我知道我可以使用
substr()
函数获取字符串的前n
个字符。不过,我想删除最后几个字符。在C语言中,是否可以像在Python中那样使用-2
或-3
作为结束位置呢?substr()
函数获取字符串的前n
个字符。不过,我想删除最后几个字符。在C语言中,是否可以像在Python中那样使用-2
或-3
作为结束位置呢?int main()
{
char s[] = "I am a string";
int len = strlen(s);
s[len-3] = '\0';
printf("%s\n",s);
}
"我是一个str"
C语言与Python不同,字符串索引并不是“智能”的。说 str[-3]
的意思就是“在开头之前三个字节的字符”;访问这块内存是未定义行为。
如果你想要将一个字符串的最后几个字符作为另一个字符串返回,只需获取指向所需第一个字符的指针即可:
char *endstr = str + (strlen(str) - 3); // get last 3 characters of the string
\0
) 即可。str[strlen(str)-3] = 0; // delete last three characters
substr()
函数的代码,包括测试代码。请注意,测试代码没有推动边界 - 缓冲区长度小于所请求的字符串或缓冲区长度为零。
#include <string.h>
extern void substr(char *buffer, size_t buflen, char const *source, int len);
/*
** Given substr(buffer, sizeof(buffer), "string", len), then the output
** in buffer for different values of len is:
** For positive values of len:
** 0 ""
** 1 "s"
** 2 "st"
** ...
** 6 "string"
** 7 "string"
** ...
** For negative values of len:
** -1 "g"
** -2 "ng"
** ...
** -6 "string"
** -7 "string"
** ...
** Subject to buffer being long enough.
** If buffer is too short, the empty string is set (unless buflen is 0,
** in which case, everything is left untouched).
*/
void substr(char *buffer, size_t buflen, char const *source, int len)
{
size_t srclen = strlen(source);
size_t nbytes = 0;
size_t offset = 0;
size_t sublen;
if (buflen == 0) /* Can't write anything anywhere */
return;
if (len > 0)
{
sublen = len;
nbytes = (sublen > srclen) ? srclen : sublen;
offset = 0;
}
else if (len < 0)
{
sublen = -len;
nbytes = (sublen > srclen) ? srclen : sublen;
offset = srclen - nbytes;
}
if (nbytes >= buflen)
nbytes = 0;
if (nbytes > 0)
memmove(buffer, source + offset, nbytes);
buffer[nbytes] = '\0';
}
#ifdef TEST
#include <stdio.h>
struct test_case
{
const char *source;
int length;
const char *result;
};
static struct test_case tests[] =
{
{ "string", 0, "" },
{ "string", +1, "s" },
{ "string", +2, "st" },
{ "string", +3, "str" },
{ "string", +4, "stri" },
{ "string", +5, "strin" },
{ "string", +6, "string" },
{ "string", +7, "string" },
{ "string", -1, "g" },
{ "string", -2, "ng" },
{ "string", -3, "ing" },
{ "string", -4, "ring" },
{ "string", -5, "tring" },
{ "string", -6, "string" },
{ "string", -7, "string" },
};
enum { NUM_TESTS = sizeof(tests) / sizeof(tests[0]) };
int main(void)
{
int pass = 0;
int fail = 0;
for (int i = 0; i < NUM_TESTS; i++)
{
char buffer[20];
substr(buffer, sizeof(buffer), tests[i].source, tests[i].length);
if (strcmp(buffer, tests[i].result) == 0)
{
printf("== PASS == %2d: substr(buffer, %zu, \"%s\", %d) = \"%s\"\n",
i, sizeof(buffer), tests[i].source, tests[i].length, buffer);
pass++;
}
else
{
printf("!! FAIL !! %2d: substr(buffer, %zu, \"%s\", %d) wanted \"%s\" actual \"%s\"\n",
i, sizeof(buffer), tests[i].source, tests[i].length, tests[i].result, buffer);
fail++;
}
}
if (fail == 0)
{
