Is it possible to print out only a certain section of a C-string, without making a separate substring?
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 f开发者_如何学JAVAirst 5 bytes only? Is there some variation of printf
that would allow for this?
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;
}
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.
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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