修改用C为const char * [英] Modify const char * in C
问题描述
我在练了采访。即时通讯目前停留在问题被逆转C.常量字符串我知道，既然是str2的常量，我可以修改的位置STR2点，而不是它的价值。我有以下称reverse_const功能。这将扭转地方为const char * str_const并打印。然而，当我尝试打印ST2从主要方法逆转后，该字符串不再转回。它像reverse_const（）暂时改变STR2的存储位置。我在做什么错在这里？
的#include＆LT;＆stdio.h中GT;
＃包括LT＆;＆string.h中GT;void反转（字符* STR）{
INT c_size = strlen的（STR）;
字符* c_begin = str中，* c_end = STR +（c_size - 1）; INT I;
对于（i = 0; I＆LT; c_size / 2;我++）{
* ^ c_begin = * c_end;
* ^ c_end = * c_begin;
* ^ c_begin = * c_end; c_begin ++;
c_end--;
}
}无效reverse_const（为const char * str_const）{
INT c_size = strlen的（str_const）;
焦炭海峡[c_size]
的strcpy（STR，str_const）;
字符* c_begin = str中，* c_end = STR +（c_size - 1）; INT I;
对于（i = 0; I＆LT; c_size / 2;我++）{
* ^ c_begin = * c_end;
* ^ c_end = * c_begin;
* ^ c_begin = * c_end; c_begin ++;
c_end--;
} str_const =海峡;
的printf（％S \\ n，str_const）;
}诠释主（）{
STR1的char [] =夺宝;
的char * str2的=肯德基;
的printf（测试：\\ nString 1 pre-逆转：％S \\ n，STR1）;
反向（STR1）;
的printf（字符串1逆转后：％S \\ n，STR1）;
的printf（常量字符串2 pre-逆转：％S \\ n，STR2）;
reverse_const（STR2）;
的printf（常量字符串2逆转后：％S \\ n，STR2）;
}
如果你想扭转 STR2 回的main（），你要么需要一个足够大的缓冲区传递到 reverse_const 持有相反的字符串，或者您需要动态地分配它存储在 reverse_const （局部变量长度数组不会做）：
的#include＆LT;＆stdlib.h中GT;
...
无效reverse_const（为const char ** str_const）
{
INT c_size = strlen的（* str_const）;
字符*海峡=释放calloc（c_size + 1，sizeof的* STR）;
的strcpy（STR，* str_const）;
字符* c_begin = str中，* c_end = STR +（c_size - 1）; INT I;
对于（i = 0; I＆LT; c_size / 2;我++）{
* ^ c_begin = * c_end;
* ^ c_end = * c_begin;
* ^ c_begin = * c_end; c_begin ++;
c_end--;
} * str_const = str中;
的printf（％S \\ n，* str_const）;
}诠释主要（无效）{ STR1的char [] =夺宝;
的char * str2的=肯德基; 的printf（测试：\\ nString 1 pre-逆转：％S \\ n，STR1）; 反向（STR1）; 的printf（字符串1逆转后：％S \\ n，STR1）;
的printf（常量字符串2 pre-逆转：％S \\ n，STR2）; reverse_const（（为const char **）及STR2）; 的printf（常量字符串2逆转后：％S \\ n，STR2）; 免费（STR2）; 返回0;
}
输出
$ ./bin/revconststr
测试：
串1 pre-逆转：印第安纳
串1后的逆转：anaidnI
常量字符串2 pre-逆转：肯塔基州
ykcutneK
常量字符串2逆转后：ykcutneK
返回指针
您有更多的选择，指针返回 STR 分配到 STR2 在的main（）。这更是你通常会期望看到的。让我知道如果你有任何问题：
的char * reverse_const2（为const char ** str_const）
{
INT c_size = strlen的（* str_const）;
字符*海峡=释放calloc（c_size + 1，sizeof的* STR）;
的strcpy（STR，* str_const）;
字符* c_begin = str中，* c_end = STR +（c_size - 1）; INT I;
对于（i = 0; I＆LT; c_size / 2;我++）{
* ^ c_begin = * c_end;
* ^ c_end = * c_begin;
* ^ c_begin = * c_end; c_begin ++;
c_end--;
} // * str_const = str中;
的printf（％S \\ n，* str_const）; 返回海峡;
}INT主要（无效）
{ STR1的char [] =夺宝;
的char * str2的=肯德基; 的printf（测试：\\ nString 1 pre-逆转：％S \\ n，STR1）; 反向（STR1）; 的printf（字符串1逆转后：％S \\ n，STR1）;
的printf（常量字符串2 pre-逆转：％S \\ n，STR2）; STR2 = reverse_const2（（为const char **）及STR2）; 的printf（常量字符串2逆转后：％S \\ n，STR2）; 免费（STR2）; 返回0;
}
I am practicing for interviews. The problem Im currently stuck on is reversing a constant string in C. I know that since str2 is const, I can modify the location str2 points to, but not its value. I have a function below called reverse_const. It will reverse the const char *str_const in place and print it. However, when I try to print st2 after reversal from the main method, the string isn't reversed anymore. Its like reverse_const() is temporarily changing the memory location of str2. What am I doing wrong here?
