How to trim the white space around a string in C program


 
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# 15  
Old 10-11-2008
Note to anybody copying code: both of calv's examples will segfault as posted. I have not had time to mess with it. On Monday I can get time to post something.

Segfaults on the condition when there are no spaces in the string, for example.
Plus undeclared variables.... maybe someone else can fix it.

edit: corrected code
Code:
void trim_copy(char *input, char *output)
{
  char *end =NULL   //  ouput;
  char c;

  // skip spaces at start
  while(*input && isspace(*input))
    ++input;

  // copy the rest while remembering the last non-whitespace
  while(*input)
  {
    // copy character
    c = *(output++) = *(input++);

    // if its not a whitespace, this *could* be the last character
    if( !isspace(c) )
      end = output;
  }

  // white the terminating zero after last non-whitespace
  if(end!=NULL) *end = 0;
}

Code:
similar changes are needed for the other example, trim_nocopy();


Last edited by jim mcnamara; 10-13-2008 at 11:03 AM..
# 16  
Old 10-17-2008
Why would they crash? Please explain to me the conditions, under which that happens. I tested them, and they don't crash. The change you made has the effect, that if the input is empty, or contains only spaces, then in the output no terminating zero is written (which makes it worse). My version crashes only in the following instances:

- the input pointer is not valid
- the input data contains no terminating zero
- the output pointer is not valid
- the output pointer points to a read-only area in memory
- the output pointer doesn't have enough memory to contain the result

all of those conditions are like in other string handling functions, like strcpy(). So if strcpy() works, trim_copy() should also work.

EDIT: oh, I see your problem now. I had a Typo in the first line, saying "char *end = ouput;". That was supposed to be "output". You probably just deleted the initialization, so that it crashed.

Last edited by calv; 10-17-2008 at 09:53 AM..
# 17  
Old 10-17-2008
error in trim_copy() was a typo: "ouput" -> "output" (but no segfault)

error in trim_nocopy() was an undeclared variable (that also needs to be initialized. segfault, if the new variable is not initialized and the string contains no nonspace characters)

btw. trim_nocopy() has no real practical reason to exist. It is only to show that a left trimming can be done without writing to the string at all, just by changing the start pointer. Also right trimming can be done by just writing a zero after the last nonspace. In fact the trim_copy() function is the one you should always use, both for in place (input==output) or copying operations. Also it makes sense to switch the parameters, so they are in sync with other c std lib string functions like strcpy() and strcat().

ok, now the fixed version:

Code:
void trim_copy(char *input, char *output)
{
  char *end = output;
  char c;

  // skip spaces at start
  while(*input && isspace(*input))
    ++input;

  // copy the rest while remembering the last non-whitespace
  while(*input)
  {
    // copy character
    c = *(output++) = *(input++);

    // if its not a whitespace, this *could* be the last character
    if( !isspace(c) )
      end = output;
  }

  // write the terminating zero after last non-whitespace
  *end = 0;
}

void trim_inplace(char *s)
{
  trim_copy(s, s);
}

char *trim_nocopy(char *s)
{
  char *start = s;

   // skip spaces at start
  while(*start && isspace(*start))
    ++start;

  char *i = start;
  char *end = start;
  // iterate over the rest remebering last non-whitespace
  while(*i)
  {
    if( !isspace(*(i++)) )
      end = i;
  }

  // write the terminating zero after last non-whitespace
  *end = 0;

  return start;
}

# 18  
Old 10-17-2008
I cut and pasted you code here - I had to make changes so the code would compile.
All my stuff is in red.

Code:

#include <string.h> 
/*  my changes are in red to get the code to compile without errors
              and to run a sample*/
char *trim_nocopy(char *s)
{
  char *start = s;
  char *end;
   // skip spaces at start
  while(*start && isspace(*start))
    ++start;

  char *i = start;
  // iterate over the rest remebering last non-whitespace
  while(*i)
  {
    if( !isspace(*(i++)) )
      end = i;
  }

  // white the terminating zero after last non-whitespace
  *end = 0;

  return start;
}

char *trim_nocopy1(char *s)
{
  char *start = s;
  char *end = NULL;
   // skip spaces at start
  while(*start && isspace(*start))
    ++start;

  char *i = start;
  // iterate over the rest remebering last non-whitespace
  while(*i)
  {
    if( !isspace(*(i++)) )
      end = i;
  }

  // white the terminating zero after last non-whitespace
  if (end !=NULL) *end = 0; 

  return start;
}


int main()
{
	char tmp[8]={0x0};
	char *test = "  ";
	
	printf("running char *trim_nocopy1(char *s)\n");
	strcpy(tmp, test);
	printf("trimmed: %s\n", trim_nocopy1(tmp));
	printf("running char *trim_nocopy(char *s)\n");
	strcpy(tmp, test);
	printf("trimmed: %s\n", trim_nocopy(tmp));

}

Code:
csaprd:/home/jmcnama> uname -a
HP-UX csaprd B.11.23 U 9000/800 52720173 unlimited-user license
csaprd:/home/jmcnama> cc t.c -g -o trim
csaprd:/home/jmcnama> trim
running char *trim_nocopy1(char *s)
trimmed:
running char *trim_nocopy(char *s)
Bus error(coredump)
csaprd:/home/jmcnama> gdb trim core
HP gdb 5.4.0 for PA-RISC 1.1 or 2.0 (narrow), HP-UX 11.00
and target hppa1.1-hp-hpux11.00.
Copyright 1986 - 2001 Free Software Foundation, Inc.
Hewlett-Packard Wildebeest 5.4.0 (based on GDB) is covered by the
GNU General Public License. Type "show copying" to see the conditions to
change it and/or distribute copies. Type "show warranty" for warranty/support.
..
Core was generated by `trim'.
Program terminated with signal 10, Bus error.

#0  0x2c60 in trim_nocopy (s=0x705f11e0 "  ") at t.c:20
20        *end = 0;

It dumps core on a string of all spaces, for example.
It will dump core on any string consisting soley of characters on which isspace returns > 0. I was in a hurry in the post above, my bad. I said incorrectly a string of all non-spaces, which is clearly wrong.
# 19  
Old 10-17-2008
Yeah, on second thought I realized that if someone adds the variable "end" without initializing it, the result can crash. However the correct initial value is not NULL, but start (after the first loop) See earlier post about that. I posted the corrected code above. Your corrected version is wrong however. You see that, when you change your initialization of temp to

char tmp[8]={'x'};
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