I have tried a lot to print a new line in lisp. I want to print a new line like java (system.out.println()), and not necessarily having anything. I know that print goes to a new line however I must have a variable inside it. Is there a function in lisp that does not need a variable and goes to a new line?
Use the TERPRI function (the name stands for "terminate printing", as it's intended to be used to terminate a line of output).
(terpri)
You could also use FRESH-LINE. This prints a newline unless you're already at the start of a line.
Related
If I run a perl program and call another perl program using backquotes, print statements from the called program don't appear at the terminal.
If I call the program using 'system', the print statements are displayed.
EG:
This is ProgA.pl
print "In ProgA.pl, about to call ProgB.pl";
my $dum=`ProgB.pl`; # print output doesn't appear
### $dum=system("ProgB.pl"); # this prints OK
print"\nBack in ProgA.pl";
print "\ndum = $dum"; # ProgB's output doesn't show here either
(No warnings or errors, perl.exe found through file association)
This is ProgB.pl:
print "\nPrinting from ProgB.pl";
What is the reason for the difference?
Why isn't the backquoted call output returned in $dum (I tried both STDOUT and STDERR)? If I call dir in backquotes, I get its output in $dum.
You have a path issue.
It works as expected ($dum is assigned the value "Printing from ProgB.pl") if I change the backticks from ProgB.pl to ./ProgB.pl. Without the explicit ./ path, it searches the system path and generates an error, as you can see if you change that line to
my $dum=`ProgB.pl` or die $!;
Which generates the output
In ProgA.pl, about to call ProgB.plNo such file or directory at ./ProgA.pl line 4.
Thus illustrating once again that you should always check the return values of your system calls for error conditions.
It appears that by failing to put a newline character at the end of the print command in ProgB, I failed to flush the buffer before returning to ProgA. Thanks to Chris Turner.
I wrote a simple parser for a .txt file with the following instructions:
my $file2 = "test.txt";
open ($process, "<",$file2) or die "couldn't manage to open the file:$file2!";
while (<$process>)
{
...
}
In some files that I am trying to parse there is a special character that is like the right arrow (->) and that I don't manage to paste here from the file.
Every time the parser hits that character (->), it exits the file without processing it till the end.
Is there a way to avoid it and continue processing the file till the very end?
I am using perl 5.6.1 (I cannot use a newer one) and the files that I need to process might have these special characters.
Thanks for your help.
I don't think it's perl that's causing your problem, but almost certainly something in that middle missing block. Are you using eval in the while block on the input from the file? This is a minimal example that shows that the stream containing -> doesn't cause difficulties:
#!/usr/bin/env perl
use warnings;
use strict;
while(<DATA>) {
print "Data[$.]: $_";
}
__DATA__
this is some data
this is also some data
-> this looks fine
foo->dingle also looks fine
This produces:
$ perl ./foo.pl
Data[1]: this is some data
Data[2]: this is also some data
Data[3]: -> this looks fine
Data[4]: foo->dingle also looks fine
So, the -> characters in perl are special:
"-> " is an infix dereference operator, just as it is in C and C++. If
the right side is either a [...] , {...} , or a (...) subscript, then
the left side must be either a hard or symbolic reference to an array,
a hash, or a subroutine respectively. (Or technically speaking, a
location capable of holding a hard reference, if it's an array or hash
reference being used for assignment.) See perlreftut and perlref.
Otherwise, the right side is a method name or a simple scalar variable
containing either the method name or a subroutine reference, and the
left side must be either an object (a blessed reference) or a class
name (that is, a package name). See perlobj.
So if I had to guess, you're definitely using eval to try to parse your content and it's failing to dereference the left side of the operator and crashing. Please provide command line error messages or the code in the while loop if you want further assistance.
I am wondering why the perl creators chose an unusual syntax for printing to a filehandle:
print filehandle list
with no comma after filehandle. I see that it's to distinguish between "print list" and "print filehandle list", but why was the ad-hoc syntax preferred over creating two functions - one to print to stdout and one to print to given filehandle?
In my searches, I came across the explanation that this is an indirect object syntax, but didn't the print function exist in perl 4 and before, whereas the object-oriented features came into perl relatively late? Is anyone familiar with the history of print in perl?
Since the comma is already used as the list constructor, you can't use it to separate semantically different arguments to print.
open my $fh, ...;
print $fh, $foo, $bar
would just look like you were trying to print the values of 3 variables. There's no way for the parser, which operates at compile time, to tell that $fh is going to refer to a file handle at run time. So you need a different character to syntactically (not semantically) distinguish between the optional file handle and the values to actually print to that file handle.
At this point, it's no more work for the parser to recognize that the first argument is separated from the second argument by blank space than it would be if it were separated by any other character.
If Perl had used the comma to make print look more like a function, the filehandle would always have to be included if you are including anything to print besides $_. That is the way functions work: If you pass in a second parameter, the first parameter must also be included. There isn't one function I can think of in Perl where the first parameter is optional when the second parameter exists. Take a look at split. It can be written using zero to four parameters. However, if you want to specify a <limit>, you have to specify the first three parameters too.