#include <stdio.h>
#include <string.h>
void reverse(char *str){
int c_size = strlen(str);
char *c_begin = str, *c_end = str + (c_size - 1);
int i;
for(i = 0; i < c_size / 2; i++){
*c_begin ^= *c_end;
*c_end ^= *c_begin;
*c_begin ^= *c_end;
c_begin++;
c_end--;
}
}
void reverse_const(const char *str_const){
int c_size = strlen(str_const);
char str[c_size];
strcpy(str, str_const);
char *c_begin = str, *c_end = str + (c_size - 1);
int i;
for(i = 0; i < c_size / 2; i++){
*c_begin ^= *c_end;
*c_end ^= *c_begin;
*c_begin ^= *c_end;
c_begin++;
c_end--;
}
str_const = str;
printf("%s\n", str_const);
}
int main(){
char str1[] = "Indiana";
char *str2 = "Kentucky";
printf("TESTS:\nString 1 pre-reversal: %s\n", str1);
reverse(str1);
printf("String 1 post-reversal: %s\n", str1);
printf("Constant string 2 pre-reversal: %s\n", str2);
reverse_const(str2);
printf("Constant string 2 post-reversal: %s\n", str2);
}
If you want the reversed str2 back in main(), you will either need to pass an adequately sized buffer to reverse_const to hold the reversed string, or you will need to dynamically allocate storage for it in reverse_const (a local variable length array won't do):
#include <stdlib.h>
...
void reverse_const (const char **str_const)
{
int c_size = strlen (*str_const);
char *str = calloc (c_size + 1, sizeof *str);
strcpy (str, *str_const);
char *c_begin = str, *c_end = str + (c_size - 1);
int i;
for (i = 0; i < c_size / 2; i++) {
*c_begin ^= *c_end;
*c_end ^= *c_begin;
*c_begin ^= *c_end;
c_begin++;
c_end--;
}
*str_const = str;
printf ("%s\n", *str_const);
}
int main (void) {
char str1[] = "Indiana";
char *str2 = "Kentucky";
printf ("TESTS:\nString 1 pre-reversal: %s\n", str1);
reverse (str1);
printf ("String 1 post-reversal: %s\n", str1);
printf ("Constant string 2 pre-reversal: %s\n", str2);
reverse_const ((const char **)&str2);
printf ("Constant string 2 post-reversal: %s\n", str2);
free (str2);
return 0;
}
Output
$ ./bin/revconststr
TESTS:
String 1 pre-reversal: Indiana
String 1 post-reversal: anaidnI
Constant string 2 pre-reversal: Kentucky
ykcutneK
Constant string 2 post-reversal: ykcutneK
Returning The Pointer
You have an additional option to return the pointer to str to assign to str2 in main(). This is more what you would normally expect to see. Let me know if you have any questions:
char *reverse_const2 (const char **str_const)
{
int c_size = strlen (*str_const);
char *str = calloc (c_size + 1, sizeof *str);
strcpy (str, *str_const);
char *c_begin = str, *c_end = str + (c_size - 1);
int i;
for (i = 0; i < c_size / 2; i++) {
*c_begin ^= *c_end;
*c_end ^= *c_begin;
*c_begin ^= *c_end;
c_begin++;
c_end--;
}
//*str_const = str;
printf ("%s\n", *str_const);
return str;
}
int main (void)
{
char str1[] = "Indiana";
char *str2 = "Kentucky";
printf ("TESTS:\nString 1 pre-reversal: %s\n", str1);
reverse (str1);
printf ("String 1 post-reversal: %s\n", str1);
printf ("Constant string 2 pre-reversal: %s\n", str2);
str2 = reverse_const2 ((const char **)&str2);
printf ("Constant string 2 post-reversal: %s\n", str2);
free (str2);
return 0;
}
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