C语言面试题（二）

　　上篇对嵌入式中C语言基本数据类型，关键字和常用操作进行了汇总，这篇我们将侧重字符串操作。请看下面的字符串处理函数：
   a.库函数
   1)将字符串src拷贝到字符数组dest内
       char *strcpy(char *dest, const char *src);
       dest 要输入的字符数组名,内存的地址
       src 要复制的字符数组名,内存地址

 1 /*************************************************************************
 2 > File Name: strcpy_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com 
 5 > Created Time: Fri 31 Jul 2015 04:29:07 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int main(void)
12 {
13     char str[]="asdfjkladfeifdfjkjfljfljlasjf";
14     char dest[BUFSIZ]="";
15     char ch[] = "ab";
16     strcpy(dest,str);
17     printf("dest = %s
",dest);
18     printf("sizeof(ch) = %ld
",sizeof(ch));//include 
19     return 0;
20 }

   2)求字符串长度
       size_t strlen(const char *s);
       字符串长度:是''前面的字母个数,不包含''
       s 要计算长度的字符串,或存字符串内存的地址

 1 /*************************************************************************
 2 > File Name: strlen_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Fri 31 Jul 2015 06:35:25 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int main(void)
12 {
13     char str[] = "qwertyuasdfgj";
14     int len;
15     len = strlen(str);
16     printf("strlen(str) = %d,%s
",len,str);//not inlcude 
17     return 0;
18 }

   3)比较字符串的大小
       int strcmp(const char *s1, const char *s2);
       s1,s2,是两个要比较的字符串
       返回的值 <0 代表s1小,=0 相等 >0 s1大

 1 /*************************************************************************
 2 > File Name: strcmp_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Fri 31 Jul 2015 06:58:19 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int main(void)
12 {
13     char *str = "adfndfkkf";
14     char *dest = "acdffgf";
15 
16     if(strcmp(dest,str) == 0)
17     {
18         printf("str == dest
");
19     }
20     else if(strcmp(dest,str) < 0)
21     {
22         printf("dest < str
");
23     }
24     else
25     {
26         printf("dest > str
");
27     }
28     
29     return 0;
30 }

   4)字符串拼接
       char *strcat(char *dest, const char *src);
       dest 目标的地址,目标的字符数组
       src 源地址,是将src字符串连接在dest字符串后面

 1 /*************************************************************************
 2 > File Name: strcat_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Fri 31 Jul 2015 07:14:01 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int main(void)
12 {
13     char *str = "abcdefg";
14     char dest[BUFSIZ] = "higklmn";
15     
16 
17     printf("strlen(str) == %d
",strlen(str));
18     printf("strlen(dest) == %d
",strlen(dest));
19     strcat(dest,str);
20     printf("String cating. . .
");
21     sleep(3);
22     printf("dest == %s
",dest);
23     printf("strlen(dest) == %d
",strlen(dest));
24     printf("sizeof(dest) == %d
",sizeof(dest));
25     return 0;
26 }

   5)在字符串中查找另一个字符串
       char *strstr(const char *haystack, const char *needle);
       haystack 一个字符串
       needle 要寻找字符串,在haystack里面查找needle这个字符串
       返回值,是在haystack内找到的字符串的地址或者说,是找到的字符数组

 1 /*************************************************************************
 2 > File Name: strstr_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Fri 31 Jul 2015 07:32:36 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int main(void)
12 {
13     char *haystack = "abcdadecdakllgd";
14     char *needle = "b";
15     char *temp;
16 
17     temp = strstr(haystack,needle);
18     printf("temp == %s
",temp);
19     return 0;
20 }

   6)从键盘上获取字符串
       char *gets(char *s);
       s 准备好的字符数组,或内存地址
　　见 7)demo
   7)将字符串输出到屏幕
       int puts(const char *s);
       s 是要输出的字符数组,或内存地址

 1 /*************************************************************************
 2 > File Name: gets_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Fri 31 Jul 2015 07:50:02 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int main(void)
12 {
13     char str[256] = "";
14 
15     gets(str);
16 //    printf("str == %s
",str);
17     puts(str);
18     return 0;
19 }

   8)将字符串按格式写入到内存,或字符数组内
       sprintf(char *str,"格式字符串",参数列表....);
       str 是准备好的字符数组
       格式字符串由%c %d %lf %f %u等组成的字符串
       char str1[256]="";
       int a=102;
       sprintf(str1,"a=%d, &a=%p, ASCII = %c ",a,&a,a);
       puts(str1);
       用途:将数值转换为字符串

