3.C/C++字符串

1.不使用库函数将整数转换为字符串

C语言提供了将几个标准库函数，可以将任意类型的（整形、浮点型、长整形）的数字转换为字符串

itoa（）:将整形值转换为字符串 
ltoa（）;将长整形转换为字符串 
ultoa（）；将无符号长整形转换为字符串 
gcvt（）；将浮点数转换为字符串，取四舍五入 
ecvt（）；将双精度浮点数转换为字符串，转换结果中不包含十进制小数点 
fcvt（）；将指定位数转换精度 
还可以使用sprintf系类函数把数字转换为字符串，这种方式比itoa系列函数速度慢

#include<stdio.h>#include<iostream>using namespace std;void int2str(int n, char*str){char buf[10] = {0};  //临时数组int i = 0;    //临时数组buf的下标int j = 0;   //str的下标int len=0;int temp = n < 0 ? -n : n; //取数字n的绝对值if (n < 0)  //首先对负数做处理{str[j] = '-';j++;}//将数字倒序放在临时数组while (temp){buf[i] = temp%10 + '0';i++;temp /= 10;}//转换len = i - 1;while (len>=0){str[j] = buf[len];j++;len--;}str[j] = '\0';}int main(){int num;char p[10];cout << "please input the number:" << endl;cin >> num;int2str(num, p);cout << "ouput: " << p << endl;return 0;}

please input the number:

-1234
ouput: -1234

2.不使用库函数将字符串转换为数字

与数字转换为字符串类似，C/C++提供了几个标准的库函数，可以将字符串转换为任意类型（整型，长整型，浮点型）的数字。下面列举其函数

atof（）；将字符串转换为双精度浮点型值 
atoi（）；将字符串转换为整型值 
atol（）；将字符串转换为长整型 
strtod（）；将字符串转换为双精度浮点型值，并报告不能被转换的所有剩下数字 
strtol（）；将字符串转换为长整型值，并报告不能被转换的所有剩下的数字 
strtoul（）；将字符串转换为无符号长整型，并报告不能被转换的所有剩下的数字

//2.不使用库函数将数字符串转换为数字#include<iostream>using namespace std;int str2int(const char * str){int temp=0;const char *ptr = str;  //ptr保存str字符串开头if (*str == '-' || *str == '+') //如果第一个字符是正负号，则移到下一个字符{str++;}while (*str != 0){if ((*str < '0') || *str > '9')  //如果当前字符不是数字，则退出循环{break;}temp = temp * 10 + (*str - '0');  //如果当前字符是数字，则计算数值，然后移到下一个字符str++;}if(*ptr=='-')  //如果字符串以‘-’开头，则转换成其相反数{temp = -temp;}return temp;}int main(){int n = 0;char p[10] = "";cout << "input: " << endl;//cin >> p[10];cin.getline(p, 20);n = str2int(p);cout << "output:" << n << endl;return 0;}

input:
-1234
output:-1234

3.编程实现strcpy函数

//3.编程实现strcpy函数#include<stdio.h>char * strcpy(char * strDest, const char * strSrc){if ((strDest == NULL) || (strSrc == NULL)){return NULL;}char *strDestCopy = strDest; //保存目标字符串的首地址int i = 0;//for (i = 0; strSrc[i] != '\0'; strDest[i] = strSrc[i], i++)while (strSrc[i]!='\0'){strDest[i] = strSrc[i]; //把strSrc字符串的内容复制到strDest中i++;}strDest[i] = '\0'; //加入结束符return strDestCopy;  //返回首地址}int main(){char strSrc[] = "hello world";char strDest[20];strcpy(strDest, strSrc);printf("strDest:%s\n", strDest);return 0;}

strDest:hello world

4.编程实现计算字符串的长度

//4.编程实现计算字符串的长度#include<stdio.h>int strlen1(const char* src){if (src == NULL)   //判断src是否有效{return 0;}int len = 0;while (*src!='\0'){*src++;len++;}return len;}int strlen2(const char* src){if (src == NULL)  //判断src是否有效{return 0;}const char *temp = src;  //保存src的首地址while (*src!='\0'){src++;}return (src - temp); //返回指针差}int main(){char p[] = "Hello World!";printf("Strlen1 len: %d\n", strlen1(p));printf("Strlen2 len: %d\n", strlen2(p));return 0;} //第二种方法效率更好

Strlen1 len: 12
Strlen2 len: 12

5.编程实现字符串中子串的查找

编写一个函数，实现strstr，即从一个字符串中，查找另一个字符串的位置，如果输入strstr("12345","34")返回值为2，即在2号位置找到字符串34.

