Is it possible to print out only a certain section of a C-string, without making a separate substring?

问题

Say I have the following:

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

Is it possible to print out only the last 5 bytes of this string? What about the first 5 bytes only? Is there some variation of printf that would allow for this?

回答1:

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;
}

回答2:

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.

回答3:

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

来源：https://stackoverflow.com/questions/7780809/is-it-possible-to-print-out-only-a-certain-section-of-a-c-string-without-making

标签

printf

c-strings