 1 /*************************************************************************
 2 > File Name: sprint_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Fri 31 Jul 2015 09:16:36 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 
10 int main(void)
11 {
12     char str[256] = "";
13     int a = 102;
14 
15     sprintf(str,"a = %d,&a = %p,ascii = %c",a,&a,a);
16 //    printf("str = %s
",str);
17     puts(str);
18     return 0;
19 }

   9)把字符串转换为整数
       #include <stdlib.h>
       int atoi(const char *nptr); 把字符串转换为整型
       long atol(const char *nptr); 转换为长整型
       double atof(const char *nptr);转换为浮点

 1 /*************************************************************************
 2 > File Name: atoi_test.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Fri 31 Jul 2015 09:29:20 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 #include <stdlib.h>
11 
12 int main(void)
13 {
14     char *str = "234";    //if str[] = "abc",after atoi,temp == 0
15     int temp;
16 
17     temp = atoi(str);
18     printf("temp == %d
",temp);
19     return 0;
20 }

b.自定义的字符串处理函数

    当涉及到对字符串操作是，一定要记得这几点：
       a)若要存储字符串，声明的变量必须指定空间大小，必要是要对其初始化。如 char [256] = "";char *str = "abdefg";
       b)操作函数中的参数要用 const 关键子限定，防止外部对存储空间地址进行修改；
   1)strcpy_me

 1 /*************************************************************************
 2 > File Name: strcpy_me.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Sat 01 Aug 2015 03:00:08 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 char *fun_cpy(char *,const char *);
12 
13 int main(void)
14 {
15     char *str = "abcdefghikjlmnopqrstuswxyz";
16     char dest[256] = "";
17     dest[26] = 'v';
18 
19     fun_cpy(dest,str);
20     printf("dest:%s
",dest);
21     printf("str :%s
",str);
22     return 0;
23 }
24 
25 
26 char *fun_cpy(char *dest,const char *str)
27 {
28     int i = 0;
29     while(*(str + i) != '')
30     {
31         *(dest++) = *(str + i);
32         i++;
33     }
34     *dest = '';
35     return dest;
36 }

2)strlen_me

 1 /*************************************************************************
 2 > File Name: strlen_me.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Sat 01 Aug 2015 03:26:22 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int my_strlen(const char *);
12 
13 int main(void)
14 {
15     char *str = "abcdefg";
16     int val;
17 
18     val = my_strlen(str);
19     printf("val == %d
",val);
20     return 0;
21 }
22 
23 int my_strlen(const char *s)
24 {
25     int i = 0;
26 
27     while(*(s+i) != '')
28     {
29         i++;
30     }
31     return i;
32 }

3)strcat_me

 1 /*************************************************************************
 2 > File Name: strcat_me.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Sat 01 Aug 2015 04:00:37 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 void my_strcat(char *,const char *);
12 
13 int main(void)
14 {
15     char *str = "abcdefg";
16     char dest[256] = "higklmnl";
17 
18     my_strcat(dest,str);
19     printf("str:%s
",str);
20     printf("dest:%s
",dest);
21     return 0;
22 }
23 
24 void my_strcat(char *dest,const char *str)
25 {
26     int i;
27 
28     while(*dest++)
29         ;
30     dest -= 1;
31     while(*(str+i) != '')
32     {
33         *dest++ = *(str+i);
34         i++;
35     }
36     *dest = '';
37 }

4)strcmp_me

 1 /*************************************************************************
 2 > File Name: strcmp_me.c
 3 > Author: shuk
 4 > Mail: wsksec@gmail.com
 5 > Created Time: Sat 01 Aug 2015 04:12:51 PM CST
 6  ************************************************************************/
 7 
 8 #include <stdio.h>
 9 #include <string.h>
10 
11 int my_strcmp(const char *,const char *);
12 
13 int main(void)
14 {
15     char *str = "abcdefgbra";
16     char *dest = "abcdefgbaa";
17     int i;
18 
19     i = my_strcmp(dest,str);
20     if(i > 0)
21     {
22         printf("dest > str
");
23     }
24     else if(i < 0)
25     {
26         printf("dest < str
");
27     }
28     else
29     {
30         printf("dest = str
");
31     }
32 
33     return 0;
34 }
35 
36 int my_strcmp(const char *dest,const char *str)
37 {
38     int i = 0;
39 
40     while(1)
41     {
42         if((*(dest + i) == *(str + i)) && (*(dest + i)!=''))
43         {
44             i++;
45         }
46         else if(*(dest + i) > *(str + i))
47         {
48             return 1;
49         }
50         else if(*(dest + i) < *(str + i))
51         {
52             return -1;
53         }
54         else
55         {
56             return 0;
57         }
58     }
59 }

　　细节不足的地方请您指教，后续我也会加以说明和修改。

wsksec@gmail.com Pressing on Toward the Goal