C语言 如何截断C char *？
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How to truncate C char*?
提问by erandros
As simple as that. I'm on C++ btw. I've read the cplusplus.com's cstdlib library functions, but I can't find a simple function for this. I know the length of the char, I only need to eraselast three characters from it. I can use C++ string, but this is for handling files, which uses char*, and I don't want to do conversions from string to C char.
就如此容易。顺便说一句，我在使用 C++。我已经阅读了 cplusplus.com 的 cstdlib 库函数，但是我找不到一个简单的函数。我知道字符的长度，我只需要从中删除最后三个字符。我可以使用 C++ 字符串，但这是用于处理使用 char* 的文件，我不想进行从字符串到 C 字符的转换。
回答by pmg
If you don't need to copy the string somewhere else and can change it
如果您不需要将字符串复制到其他地方并且可以更改它
/* make sure strlen(name) >= 3 */
namelen = strlen(name); /* possibly you've saved the length previously */
name[namelen - 3] = 0;
If you need to copy it (because it's a string literal or you want to keep the original around)
如果您需要复制它（因为它是一个字符串文字或者您想保留原始内容）
/* make sure strlen(name) >= 3 */
namelen = strlen(name); /* possibly you've saved the length previously */
strncpy(copy, name, namelen - 3);
/* add a final null terminator */
copy[namelen - 3] = 0;
回答by thelionroars1337
I think some of your post was lost in translation.
我认为您的一些帖子在翻译中丢失了。
To truncate a string in C, you can simply insert a terminating null character in the desired position. All of the standard functions will then treat the string as having the new length.
要在 C 中截断字符串，只需在所需位置插入终止空字符即可。然后，所有标准函数都会将字符串视为具有新长度。
#include <stdio.h>
#include <string.h>
int main(void)
{
char string[] = "one one two three five eight thirteen twenty-one";
printf("%s\n", string);
string[strlen(string) - 3] = 'const char* mystring = "abc123";
const int len = 6;
const char* substring = mystring + len - 3;
';
printf("%s\n", string);
return 0;
}
回答by Anders Abel
If you know the length of the string you can use pointer arithmetic to get a string with the last three characters:
如果您知道字符串的长度，您可以使用指针算术来获取包含最后三个字符的字符串：
bool TakeOutLastThreeChars(char* src, int len) {
if (len < 3) return false;
memset(src + len - 3, 0, 3);
return true;
}
Please note that substringpoints to the same memory as mystringand is only valid as long as mystringis valid and left unchanged. The reason that this works is that a c string doesn't have any special markers at the beginning, only the NULLtermination at the end.
请注意，substring指向与相同的内存mystring并且仅mystring在有效且保持不变时才有效。这样做的原因是 ac 字符串在开头没有任何特殊标记，只有NULL结尾处有终止符。
I interpreted your question as wanting the last three characters, getting rid of the start, as opposed to how David Heffernan read it, one of us is obviously wrong.
我将你的问题解释为想要最后三个字符，去掉开头，而不是大卫赫弗南如何阅读它，我们中的一个人显然是错误的。
回答by cdiggins
char* str = "theFile.nam";
I assume mutating the string memory is safe since you did say erasethe last three characters. I'm just overwriting the last three characters with "NULL" or 0.
我认为改变字符串内存是安全的，因为您确实说过擦除最后三个字符。我只是用“NULL”或 0 覆盖了最后三个字符。
回答by trutheality
It might help to understand how C char*"strings" work:
它可能有助于理解 C char*“字符串”的工作原理：
You start reading them from the char that the char* points to until you hit a \0char (or simply 0).
您从 char* 指向的字符开始读取它们，直到遇到\0字符（或只是 0）。
So if I have
所以如果我有
char str2[9];
strncpy (str2,str,8); // now str2 contains "theFile.#" where # is some character you don't know about
str2[8]='##代码##'; // now str2 contains "theFile.##代码##" and is a proper char* string.
then str+3represents the string File.nam.
然后str+3代表字符串File.nam。
But you want to remove the last three characters, so you want something like:
但是您想删除最后三个字符，因此您需要以下内容：
##代码##