printf("== PASS == %d tests passed\n", NUM_TESTS);
return(0);
}
else
{
printf("!! FAIL !! %d tests out of %d failed\n", fail, NUM_TESTS);
return(1);
}
}
#endif /* TEST */
函数声明应该在适当的头文件中。变量sublen
可帮助代码在以下情况下编译得更加干净:
gcc -O3 -g -std=c99 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
-Wold-style-definition -Werror -DTEST substr.c -o substr
测试结果:
== PASS == 0: substr(buffer, 20, "string", 0) = ""
== PASS == 1: substr(buffer, 20, "string", 1) = "s"
== PASS == 2: substr(buffer, 20, "string", 2) = "st"
== PASS == 3: substr(buffer, 20, "string", 3) = "str"
== PASS == 4: substr(buffer, 20, "string", 4) = "stri"
== PASS == 5: substr(buffer, 20, "string", 5) = "strin"
== PASS == 6: substr(buffer, 20, "string", 6) = "string"
== PASS == 7: substr(buffer, 20, "string", 7) = "string"
== PASS == 8: substr(buffer, 20, "string", -1) = "g"
== PASS == 9: substr(buffer, 20, "string", -2) = "ng"
== PASS == 10: substr(buffer, 20, "string", -3) = "ing"
== PASS == 11: substr(buffer, 20, "string", -4) = "ring"
== PASS == 12: substr(buffer, 20, "string", -5) = "tring"
== PASS == 13: substr(buffer, 20, "string", -6) = "string"
== PASS == 14: substr(buffer, 20, "string", -7) = "string"
== PASS == 15 tests passed
假设您删除了为什么这样不行:
memcpy(new_string, old_string, strlen(old_string) - 3; &new_string[strlen(old_string) - 3] = '\0'
假设new_string
和old_string
都是char
指针且strlen(old_string) > 3
?
&
,插入了缺失的)
和;
,指针指向有效的非重叠位置,并且长度条件得到满足,那么就可以将旧字符串除最后3个字符外的所有内容复制到新字符串中。您可以通过将其嵌入到一些测试代码中进行测试。它并没有尝试处理复制旧字符串的最后三个字符,这似乎是问题的主要关注点。#include <string.h>
#include <stdio.h>
int main(void)
{
char new_string[32] = "XXXXXXXXXXXXXXXX";
char old_string[] = "string";
memcpy(new_string, old_string, strlen(old_string) - 3);
new_string[strlen(old_string) - 3] = '\0';
printf("<<%s>> <<%s>>\n", old_string, new_string);
return(0);
}
输出:
<<string>> <<str>>
但要小心棘手的巧合;我选择了一个6个字符长的旧字符串样本,-3也会使'length -3'等于3。要获取最后N个字符,您需要编写更像这样的代码:
#include <assert.h>
#include <string.h>
#include <stdio.h>
int main(void)
{
int N = 3;
char new_string[32] = "XXXXXXXXXXXXXXXX";
char old_string[] = "dandelion";
int sublen = strlen(old_string) - N;
assert(sublen > 0);
memcpy(new_string, old_string + sublen, N);
new_string[N] = '\0';
printf("<<%s>> <<%s>>\n", old_string, new_string);
return(0);
}
输出:
<<dandelion>> <<ion>>
substr(strlen(str)-4,3);
请记住,字符串的下标从0开始,因此这会给你最后3个字符。
因此,一般的技巧是
substr(strlen(str)-n-1,n);
(当然,字符串必须比n
更长)
如果您想获取最后3个字符,请使用以下代码:
substr(0,strlen(str)-4);
substr(0,strlen(str)-n-1);
我注意到substr
不是标准的C函数,因此在C中使用它是无效的。因此,为了通过消除最后几个字符来查找子字符串,可以使用memcpy(new_string, old_string, strlen(old_string) - 3)
strlen(old_string) < 3
,它也会遇到问题。 - Jonathan Lefflermemcpy
或其他方式获取一个空终止字符串? - TheRookierLearnermemcpy(new_string, old_string, strlen(old_string) - 3; &new_string[strlen(old_string) - 3] = '\0'
假设 new_string
和 old_string
都是 char 指针且 strlen(old_string) > 3。 - TheRookierLearner
s
是一个字符缓冲区而不是一个常量字符串。 - nneonneo