Is there a way to print a range of a char array without a for loop?

trist007 · 11-06-2010, 07:58 PM

Program in C
Say I have a char array of 1024 bytes called buf1.

But I only want to print the chars in index 0 up to index 30. I know I could do this with a for loop. But is there any other way? What about maybe storing from 31-1024 to another char array say buf2 with strcpy and somehow popping 31+ out of the buf1 char array?

What if I wanted to print chars in index 30 to 36? Any quick way without a for loop?

aluser · 11-06-2010, 08:13 PM

Code:

#include <stdio.h>

int main()
{
        char a[1024] = "abcdefghijklmnopqrstuvwxyz";
        printf("Characters 0 through 2 of a[]: %.3s\n", a);
        printf("Characters 10 through 15 of a[]: %.6s\n", &a[10]);
        return 0;
}

Prints this:

Code:

Characters 0 through 2 of a[]: abc
Characters 10 through 15 of a[]: klmnop

The "%.Ns" notation in the printf string is "%s" with a precision specifier. It's buried in the geek-speak in the printf(3) man page

trist007 · 11-06-2010, 08:21 PM

That's awesome, thank you.

wje_lq · 11-07-2010, 09:15 AM

To demonstrate that the fancy format string saves not only code complexity but actual time, run this shell script. You may wish to play with its first line, and possibly its second line.

I got this output.

Code:

running with compilation optimization O0
format string is %.25s
loop   way is 8 seconds
format way is 4 seconds
running with compilation optimization O1
format string is %.25s
loop   way is 7 seconds
format way is 5 seconds
running with compilation optimization O2
format string is %.25s
loop   way is 8 seconds
format way is 4 seconds
running with compilation optimization O3
format string is %.25s
loop   way is 7 seconds
format way is 4 seconds

Here is the script.

Code:

export iterations=10000000
export length=25
rm -f O0 O1 O2 O3; cat > 1.c <<EOD; \
gcc -O0 -Werror -Wall 1.c -o O0; \
gcc -O1 -Werror -Wall 1.c -o O1; \
gcc -O2 -Werror -Wall 1.c -o O2; \
gcc -O3 -Werror -Wall 1.c -o O3; \
./O0 O0; \
./O1 O1; \
./O2 O2; \
./O3 O3
#include <sys/stat.h>
#include <sys/times.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc,char **argv)
{
  int         jndex;

  long        iterations;
  long        length;
  long        single_iteration;
  long        ticks_per_second;

  char       *theoretical_string;

  char        format_string[80];

  clock_t     loop_way;
  clock_t     format_way;

  struct tms  the_beginning;
  struct tms  the_middle;
  struct tms  the_end;

  FILE       *phyle;

  phyle=fopen("/dev/null","a");

  if(phyle==NULL)
  {
    fprintf(stderr,"couldn\'t open /dev/null\n");

    exit(1);
  }

  printf("running with compilation optimization %s\n",
         argv[1]
        );

  iterations=strtol(getenv("iterations"),NULL,0);
  length    =strtol(getenv("length"    ),NULL,0);

  theoretical_string=malloc(length+2);

  if(theoretical_string==NULL)
  {
    fprintf(stderr,"string is theoretically too long\n");

    exit(1);
  }

  memset(theoretical_string,'A',length+1);
  theoretical_string[length+1]=0;

  sprintf(format_string,"%%.%lds",length);
  printf("format string is %s\n",format_string);

  loop_way  =0;
  format_way=0;

  for(single_iteration=0;
      single_iteration<iterations;
      single_iteration++
     )
  {
    times(&the_beginning);

    for(jndex=1;jndex<length+1;jndex++)
    {
      fprintf(phyle,"%c",theoretical_string[jndex]);
    }

    times(&the_middle);

    fprintf(phyle,format_string,theoretical_string+1);

    times(&the_end);

    loop_way+=(the_middle   .tms_utime+the_middle   .tms_stime)
             -(the_beginning.tms_utime+the_beginning.tms_stime);

    format_way+=(the_end   .tms_utime+the_end   .tms_stime)
               -(the_middle.tms_utime+the_middle.tms_stime);
  }

  ticks_per_second=sysconf(_SC_CLK_TCK);

  printf("loop   way is %ld seconds\n",  loop_way/ticks_per_second);
  printf("format way is %ld seconds\n",format_way/ticks_per_second);

  return 0;

} /* main() */
EOD