CS107编程范式-学习笔记-5

第五讲

看下面这个swap函数：

void *lsearch3(void *key, void *base, int n, int elemSize, int (*cmpfn)(void *, void *)) {
    for (int i = 0; i < n; i++ ) {
        void *elemAddr = (char*)base + i * elemSize;
        if (cmpfn(key, elemAddr) == 0) {
            cout << i << endl;
            return elemAddr;
        }
    }
    return NULL;
}

这里出现了函数指针，cmpfn是一个指向函数的指针。

对于int类型的查找，我们编写intCmp函数如下：

int intCmp(void *elem1, void *elem2) {
    int *ip1 = (int*)elem1;
    int *ip2 = (int*)elem2;
    return *ip1 - *ip2;
}

运行以下代码： 

int array[] = {4, 2, 3, 7, 11, 6};
int size = 6;
int number = 7;
int *found = (int*)lsearch3(&number, array, size, sizeof(int), intCmp);
if (found == NULL) {
    cout << ":(" << endl;
}
else {
    cout << ":)" << endl;
}

运行结果如下： 

也就是在array[3]时找到了number。

老师源码中的部分如下：

int *ip1 = elem1;
int *ip2 = elem2;

int *found = lsearch3(&number, array, size, sizeof(int), intCmp);

编译器中有如下报错，应该是void*泛型转换不允许了。 

再看char*类型的查找：

int strCmp(void *vp1, void *vp2) {
    char *s1 = *(char**)vp1;
    char *s2 = *(char**)vp2;
    return strcmp(s1, s2);
}

char *notes[] = {"Ab", "F#", "B", "Gb", "D"};
char *favorateNode = "Eb";
char **found = (char**)lsearch3(&favorateNode, notes, 5, sizeof(char*), strCmp);
if (found == NULL) {
    cout << ":(" << endl;
}
else {
    cout << ":)" << endl;
}

 内存如下：

运行结果：

这段代码中，strCmp的：

char *s1 = *(char**)vp1;
char *s1 = (char*)vp1;

有什么区别？

这里实际在内存中执行向前两跳的操作，即指向vp1所对应的字符串首地址，然后解引用。

如果直接char *s1 = (char*)vp1，那么s1就是vp1所在的内存地址所对应“字符串”了。

如果要实现，该怎么做呢？

那么我们对内存执行一跳就好了：

注意哪些地方有改动

int strCmp(void *vp1, void *vp2) {
    char *s1 = (char*)vp1;
    char *s2 = (char*)vp2;
    return strcmp(s1, s2);
}

char *notes[] = {"Ab", "F#", "B", "Gb", "D"};
char *favorateNode = "Eb";
char **found = (char**)lsearch3(favorateNode, notes, 5, sizeof(char), strCmp);
if (found == NULL) {
    cout << ":(" << endl;
}
else {
    cout << ":)" << endl;
}

 运行结果：

二分搜索

上面说的都是线性搜索，编写一个二分搜索的泛型函数：

我自己实现了一下，老师只给了模板：

void *bsearch(void *key, void *base, int b, int n, int elemSize, int (*cmp)(void *, void *)) {
    int low = b, high = n, middle;
    middle = (low + high) / 2;
    void *elemAddr = (char*)base + middle * elemSize;
    if (cmp(key, elemAddr) == 0) {
        cout << middle << endl;
        return elemAddr;
    }
    else if (low < high) {
        if (cmp(key, elemAddr) > 0) {
            return bsearch(key, base, middle, n, elemSize, cmp);
        }
        else if (cmp(key, elemAddr) < 0) {
            return bsearch(key, base, b, middle, elemSize, cmp);
        }
    }
    return NULL;
}

int array2[] = {2, 3, 4, 6, 7, 11};
int size = 6;
int number = 7;
int *found2 = (int*)bsearch(&number, array2, 0, size, sizeof(int), intCmp);
if (found2 == NULL) {
    cout << ":(" << endl;
}
else {
    cout << ":)" << endl;
}

运行结果：

栈

来使用内存操作的思想，编写一个栈。

我们写一个stack.h的头文件如下，定义一下栈的结构和常用操作：

typedef struct {
    int *elems;
    int logicalLength;
    int allocLength;
}stack;
void StackNew(stack *s);
void StackDispose(stack *s);
void StackPush(stack *s, int value);
void StackPop(stack *s);

函数实现如下： 

void StackNew(stack *s) {
    s->logicalLength = 0;
    s->logicalLength = 4;
    s->elems = (int*)malloc(4 * sizeof(int));
    assert(s->elems != NULL);
}