一、字符串的复制

1、指针形式

void Mystrcpy(char *des, const char *src)
{//1，/*while(*src!='\0'){*des = *src;des++;src++;}*///2，/*while (*src != '\0'){*des++ = *src++;//优先级： ++ 高于 * 高于 =}*///3，//while (*des++ = *src++)*des = '\0';
}
int main()
{char str1[] = "abcdefg";//不能写成char *str1 = "abcdefg";//因为指针不是自己的空间，是指向字符常量的，不能被修改char *str2 = "hello";Mystrcpy(str1, str2);//4，strcpy(str1, str2);printf("%s\n", str1);//5，//printf("%s\n", strcpy(str1, str2));//支持链式表达return 0;
}

2、数组形式

void Mystrcpy(char *des, char *src)
{int i;for (i = 0; src[i] != '\0'; i++){des[i] = src[i];}des[i] = '\0';//加上\0后表示无论arr有多长，都可以结束复制
}
int main()
{char arr[] = "hellojfksd";char brr[] = "world";Mystrcpy(arr, brr);printf("%s\n", arr);return 0;
}

二、字符串的连接

char *Mystrcat(char *str1,char*str2)
{char *begin = str1;while( *str1!='\0' ){str1++;}while (*str2 != '\0'){*str1++= *str2++;}*str1 = '\0';//字符串遇见‘\0’才算结束return begin;
}
int main()
{char str1[20] = "abcde";char *str2 = "hello";Mystrcat(str1, str2);printf("%s\n", str1);return 0;
}

三、字符串的比较

字符串比较的是ASCII的值，并不是字符串的长短

int Mystrcmp(const char *str1, const char*str2)
{while (*str1 == *str2){if (*str1 == '\0'){return 0;}str1++;str2++;}if (*str1 > *str2){return 1;}else{return -1;}
}
int main()
{char *str1 = "abcdef";char *str2 = "abcde";printf("%d\n", Mystrcmp(str1, str2));return 0;
}

四、字符串与数字之间的转换

1、将数字转为字符串形式//1234->“1234”

char *Myitoa(char *str, int num)
{int i = 0;while (num != 0){str[i] = num % 10 + 48;//48是‘\0’的ASCII的值i++;num /= 10;}str[i] = '\0';//此时str中是数组的逆置i--;//--后才是数组的下标char tmp = 0;for (int j = 0; j < i; i--, j++){tmp = str[i];str[i] = str[j];str[j] = tmp;}return str;
}
int main()
{char src[100];int a = 1234;Myitoa(src, a);printf("%s\n", src);return 0;
}

2、将数字字符串转为数字形式 //“1234”->1234,“a123”->0,“12a34”->12

int Myatoi(const char *str)
{int flg = 1;int tmp = 0;assert( str != NULL);//判空while (*str == ' '){str++;}if (*str == '-'){flg = -1;str++;}else if (*str == '+'){flg = 1;str++;}while (isdigit(*str))//isdigit函数，主要用于检查其参数是否为十进制数字字符//C语言中需要加上头文件#include <ctype.h>，C++加上#include<cctype>{tmp = tmp * 10 + *str - '0';str++;}return flg *tmp;
}
int main()
{char str1[100] = "12a 34";char str2[100] = "a1223";char str3[100] = "-123";printf("%d\n", Myatoi(str1));printf("%d\n", Myatoi(str2));printf("%d\n", Myatoi(str3));return 0;
}