need help ...

[Nikola]

I need a function that reads from a txt file and randomly chooses a line
from which retrieves a string (with spaces) and returns it to main function.

thx

[osmium]

Nikola writes:

I need a function that reads from a txt file and randomly chooses a line
from which retrieves a string (with spaces) and returns it to main

function.

I assume this is a student problem. Try breaking the problem down. You can
not choose a random line until you know how many lines there are. So you
must start by counting the lines. To count the lines you will have to read
the lines. To read the lines you will have to open the file. To open the
file you must know the file name. And so on. Writing things down in some
form can be a great help. The preferred method is called pseudocode but any
form at all is better than just fussing and flailing.

[those who know me have no need of my name]

in comp.lang.c i read:

I need a function that reads from a txt file and randomly chooses a line
from which retrieves a string (with spaces) and returns it to main function.

i bet you do. when is your homework due?

--
a signature

[Eric Sosman]

osmium wrote:

Nikola writes:

I need a function that reads from a txt file and randomly chooses a line
from which retrieves a string (with spaces) and returns it to main

function.

I assume this is a student problem. Try breaking the problem down. You can
not choose a random line until you know how many lines there are.

There are perfectly good ways to choose a line at random
without knowing in advance how many there are. In fact, you can
choose a random sample of N lines without advance knowledge; the
case N==1 is particularly easy, but N>1 isn't difficult. See
Knuth "The Art of Computer Programming, Volume II: Seminumerical
Algorithms," section 3.4.2, Algorithm R.

(Well, I guess you do in fact need *some* advance knowledge.
Specifically, you need to know that the source of random numbers
can deliver a different value for every line in the file, without
repetitions. A PRNG that will deliver two billion distinct
"good" numbers is easy to write in portable C; four billion is
also easy at some small sacrifice of "goodness." So the advance
knowledge amounts to knowing that the file holds fewer than four
billion lines; I think this may be considered axiomatic in the
context of a simple homework assignment.)

--
Er*********@sun .com

[Nikola]

No, it's not a home work and yes, I am a begginer (weren't we all??) and
trying to do a little code of my own - was told that ambition is a virtue
:-)

[Arthur J. O'Dwyer]

On Fri, 14 May 2004, Eric Sosman wrote:

osmium wrote:

Nikola writes:

I need a function that reads from a txt file and randomly chooses a line
from which retrieves a string (with spaces) and returns it to main
function.

<snip> You cannot choose a random line until you know how many lines there are.
There are perfectly good ways to choose a line at random
without knowing in advance how many there are.

<snip> (Well, I guess you do in fact need *some* advance knowledge.
Specifically, you need to know that the source of random numbers
can deliver a different value for every line in the file, without
repetitions.

...Or, I think more generally, you can be satisfied with a RNG
that will return an unbiased random number in the range 1..n, for
every n in the range 1..N where N is the number of lines in the
file. That may be what you were getting at; after all, if the
RNG can deliver a number in the range 1..N, then in some sense it
must be able to deliver N different numbers "without repetition."
Of course, it doesn't need to generate its random numbers without
repeating itself --- that would be a pretty bad RNG! ;)

Okay, enough hints from me now. :)

-Arthur

[Default User]

Nikola wrote:

No, it's not a home work and yes, I am a begginer (weren't we all??) and
trying to do a little code of my own - was told that ambition is a virtue
:-)

Then you need to show us this "little code". Just giving us the problem
spec and requesting an answer doesn't really display much in the way of
ambition.

What have you figured out so far? What is your analysis of the problem?
Do you know how to open a file? Read out a line? How would you choose a
random line?

Brian Rodenborn

[Nikola]

> What have you figured out so far? What is your analysis of the problem?

Do you know how to open a file? Read out a line? How would you choose a
random line?

Well, nothing anyone said didn't help so I kept trying ...
If there are some noncence-mistakes in my code that no self-respectfull
programmer would make
keep in mind that I'm just 19, meaning, a begginer
this code should read the random line of an file
commented parts were just a unsuccessfull toughts....
it's not in english this is the translation of some words:

znak-character
br_redaka-number of lines
brojac-counter
pojam-randomly chosen line (string)

this is ment to be a function that returns a string ...