#include<stdio.h>int strstr(const char* src, const char* sub){const char *bp = src; //定义指针bp指向src首地址const char *sp = sub;int flag = 0;const char *temp = src; //定义一个临时指针temp，暂时先指向src首地址if (src == NULL || sub == NULL)  //判断src和sub是否有效{return 0;}while (*src)  //遍历src{sub = sp;while (*src == *sub) //如果src与sub字符相等进入，如果不等，src++{src++;  //然后src和sbu都加1，比较下一个字符是否相等sub++;flag = 1;if (*sub == '\0')  //如果sub结束了，那就是找到了子串{return (temp - bp);  //输出子串的位置，bp是首地址，temp存储的是第一个相等字符的前一个break;                   //字符的位置，然后与首地址相减再加1，就得到最终结果}}if (flag == 1) //flag的作用是如果在上一个while循环中src执行了+1，这里就不需要再执行一次了{              //所有先对src--，然后在src++src--;flag = 0;}src++;  //src加1，比较下一个字符temp = src;if (*src == '\0') //如果src下一个字符是空，那就结束，{return 0;}}}int main(){char p[] = "123345";char q[] = "34";printf("the position of sub is : %d\n", strstr(p, q));return 0;}

the position of sub is : 3

6.编程判断字符串是否是回文，例如“level”是回文

//6.编程判断字符串是否是回文//例如“level”是回文#include<iostream>using namespace std;int IsRevStr(char * str){int i, len;int found = 1;  //1表示是回文，0表示不是回文if (str == NULL){return -1;}len = strlen(str);for (i = 0; i < len / 2; i++){if (str[i] != str[len - i - 1])  //不是回文{found = 0;  break;}}return found;}int main(){char str1[] = "1234321";char str2[] = "1234221";int test1 = IsRevStr(str1);int test2 = IsRevStr(str2);cout << "str1 is " << (test1 == 1 ? "" : "not ") << "reverse string." << endl;cout << "str2 is " << (test2 == 1 ? "" : "not ") << "reverse string." << endl;return 0;}

str1 is reverse string.
str2 is not reverse string.

7.编程实现stcmp库函数
对于两个字符串str1，str2，若相等，则返回0，若str1大于str2，则返回1，若str1小于str2，则返回-1

//7.编程实现stcmp库函数//对于两个字符串str1，str2，若相等，则返回0，若str1大于str2，则返回1，若str1小于str2，则返回-1#include<iostream>using namespace std;int mystrcmp(const char*src, const char* dst){int ret = 0;while (!(ret=*(unsigned char *)src-*(unsigned char *)dst) && *dst){src++;  //循环比较两个字符串是否相等，如果不等或者dst字符串到了末尾，退出循环，dst++;  //与后面的*dst和*src有一个在就可以，如果src到了结尾，而dst没到结尾，则!ret=0,同样会结束循环}if (ret < 0)ret = -1;else if (ret > 0)ret = 1;return ret;}int main(){char str[10] = "1234567";char str1[10] = "1234567";char str2[10] = "12345678";char str3[10] = "1234566";int test1 = mystrcmp(str, str1);int test2 = mystrcmp(str, str2);int test3 = mystrcmp(str, str3);cout << "test1 = " << test1 << endl;cout << "test2 = " << test2 << endl;cout << "test3 = " << test3 << endl;return 0;}

test1 = 0
test2 = -1
test3 = 1

8.编写字符串反转函数strrev

如：输入“abcd”,输出应为“dcba”

//8.编写字符串反转函数strrev#include<iostream>using namespace std;char* strrev1(char* str)  //遍历字符串{                         //第一个与最后一个交换，依次循环int len = strlen(str);char *tmp = new char[len + 1];strcpy_s(tmp,len+1,str);for (int i = 0; i < len / 2; i++){char c = tmp[i];tmp[i] = tmp[len - i-1];tmp[len - i - 1] = c;}return tmp;} int main(){char * str = "123456";cout << str << endl;char* str1 = strrev1(str);cout << "strrev1(str) = " << str1 << endl;return 0;}

