《手写代码必备手册》-实现几个字符串API

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最新推荐文章于 2024-07-31 05:00:00 发布

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分类专栏：笔试

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1. size_t strlen(const char *str)

size_t strlen(const char *str)
{
    const char *s;
    for(s = str; *s; ++s) { }
    return (s - str);
}

2. char* strcpy(char *to, const char *from)

char* strcpy(char *to, const char *from)
{
    assert(to != NULL && from != NULL);
    char *p = to;
    while( (*p++ = *from++) != '\0');
    return to;
}

3. char* strstr(const char *haystack, const char *needle)

char *strstr(const char *haystack, const char *needle)
{
    if(!*needle) return (char*)haystack;
    const char *p1;
    const char *p2;
    const char *p1_advance = haystack;
    for(p2 = &needle[1]; *p2; ++p2)
    {
        p1_advance++;
    }
    for(p1 = haystack; *p1_advance; p1_advance++)
    {
        char *p1_old = (char*)p1;
        p2 = needle;
        while(*p1 && *p2 && *p1 == *p2)
        {
            p1++; p2++;
        }
        if(!*p2) return p1_old;
        p1 = p1_old + 1;    
    }
    return NULL;
}