有没有可能只打印出 C 字符串的某一部分，而不制作一个单独的子字符串？

假设我有以下几点:

char* string = "Hello, how are you?";

有没有可能只打印出这个字符串的最后5个字节？那么前5个字节呢？是否有一些 printf的变化可以允许这种情况发生？

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最佳答案

Is it possible to print out only the last 5 bytes of this string?

Yes, just pass a pointer to the fifth-to-the-last character. You can determine this by string + strlen(string) - 5.

What about the first 5 bytes only?

Use a precision specifier: %.5s

#include <stdio.h>
#include <string.h>
char* string = "Hello, how are you?";


int main() {
/* print  at most the first five characters (safe to use on short strings) */
printf("(%.5s)\n", string);


/* print last five characters (dangerous on short strings) */
printf("(%s)\n", string + strlen(string) - 5);


int n = 3;
/* print at most first three characters (safe) */
printf("(%.*s)\n", n, string);


/* print last three characters (dangerous on short strings) */
printf("(%s)\n", string + strlen(string) - n);
return 0;
}
小开

Yes, the last five bytes of that string can be done with:

printf ("%s\n", &(string[strlen (string) - 5]));

The first five can be done with:

printf ("%.5s\n", string);

You can combine the two to get substrings within the string as well. The word how can be printed with:

printf ("%.3s\n", &(string[strlen (string) + 7]));

You do have to be careful that the string is long enough for this to work. Printing the last five characters of a one-character string will cause undefined behaviour since the index ends up at -4. In other words, check the string length before attempting this.

小开

Two solutions:

Say given a predicatable string with same length - I will use date as an example and asked to split into HH:MM:SS.DDDDDDD

char date[14] = "2359591234567";

[1] Readable Implementation:

char hh[3] = {0};
char mm[3] = {0};
char ss[3] = {0};
char dec[8] = {0};
strncpy ( hh, date, 2 );
strncpy ( mm, date+2, 2 );
strncpy ( ss, date+4, 2 );
strncpy ( dec, date+6, 7 );


printf("%s:%s:%s.%s\n", hh, mm, ss, dec);

[2] Short Implementation:

Either:

printf("%.2s:%.2s:%.2s.%.7s\n", date, date+2, date+4, date+6);

or:

printf("%2.2s:%2.2s:%2.2s.%7.7s\n", date, date+2, date+4, date+6);

Should work.

Instead of printf - you can use sprintf and copy to a buffer. I would also check for the correct length to avoid unpredictable behavior.

In either case - the output will be:

23:59:59.1234567

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