File Lines

Aiwrtcode

Hey,

I was wondering if anyone knows how to jump to a certian line in a text file. I have found ways to read the file line by line from the start and how to just a certian number of bytes into a file but have not been able to jump a certian number of lines into it.

nashirak

You havent found anything cause there is none. The operating system itself knows nothing about how many lines the file actually has. It can tell you the number of bites but not lines. Why? When you make the call fgets($fp,4096) it works like the fgets() function in C. It reads all the bytes in a file until it hits the \n character or the buffer runs out. So when you call fgets, its doing a bit of logic. It asks itself everytime it pulls a charcter do I have a \n character and if so stop reading.
So this is a long explination to tell you...no PHP has nothing to skip lines. It would require one to run through a file and figure out how many bytes there are till you hit a newline (for every line) and Store those values in some data structure. By that time its absolutly usless cause you have just run through the entire file. Since I am assuming you want to do this to enhance search times or something (in a file). Moving X many bytes in a file is different, cause all you move is instead of just incrementing a file pointer (as you would do with fgetc()) you set the file pointer equal to some value (essentially memory).

Hope this Helps and explains 🙂!

Aiwrtcode

Thanks for confirming what I was worried about.

ryza_

Aiwrtcode

very easy to do. ignore everything nashirak said

$theFile="./database.txt"; #the file
$data=file($theFile); # put in array
$lines=count($data); # total lines.

now you have your file in a readable array, and you know how many lines it contains.

ryza

nashirak

Hey raza_ what are you doing? Lets see:

reading the entire file into an array; What does this function actually do?

your reading the entire file into an array. Guess what... on the OS level you just read character by character telling the OS to read the character by character till it hits the newline character increment the array number (or allocate new memory) then do the same thing over again. Then you are essenitally calling the sizeof() the array which tells you how much memory is allocated. Now, what you just did is something far different than what this guy wants. He wants something like fseek() that will actually take that file pointer that is used and move it N bytes in the file. The OS will gladly do this. Why does this way work? What essentially you are doing when you call fgets() as I said before it calls loops through fgetc() until a newline is hit. Well what does fgetc() do. Well if you cut through all the PHP and all the C... at the assembly level it calls an interrupt vector like:
int 21h (in dos)
What does this do? Well internally it does some stuff but when you exit (depends on architecture) it will send back an incremented variable (hex value) or in some systems you increment it yourself. So what you do when you call fseek() is essentially change the value of that register so that when the interrupt vector gets called again it reads from that byte or area in the file. Now, did this read the entire file in? No.. this just increments the fp to read some other byte in the file.

Your solution would work. But how would this cutdown on search time? You have to read in entire file to do this. So essentially it (in big O) O(N). Now using fseek() or something (which is what he was asking except he wanted one that new about lines). You can create algorithms that move the file pointer ahead to a specific location and read from there. Did you just read in the entire file? No. This is essentially what a database does. It uses complicatated hashing + B-Tree implementations to do figure out at the byte level how far it needs to seek to grab what it needs.

So that is a long explination, just to tell you I do know what I am talking about. If you would like to ignore me however, that is your buisness.

For more on this subject do a search on the internet, or mabye right some assembly language. 🙂

Aiwrtcode

Actually what he said helps. The problem was that it took too long to sort the data into a SQL database. Sometime tomorrow (wed) I'll try it and see if it works, Thanks.