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May 5th, 2009

remove newline character from a string

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hi all.

i have read a text line from a file using

C Syntax (Toggle Plain Text)

fgets()

into a char array

C Syntax (Toggle Plain Text)
char astr[30]
char astr[30]

then i did

C Syntax (Toggle Plain Text)
strcat(astr,"secondstring")
strcat(astr,"secondstring")

The output of

C Syntax (Toggle Plain Text)
printf (astr)
printf (astr)

is appearing on 2 lines instead on same line.

plz advise me wat i did wrong.
thanks.

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CoolAtt

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May 5th, 2009

Re: remove newline character from a string

fgets places '\n' char at the end of string. Overwrite it by zero byte (end of C-string) - that's all:

c Syntax (Toggle Plain Text)
if (fgets(astr,sizeof astr,f)) {
    char* p = strchr(astr,'\n');
    if (p)
      *p = '\0';
    ...
} else { /* eof or i/o error */
    ...
}
if (fgets(astr,sizeof astr,f)) {
    char* p = strchr(astr,'\n');
    if (p)
      *p = '\0';
    ...
} else { /* eof or i/o error */
    ...
}

ArkM

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May 5th, 2009

Re: remove newline character from a string

>fgets places '\n' char at the end of string.
To the OP: ArkM's code does the right thing, but the quoted statement is incomplete. fgets preserves the newline, but not if the buffer is filled. So solutions like the following distressingly common one are incorrect:

c Syntax (Toggle Plain Text)
if ( fgets ( buf, sizeof buf, stdin ) != NULL ) {
  buf[strlen ( buf ) - 1] = '\0';
 
  /* ... */
}
if ( fgets ( buf, sizeof buf, stdin ) != NULL ) {
  buf[strlen ( buf ) - 1] = '\0';

  /* ... */
}

Narue

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May 5th, 2009

Re: remove newline character from a string

ArkM's method is sufficient, but i prefer this way.

c Syntax (Toggle Plain Text)
buf[strcspn(buf,'\n')] = '\0';
buf[strcspn(buf,'\n')] = '\0';

it's just a single line of code that will replace the newline with a NULL, if a newline is found, or will do nothing if one is not there

.

Last edited by jephthah; May 5th, 2009 at 11:29 am.

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May 5th, 2009

Re: remove newline character from a string

>ArkM's method is sufficient, but i prefer this way.
Keep in mind that of the potential ways to do this, strcspn is among the least efficient while strchr is among the most efficient.

Narue

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May 5th, 2009

Re: remove newline character from a string

oh, i wasnt aware of that.

i guess the only time i ever concern myself with reducing cycle times is in embedded applications, and there i rarely deal with strings and newlines.

but thats useful to know. thanks.

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May 5th, 2009

Re: remove newline character from a string

I prefer using strcspn myself, but knowing the performance characteristics is important. However, none of the methods for stripping a newline will change the fact that the whole operation is I/O bound because of fgets. You can save cycles when stripping the newline, but it isn't likely to have a noticeable effect on performance as a whole.

Just food for thought.

Narue

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May 6th, 2009

Re: remove newline character from a string

Narue,
Because fgets() reads a line up to the \n and loads everything up to the RETURN, isn't it simply a matter of testing the last character in the buffer with \n and if it isn't, they entered more than the requested data? Simple use of strlen() and a test?

WaltP

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May 6th, 2009

Re: remove newline character from a string

Click to Expand / Collapse Quote originally posted by WaltP ...

Narue,
Because fgets() reads a line up to the \n and loads everything up to the RETURN, isn't it simply a matter of testing the last character in the buffer with \n and if it isn't, they entered more than the requested data? Simple use of strlen() and a test?

Yes, what's your point? If all you want to do is determine if a partial line was read, then looking for a newline is trivial. Ideally one would write code that can handle both cases where fgets reads a newline and where it doesn't. For example:

c Syntax (Toggle Plain Text)
#include <stdio.h>
 
int main ( void )
{
    char buffer[5]; // Arbitrary small size
    char *s = NULL;
    size_t n = 0;
 
    while ( fgets ( buffer, sizeof buffer, stdin ) != NULL ) {
        size_t len = strlen ( buffer );
        char *save = realloc ( s, n + len + 1 );
 
        if ( save == NULL ) {
            perror ( "Insufficient memory" );
            break;
        }
 
        strcpy ( save + n, buffer );
        s = save;
        n += len;
 
        if ( buffer[len - 1] == '\n' ) {
            s[n - 1] = '\0';
            break;
        }
    }
 
    printf ( ">%s<\n", s != NULL ? s : "(null)" );
    free ( s );
 
    return 0;
}
#include <stdio.h>

int main ( void )
{
    char buffer[5]; // Arbitrary small size
    char *s = NULL;
    size_t n = 0;

    while ( fgets ( buffer, sizeof buffer, stdin ) != NULL ) {
        size_t len = strlen ( buffer );
        char *save = realloc ( s, n + len + 1 );

        if ( save == NULL ) {
            perror ( "Insufficient memory" );
            break;
        }

        strcpy ( save + n, buffer );
        s = save;
        n += len;

        if ( buffer[len - 1] == '\n' ) {
            s[n - 1] = '\0';
            break;
        }
    }

    printf ( ">%s<\n", s != NULL ? s : "(null)" );
    free ( s );

    return 0;
}

But if you're not worrying about long lines, you still need to consider the case where a newline may not be present, which is what I was talking about with the usual naive strlen solution:

c Syntax (Toggle Plain Text)
buffer[strlen ( buffer ) - 1] = '\0';
buffer[strlen ( buffer ) - 1] = '\0';

Unless this is qualified with a test to verify that a newline is present at that index, you would be removing valid data. An extra test is needed to make it work for long lines:

c Syntax (Toggle Plain Text)
size_t last = strlen ( buffer ) - 1;
 
if ( buffer[last] == '\n' )
    buffer[last] = '\0';
size_t last = strlen ( buffer ) - 1;

if ( buffer[last] == '\n' )
    buffer[last] = '\0';

Narue

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May 6th, 2009

Re: remove newline character from a string

>Simple use of strlen() and a test?
strlen() has to itinerate through the string to give the length, to that
the check has to be performed to find that '\n'.

strchr() has to itinerate through the string looking for the given char.
Hard for me to see which is faster.

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