Print specific pattern line in c++

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# 8  
Old 07-06-2011
It reads a line every loop, but never prints anything unless it finds @HWI. When it does, it prints both that line (after subbing # for @) and the line after it (by reading and immediately printing another line).

So any lines that aren't @HWI or don't immediately follow @HWI get read but not printed.
# 9  
Old 07-06-2011
Many thanks for explanation in detail.
I'm still need time to 'digest' it Smilie
Will ask your experience once got problem.
Thanks first.
# 10  
Old 07-06-2011
From 'man fgets' :

gets() and fgets() return s on success, and NULL on error or when end of file occurs while no characters have been read.

   while(fgets(buf, 4096, fin)) //keep reading line-by-line into buffer
               if(strncmp(buf, "@HWI", 4) == 0) //if buffer starts with '@HWI' 
                       buf[0]='#';  //set the first char to '#'
                       fputs(buf, fout);   //and print the whole line
                       if(fgets(buf, 4096, fin) == NULL) continue;   //read next line; check for failure or eof
                       fputs(buf, fout);  //print the line 

This piece of code calls fputs() twice -- once it prints the modified '@HWI' line and the second time it prints the next line after. Then, the loop gets in next iteration and keeps reading in lines, until while condition is satisfied, that is, until '@HWI' is encountered. The third and fourth line in your input get lost in the while iterations, not in the if test.

@Corona: Any particular reason why you chose to use buffer of size 4096?
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# 11  
Old 07-07-2011
Originally Posted by mirni
@Corona: Any particular reason why you chose to use buffer of size 4096?
Not really. 4096 is just a "nice round number" in computing terms, a power of two, and plenty long enough for most text lines. 'most' may not mean yours, adjust to taste. Anything under a couple of megs in size will work. If you need really large buffers you have to make them with malloc:

unsigned long size=(1024L*1024L*64L); // 64 megabytes
char *buf=malloc(size);

You use buf the same way as you would a stack var like char buf[64]; since an array and a pointer amount to the same thing in the end, as far as the processor's concerned -- a starting point in memory.
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