123456
strrev1(str) = 654321

9.编程实现任意长度的两个正整数相加

//9.编程实现任意长度的两个正整数相加#include<stdio.h>#include<string.h>#include<stdlib.h>#include<math.h>char * addBigInt(char * num1, char *num2){int c = 0;// 进位，开始最低进位为0int i = strlen(num1)-1;   //指向第一个加数的最低位int j = strlen(num2)-1;   //指向第二个加数的最低位//得到两个数中较大数的位数并加1，作为结果的位数int maxLength = strlen(num1) >= strlen(num2) ? strlen(num1) + 1 : strlen(num2) + 1; char *rst = (char*)malloc(maxLength + 1);  //保存结果int k;if (rst == NULL){printf("malloc error\n");exit(1);}    rst[maxLength] = '\0';  //字符串最后一位为'\0';k = strlen(rst) - 1;   //指向结果数组的最低位while ((i>=0)&&(j>=0)){rst[k] = ((num1[i] - '0') + (num2[j] - '0') + c) % 10 + '0';c = ((num1[i] - '0') + (num2[j] - '0') + c) / 10;i--;j--;k--;}while (i>=0){rst[k] = ((num1[i] - '0')+ c) % 10 + '0';c = ((num1[i] - '0')+ c) / 10;i--;k--;}while (j>=0){rst[k] = ((num2[j] - '0') + c) % 10 + '0';c = ((num2[j] - '0') + c) / 10;j--;k--;}rst[0] = c +'0';if (rst[0] !='0'){return rst;}else{return rst + 1;   //如果最高位等于零，那么就从结果str的第二个元素开始输出}}int main(){char num1[] = "123";char num2[] = "996";char *result = NULL;result = addBigInt(num1, num2);printf("%s + %s = %s\n", num1, num2, result);return 0;}

123 + 996 = 1119

10.编程实现字符串的循环右移

编写函数能把一个char组成的字符串循环右移n个，例如原来是“abcdefghi”，如果n=2，移位后应该是“hiabcdefg”

//10.编程实现字符串的循环右移#include<stdio.h>#include<stdlib.h>#include<string.h>void loopMove(char* str, int n){int i = 0;char *temp = NULL;int strLen = strlen(str);char *head = str;n = n%strLen;temp = (char*)malloc(n);for (i = 0; i < n; i++){temp[i] = head[strLen - n + i];}for (i = strLen - 1; i >= n; i--){head[i] = head[i - n];}for (i = 0; i < n; i++){head[i] = temp[i];}free(temp);}int main(){char string[] = "abcdefghi";int steps = 0;printf("string: %s\n", string);printf("input steps: ");scanf_s("%d", &steps);loopMove(string,steps);printf("after loopMove %d: %s\n", steps, string);return 0;}

string: abcdefghi
input steps: 2
after loopMove 2: hiabcdefg

11.删除制定长度的字符

例如：假设一个字符串“abcdefg”，如果要从第二个开始（索引为1），删除两个字符，则删除后的字符串是“adefg”

//11.删除制定长度的字符#include<stdio.h>#include<string.h>#include<iostream>using namespace std;char *deleteChars(char* str, int pos, int len){char *p = str + pos - 1;//指向pos位置字符int tt = strlen(str);   //计算字符长度if ((pos < 1) || (p - str) > tt) //检查pos是否大于1，或者pos超出字符串的长度{return str;}if ((p + len - str) > tt)  //len大于pos后剩余的字符个数{while (*p)  //那就把pos后面的全部删掉，直到结束{*p = '\0';p++;}}while (*p && *(p+len)){*p = *(p + len);p++;}*p = '\0';return str;}int main(){char string[] = "abcdefg";int pos = 0;int len = 0;int steps = 0;printf("string: %s\n", string);printf("input pos: ");cin >> pos;cout << "input len: ";cin >> len;deleteChars(string, pos, len);printf("after delete %d chars behind pos %d: %s\n", len, pos, string);return 0;}

string: abcdefg

input pos: 2
input len: 2
after delete 2 chars behind pos 2: adefg

12.编程实现删除字符串中所有指定的字符

假设字符串为“cabcdefcgchci”，把该字符串中所有的字符'c'删除后，结果为“abdefghi”

//12.编程实现删除字符串中所有制定的字符#include<stdio.h>#include<iostream>using namespace std;char * deleteChar(char* str, char c){char* head = str;char* p = str;if (str == NULL)  //检查str的有效性{return NULL;}while (*p){if (*p != c){*str = *p; //如果不等于c的值，则记录在str中str++;}p++;}*str = '\0';  //结束符return head;}int main(){char string[] = "cabcdefcgchci";char c = 0;cout << "input char: ";cin >> c;cout << "before delete: " << string << endl;deleteChar(string, c);  //删除所有的ccout << "after delete: " << string << endl;return 0;}

input char: c

before delete: cabcdefcgchci
after delete: abdefghi

13.编程实现库函数strcat，实现字符串的拼接

//13.编程实现库函数strcat，拼接函数#include<stdio.h>#include<stdlib.h>#include<iostream>using namespace std;char *mystrcat(char *dest, const char* src){char *ret = dest;//保存目的字符串的首地址，以便返回while (*dest){dest++;}while (*src){*dest = *src;src++;dest++;}*dest = '\0'; //结束符return ret;}int main(){char *dest = NULL;char *str1 = "Hello ";char *str2 = "World!";dest = (char*)malloc(256);*dest = '\0'; //dest里面是空mystrcat(mystrcat(dest, str1), str2);cout << "dest: " << dest << endl;free(dest);dest = NULL;return 0;}

dest: Hello World!