# How-to line\_count **URL:** \ **Category:** Help!\ **Tags:** how-to\ **Created:** [April 9, 2020, 9:42pm UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136 "2020-04-09T21:42:25Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![vincent](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/vincent/32/962_2.png) [@vincent](https://swi-prolog.discourse.group/u/vincent)\ **Post date:** [April 9, 2020, 9:42pm UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/1 "2020-04-09T21:42:25Z")
I’m using: SWI-Prolog version 8.0.2 I want the code to: count the lines in any file I am trying line\_count/2 and it shows 1 no mater what file I give it. I also read that it doesn’t work properly. So I am trying to implement one but have difficulties with the counting the occurrence of the ‘newline’. I want in between get\_char(’\n’) and get\_char(‘end\_of\_file’) a counting procedure that counts the ‘newline’-s. I need help with what should I put in the place of 'increase Lines by 1. Thanks! My code looks like this: ```prolog % your code here line_co(Lines) :- repeat, get_char('\n'), increase Lines by 1, get_char('end_of_file'). ``` ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [April 10, 2020, 5:53am UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/2 "2020-04-10T05:53:57Z")
It is too much effort to do this with `get_char` and `repeat`. At the very least, look at the code sample at the bottom of the [docs for `repeat`](https://www.swi-prolog.org/pldoc/doc_for?object=repeat/0): ```prolog repeat, read(Term), ( Term == end_of_file -> ! ; process(Term), fail ). ``` You’d also need to use a mutable variable for the counting. You can see examples of mutable variables in the [implementation of `aggregate_all`](https://www.swi-prolog.org/pldoc/doc/_SWI_/library/aggregate.pl?show=src#aggregate_all/3). Putting those together I get: ```prolog line_count(Filename, N) :- setup_call_cleanup(open(Filename, read, In), stream_line_count(In, N), close(In)). stream_line_count(In, N) :- Count = count(0), repeat, get_char(In, X), ( X == end_of_file -> ! ; ( X == '\n' -> arg(1, Count, N0), N1 is N0 + 1, nb_setarg(1, Count, N1) ), fail ), arg(1, Count, N). ``` and then: ```prolog ?- line_count("line_count.pl", N). N = 19. ``` One easier way is using `read_string/5` and using a loop instead of `repeat`: ```prolog line_count(Filename, N) :- setup_call_cleanup(open(Filename, read, In), stream_line_count(In, 0, N), % note the extra argument! close(In)). stream_line_count(In, N0, N) :- read_string(In, "\n", "", Sep, _), Sep \== -1, !, N1 is N0 + 1, stream_line_count(In, N1, N). stream_line_count(_, N, N). ``` Instead of `read_string/5` you could have used one of the predicates in [library(readutil)](https://www.swi-prolog.org/pldoc/man?section=readutil). Or maybe you can use a DCG: ```prolog :- use_module(library(dcg/basics)). line_count(N0, N) --> string_without("\n", _), "\n", !, { N1 is N0 + 1 }, line_count(N1, N). line_count(N, N) --> []. ``` and then: ```prolog ?- phrase_from_file(line_count(0, N), "line_count.pl"). N = 8. ``` There are endless possibilities (do you have to do it in constant memory? or can you just read the whole file with `read_string/3` and count the newlines in this? and so on). ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [April 10, 2020, 7:55am UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/3 "2020-04-10T07:55:53Z")
Now I also have a question for everyone: if I wanted to count the newlines in a stream (or any single character, really), and I decided to use `aggregate_all(count, stream_newline(In), N)`. How would I define `stream_newline/1`? This is one attempt that seems to work: ```prolog stream_newline(In) :- repeat, get_char(In, C), ( C == end_of_file -> !, fail ; C == '\n' ; fail ). ``` Is there an easier way to achieve the same? ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [April 10, 2020, 4:02pm UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/4 "2020-04-10T16:02:32Z")
Just for Easter fun … ```prolog file_nl_count(File, Count) :- setup_call_cleanup( open(File, read, In), stream_nl_count(In, Count), close(In)). stream_nl_count(In, Count) :- stream_to_lazy_list(In, List), aggregate_all(count, member(0'\n, List), Count). ``` It is even pretty fast. Takes 16ms for a file holding 186713 chars and 7331 newlines. Your version uses 31ms and Unix `wc` does the job in 2ms (3ms for this file and 1ms for a really short file). We can also do the counting as below. Performs equal to the above. ```prolog count_nl(List, Count) :- count_nl(List, 0, Count). count_nl([], Count, Count) :- !. count_nl([H|T], Count0, Count) :- ( H == 0'\n -> Count1 is Count0+1, count_nl(T, Count1, Count) ; count_nl(T, Count0, Count) ). ``` ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [April 10, 2020, 7:33pm UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/5 "2020-04-10T19:33:29Z")
@jan this is brilliant! So what this tells me is the lazy list is the real magic here: it makes a stream look exactly like a list _and_ it is more efficient! ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [April 11, 2020, 8:08am UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/6 "2020-04-11T08:08:38Z")
Roughly it gets its performance from using _block_ reading rather than character-by-character, avoiding all the argument validation and locking for each character. It is of course also nice that you can use normal list predicates, but be aware that not all list predicates always work. That is the nasty part: it is not very easy to describe where the limits are and you must understand what happens to see where things can go wrong. ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [April 17, 2020, 7:34am UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/7 "2020-04-17T07:34:36Z")
For my own piece of mind, I did run different version of the “line counting” program and for the record: Using `string/5` with a newline as the separator beats everything else (like, about 4-5 times faster than using a lazy list). As your experiment showed, using `aggregate_all` is roughly the same as writing a classical tail-recursive loop. I can paste my full code and results if there is interest. Still, I cannot thank you enough for bringing `stream_to_lazy_list/2` to my attention! ---
**Author:** ![EricGT](https://avatars.discourse-cdn.com/v4/letter/e/f1d935/32.png) [@EricGT](https://swi-prolog.discourse.group/u/EricGT)\ **Post date:** [April 17, 2020, 10:09am UTC](https://swi-prolog.discourse.group/t/how-to-line-count/2136/8 "2020-04-17T10:09:44Z")
> [@Boris](#): > > I can paste my full code and results if there is interest. Please do.