Remove the last new line in C

Question

I want to remove the last new line from the following function:

void WriteToFile(node *tree)
{
    void Write(node*);
    fp = fopen("dictionary.txt","w");
    if(fp==NULL)
    {
        printf("Cannot open file for writing data...");
    }
    else //if(tree==NULL)
    {
        if(tree!=NULL)
        {
            Write(tree);
        }
        fclose(fp);  
    }
}
void Write(node *tree)
{
    if(tree!=NULL)
    {
        fprintf(fp,"%s:%s\n",tree->word,tree->meaning);
        Write(tree->left);
        Write(tree->right);
    }
}

I am using this function to write to a text file the contents of a BST, and I dont want it to write the last new line, how can I remove it?

I am curious. Why don't you want to write a newline for the last line? Conventionally, all lines of a text file should be terminate with a newline, including the last one. — CB Bailey, Aug 07 '11 at 10:49

pmg · Accepted Answer · 2011-08-07T11:42:06.413

In a recursive routine like yours, you cannot easily know which is the last call, but you can know what is the first call.

Rather than writing data and, optionally, a newline, consider writing an optional newline followed by data:

void FirstWrite(node *tree, FILE *fp)
{
    if (tree)
    {
        fprintf(fp, "%s%s", tree->word, tree->meaning);
        Write(tree->left, fp);
        Write(tree->right, fp);
    }
}
void Write(node *tree, FILE *fp)
{
    if (tree)
    {
        fprintf(fp, "\n%s%s", tree->word, tree->meaning);
        Write(tree->left, fp);
        Write(tree->right, fp);
    }
}

But consider the most used definition of a line for text mode files is: a sequence of 0 or more characters folllowed by and including a newline. According to this definitilon all lines have a newline; and the last piece of data in your file is not a line.

^{At least one badly written program I've used fails to process the whole input because of that. My guess is that it does something like}

fgets(buf, sizeof buf, inf);
inlen = strlen(buf);
if (buf[inlen - 1] == '\n') processline(buf);

^{This badly written program would work if the last piece of data included a newline.}

score 3 · Answer 2 · answered Aug 07 '11 at 10:47

Modify your function as follows:

void Write(node *tree, int isFarRight)
{
    if(tree!=NULL)
    {
        fprintf(fp,"%s:%s",tree->word,tree->meaning);
        if (!isFarRight || tree->left || tree->right)
        {
            fprintf(fp, "\n");
        }
        Write(tree->left, 0);
        Write(tree->right, isFarRight);
    }
}

...

Write(tree, 1);

The isFarRight variable keeps track of whether you're currently on the far right-hand side of the tree. You only print the '\n' if the current node has children, or if you're not on the far right-hand side.

Charles Ma · Answer 3 · 2011-08-07T12:34:26.033

3

Once you've already written to the file, you can use ftruncate to truncate the file to size - 1

size_t size = ftell(fp);
ftruncate(fp, size - 1);

On windows, use _chsize instead of ftruncate

_chsize(fd, size - 1);

edited Aug 07 '11 at 12:34

answered Aug 07 '11 at 10:48

Charles Ma

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22
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Your answer should point out that this will only work on POSIX-compliant systems. – Oliver Charlesworth Aug 07 '11 at 10:53
Thanks Oli, I've amended my answer – Charles Ma Aug 07 '11 at 12:34

score 1 · Answer 4 · edited May 23 '17 at 11:55

You can truncate the file by 1 byte, after writing whole tree. How to truncate a file in C might help you with that.

Normally, you could try to not write the last \n by knowing that you are in the last node, ~~but considering this implementation of tree, you would have to make few changes to the code, so generally, the 1st approach might be quicker for you.~~ - sample how to do that has been given by Oli Charlesworth, this approach I think is nicer then truncating (not doing harm, instead of fixing it), both will work though.

score 1 · Answer 5 · answered Aug 07 '11 at 10:48

1

You could do it this way:

void Write(node *tree, int first) {
  if (tree!=NULL) {
    if (!first)
      fprintf(fp, "\n");
    fprintf(fp,"%s:%s",tree->word,tree->meaning);
    Write(tree->left, 0);
    Write(tree->right, 0);
  }
}

And call Write(tree, 1); in your main. Not pretty, but should work.

answered Aug 07 '11 at 10:48

Mat

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I'd suggest to have a default value for first ( = 1) to make it prettier. The caller shouldn't care about implementation details of Write – Armen Tsirunyan Aug 07 '11 at 10:52
That would be nice, but would require a second function - I don't think you can have default values in C like you can in C++. [pmg](http://stackoverflow.com/users/25324/pmg) posted something like that. – Mat Aug 07 '11 at 10:55

score 0 · Answer 6 · edited Aug 07 '11 at 12:41

0

Does this print the tree in order, i.e. from left to right?

I think that the recursive write tree should be defined as follows:

write(tree *node)
{
    write(node->left);
    printf(.....);
    write(node->right);
}

Are you sure that your function prints the tree in order?

edited Aug 07 '11 at 12:41

Louis Marascio

2,629
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answered Aug 07 '11 at 11:22

score 0 · Answer 7 · answered Aug 07 '11 at 11:25

Note that in your code, you text twice for the root of the tree to be NULL. So without changing your function definition:

void WriteToFile(node *tree)
{
  void Write(node*);
  fp = fopen("dictionary.txt","w");
  if(fp==NULL)
  {
    printf("Cannot open file for writing data...");
  }
  else //if(tree==NULL)
  {
    if(tree!=NULL)
    {
      fprintf(fp,"%s:%s",tree->word,tree->meaning);
      Write(tree);
    }
    fclose(fp);  
  }
}
void Write(node *tree)
{
  if (tree->left)
  {
    fprintf(fp,"\n%s:%s",tree->left->word,tree->left->meaning);
    Write(tree->left);
  }

  if (tree->right)
  {
    fprintf(fp,"\n%s:%s",tree->right->word,tree->right->meaning);
    Write(tree->right);
  }
}

Remove the last new line in C

7 Answers7