(seqsearch3(p[i]) == p[i]); break;
}
}
}
clicks = clock() - start;
printf("%d\t%d\t%d\t%d\t%g\n",
algnum, n, numtests, clicks,
1e9*clicks/((float) CLOCKS_PER_SEC*n*numtests));
}
}

/* Main */

int main()
{	/* probe1(); */
/* test(25); */
timedriver();
return 0;
}