FILE *dat;
char pojam[50],c,znak;
int br_redaka=1,bro jac=1,rand_br;
dat=fopen("film .txt","r");
while(c!=char(E OF))
{
c=getc(dat);
if(c==char(10)) //10 - ascii for new line
br_redaka=br_re daka+1;
}
//generiranje nasumicnog broja od 0 do broja redaka //moguci BUG zbog
nule!
srand(time(NULL ));
rand_br=rand() % br_redaka;
c='b';
while(c!=EOF)
{
/*c=getc(dat);
if(c==char(10)& &brojac==rand_b r) //10-ascii kod za novi redak
{
brojac=brojac+1 ; //rand_br od 0 do br_redaka
fgets(pojam,50, dat);
}*/
c=getc(dat);
if(c==char(10)& &brojac==rand_b r)
{
brojac=brojac+1 ;
while(znak!='\\ ')
//strcat(pojam,zn ak);
pojam[brojac-2]=c;

}
}
printf("%s",poj am);

[Eric Sosman]

Arthur J. O'Dwyer wrote:

On Fri, 14 May 2004, Eric Sosman wrote:

osmium wrote:

Nikola writes:
I need a function that reads from a txt file and randomly chooses a line

(Well, I guess you do in fact need *some* advance knowledge.
Specificall y, you need to know that the source of random numbers
can deliver a different value for every line in the file, without
repetitions .

...Or, I think more generally, you can be satisfied with a RNG
that will return an unbiased random number in the range 1..n, for
every n in the range 1..N where N is the number of lines in the
file. That may be what you were getting at; after all, if the
RNG can deliver a number in the range 1..N, then in some sense it
must be able to deliver N different numbers "without repetition."

The algorithm I'm thinking of actually needs N distinct
values. Simply knowing that the range of generated numbers
contains more than N values is not good enough, nor is knowing
that the period of the PRNG exceeds N. You actually need a
"no duplicates among the first N values" guarantee.

At the risk of becoming almost topical: rand() itself lacks
such a guarantee, but it would be possible to write a wrapper
around rand() to get this behavior.
Of course, it doesn't need to generate its random numbers without
repeating itself --- that would be a pretty bad RNG! ;)

I don't understand what this means.

--
Er*********@sun .com

[Nick Austin]

On Fri, 14 May 2004 19:38:17 +0200, "Nikola" <az*****@inet.h r> wrote:

Well, nothing anyone said didn't help so I kept trying ...
If there are some noncence-mistakes in my code that no self-respectfull
programmer would make
keep in mind that I'm just 19, meaning, a begginer
this code should read the random line of an file
commented parts were just a unsuccessfull toughts....
it's not in english this is the translation of some words:

znak-character
br_redaka-number of lines
brojac-counter
pojam-randomly chosen line (string)

this is ment to be a function that returns a string ...

FILE *dat;
char pojam[50],c,znak;
int br_redaka=1,bro jac=1,rand_br;
dat=fopen("film .txt","r");
while(c!=char(E OF))
I see three problems.

Firstly char(EOF) is a syntax error.
Secondly getc() returns an int, not char.
Thirdly object c is used before it has been initialised.

Also testing for EOF before calling getc() is bad style. It
results in the loop executing one too many times, although
in your case the result of this is harmless. It is better
to write this loop as:

while (c=getc(dat), c!=EOF)
{
/* ... */
}

This also ensures that object c is initialised before the
comparision.
{
c=getc(dat);
if(c==char(10)) //10 - ascii for new line
This is also bad style. It needlessly restricts your program to
only working on ASCII implementations . Since the file is open
in text mode you can write:

if (c=='\n')
br_redaka=br_re daka+1;
}
//generiranje nasumicnog broja od 0 do broja redaka //moguci BUG zbog
nule!
srand(time(NULL ));
rand_br=rand() % br_redaka;
See Q13.16 in the FAQ:

http://www.eskimo.com/~scs/C-faq/q13.16.html

You also need to reset the file pointer here:

fseek( dat, 0, SEEK_SET );
c='b';
while(c!=EOF)
{
/*c=getc(dat);
if(c==char(10)& &brojac==rand_b r) //10-ascii kod za novi redak
{
brojac=brojac+1 ; //rand_br od 0 do br_redaka
fgets(pojam,50, dat);
}*/
c=getc(dat);
if(c==char(10)& &brojac==rand_b r)
{
brojac=brojac+1 ;
while(znak!='\\ ')
//strcat(pojam,zn ak);
pojam[brojac-2]=c;
This while loop would appear to be infinite. I don't think it's
even needed.
}
}
You need to null terminate the char array to turn it into a valid
string for display:

projam[brojac-1] = '\0';
printf("%s",poj am);

Nick.