c++课后作业

把字符串转换为整数

int main()
{char pn[21];cout << "请输入一个由数字组成的字符串： ";cin >> pn;int last = 0;int res[10];int j = strlen(pn);int idx = 2;cout << "请选择（2-二进制，10-十进制）： ";cin >> idx;for (int i = 0,len = strlen(pn);i < len; i++){if (pn[i] < '0' || pn[i] > '9') { cout << "非法字符！！！"; break; }last += (pn[i] - '0') * pow(idx,j-1);j = j - 1 ;}cout << last;
}

重写strcat和strncat

char* mystrcat(char* dest, const char* src)
{memcpy(dest + strlen(dest), src, strlen(src) + 1);return dest;
}char* mystrncat(char* dest, const char* src, const size_t n)
{size_t len = strlen(dest);memcpy(dest + len, src, n);*(dest + len + n) = 0;return dest;}

重写strchr和strrchr

注意⚠️：这里不能从后往前找，因为想从后往前找就只能使用strlen，而strlen本质就是遍历一遍字符串，这样就更慢了。

const char* mystrchr(const char* s, int c)
{char* p = (char*)s;while (*p){if (*p == c) return p;p++;}return 0;
}const char* mystrrchr(const char* s, int c) //查找字符
{char* p = (char*)s;char* p1 = 0;while (*p){if (*p == c){p1 = p;}p++;}return p1;}

重写strcmp和strncmp

int mystrcmp(const char* str1, const char* str2)
{char* p1 = (char*)str1;char* p2 = (char*)str2;while (*p1 && *p2){if (*p1 != *p2){if (*(p1) > *(p2)) return -1;else return 1;}p1++;p2++;}return 0;
}int mystrncmp(const char* str1, const char* str2, size_t n)
{char* p1 = (char*)str1;char* p2 = (char*)str2;for (int i = 0; i < n; i++){if (*p1 != *p2){if (*(p1) > *(p2)) return -1;else return 1;}p1++;p2++;}return 0;}

重写strstr

char* caijimystrstr(const char* haystack, const char* needle)
{char* p1 = (char*)haystack;char* p2 = (char*)needle;char* tmp1;char* tmp2 = p2;while (*p1){while (*p1 == *p2){tmp1 = p1;for (int i = 0; i < strlen(needle); i++){if (*p1 != *p2){continue;}p1++;p2++;if (*p2 == 0){return p1-strlen(needle);}}continue;}p2 = tmp2;p1++;}return nullptr;}const char* dalaomystrstr(const char* haystack, const char* needle)
{size_t ii = 0, jj = 0; size_t len = strlen(haystack), slen = strlen(needle);while ((ii < len) && (jj < slen)){if (haystack[ii] == needle[jj]){ii++, jj++;}else{ii = ii - jj;jj = 0;ii++;}}if (jj == slen) return (haystack+(ii-jj));return 0;
}

解析XML

bool xmlbuffer(const char* xmlbuf, const char* fieldname, char* value = 0, const size_t ilen = 0)
{if (xmlbuf == 0 || fieldname == 0) return false;//创建字段int lenfieldname = strlen(fieldname);//开辟空间char* sfieldname = new char[lenfieldname + 3];char* efieldname = new char[lenfieldname + 4];//初始化memset(sfieldname, 0, lenfieldname +3);  // 错误代码 memset(sfieldname, 0, sfieldname); 原因：new出来的空间不能用memsetmemset(efieldname, 0, lenfieldname +4);//创建字段strcpy(sfieldname, "<"); strcat(sfieldname, fieldname); strcat(sfieldname,">");strcpy(efieldname, "</"); strcat(efieldname, fieldname); strcat(efieldname, ">");//找字段；char* start;char* end;start = (char*)strstr(xmlbuf, sfieldname);end = (char*)strstr(xmlbuf, efieldname);//没找到if (start == 0 || end == 0){delete[] sfieldname;delete[] efieldname;return 0;}size_t vallen = (end - start - lenfieldname - 2);  //指针减去指针等于数值，表示的是从 start 到 end 的距离减去了 lenfieldname（<fieldname> 的长度）和 2（< 和 > 的长度）。if (ilen == 0){strncpy(value, start + lenfieldname + 2, vallen);value[vallen] = 0;}else if (vallen <= ilen){strncpy(value, start + lenfieldname + 2, vallen);value[vallen] = 0;}else{strncpy(value, start + lenfieldname + 2, ilen);value[ilen] = 0;}delete[] sfieldname;delete[] efieldname;return 1;}

不重复的随机数

void rrand(int arr[], const size_t len, const int minvalue = 0)
{srand(time(0));for (int i = 0; i < len; i++){while (1){int tmp = rand() % len + minvalue;int j = 0;for (j = 0; j < i; j++){if (tmp == arr[j]) break;}if (j == i){arr[i] = tmp;break;}}}
}

重写strcpy和strncpy

char* my2strcpy(char* dest, const char* src)
{int i = 0;while (src[i]){dest[i] = src[i];i++;}dest[i] = 0;return dest;
}char* my2strncpy(char* dest, const char* src , const size_t n )
{if (n < 0) return 0;size_t lens = strlen(src);if (n > lens){my2strcpy(dest, src);}int i = 0;for (i = 0; i < n; i++){dest[i] = src[i];}dest[i++] = 0;return dest;}