If you look at other languages, they all include two different ways ways to print: One if you want STDOUT, and another if you're printing to something besides STDOUT. Thus, Python has both print and write. C has both printf and fprintf. However, Perl can do this with just a single statement.
Let's look at the print statement a bit more closely -- thinking back to 1987 when Perl was first written.
You can think of the print syntax as really being:
print <filehandle> <list_to_print>
To print to OUTFILE, you would say:
To print to this file, you would say:
print OUTFILE "This is being printed to myfile.txt\n";
The syntax is almost English like (PRINT to OUTFILE the string "This is being printed to myfile.txt\n"
You can also do the same with thing with STDOUT:
print STDOUT "This is being printed to your console";
print STDOUT " unless you redirected the output.\n";
As a shortcut, if the filehandle was not given, it would print to STDOUT or whatever filehandle the select was set to.
print "This is being printed to your console";
print " unless you redirected the output.\n";
select OUTFILE;
print "This is being printed to whatever the filehandle OUTFILE is pointing to\n";
Now, we see the thinking behind this syntax.
Imagine I have a program that normally prints to the console. However, my boss now wants some of that output printed to various files when required instead of STDOUT. In Perl, I could easily add a few select statements, and my problems will be solved. In Python, Java, or C, I would have to modify each of my print statements, and either have some logic to use a file write to STDOUT (which may involve some conniptions in file opening and dupping to STDOUT.
Remember that Perl wasn't written to be a full fledge language. It was written to do the quick and dirty job of parsing text files more easily and flexibly than awk did. Over the years, people used it because of its flexibility and new concepts were added on top of the old ones. For example, before Perl 5, there was no such things as references which meant there was no such thing as object oriented programming. If we, back in the days of Perl 3 or Perl 4 needed something more complex than the simple list, hash, scalar variable, we had to munge it ourselves. It's not like complex data structures were unheard of. C had struct since its initial beginnings. Heck, even Pascal had the concept with records back in 1969 when people thought bellbottoms were cool. (We plead insanity. We were all on drugs.) However, since neither Bourne shell nor awk had complex data structures, so why would Perl need them?
Answer to "why" is probably subjective and something close to "Larry liked it".
Do note however, that indirect object notation is not a feature of print, but a general notation that can be used with any object or class and method. For example with LWP::UserAgent.
use strict;
use warnings;
use LWP::UserAgent;
my $ua = new LWP::UserAgent;
my $response = get $ua "http://www.google.com";
my $response_content = decoded_content $response;
print $response_content;
Any time you write method object, it means exactly the same as object->method. Note also that parser seems to only reliably work as long as you don't nest such notations or do not use complex expressions to get object, so unless you want to have lots of fun with brackets and quoting, I'd recommend against using it anywhere except common cases of print, close and rest of IO methods.
Why not? it's concise and it works, in perl's DWIM spirit.
Most likely it's that way because Larry Wall liked it that way.
I'm trying to wrap my head around the way Perl handles the parsing of arguments to print.
Why does this
print $fh $stufftowrite
write to the file handle as expected, but
print($fh, $stufftowrite)
writes the file handle to STDOUT instead?
My guess is that it has something to do with the warning in the documentation of print:
Be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print; put parentheses around all arguments (or interpose a + , but that doesn't look as good).
Should I just get used to the first form (which just doesn't seem right to me, coming from languages that all use parentheses around function arguments), or is there a way to tell Perl to do what I want?
So far I've tried a lot of combination of parentheses around the first, second and both parameters, without success.
On lists
The structure bareword (LIST1), LIST2 means "apply the function bareword to the arguments LIST1", while bareword +(LIST1), LIST2 can, but doesn't neccessarily mean "apply bareword to the arguments of the combined list LIST1, LIST2". This is important for grouping arguments:
my ($a, $b, $c) = (0..2);
print ($a or $b), $c; # print $b
print +($a or $b), $c; # print $b, $c
The prefix + can also be used to distinguish hashrefs from blocks, and functions from barewords, e.g. when subscripting an hash: $hash{shift} returns the shift element, while $hash{+shift} calls the function shift and returns the hash element of the value of shift.
Indirect syntax
In object oriented Perl, you normally call methods on an object with the arrow syntax:
$object->method(LIST); # call `method` on `$object` with args `LIST`.
However, it is possible, but not recommended, to use an indirect notation that puts the verb first:
method $object (LIST); # the same, but stupid.
Because classes are just instances of themselves (in a syntactic sense), you can also call methods on them. This is why
new Class (ARGS); # bad style, but pretty
is the same as
Class->new(ARGS); # good style, but ugly
However, this can sometimes confuse the parser, so indirect style is not recommended.
But it does hint on what print does:
print $fh ARGS
is the same as
$fh->print(ARGS)
Indeed, the filehandle $fh is treated as an object of the class IO::Handle.
(While this is a valid syntactic explanation, it is not quite true. The source of IO::Handle itself uses the line print $this #_;. The print function is just defined this way.)
Looks like you have a typo. You have put a comma between the file handle and the argument in the second print statement. If you do that, the file handle will be seen as an argument. This seems to apply only to lexical file handles. If done with a global file handle, it will produce the fatal error
No comma allowed after filehandle at ...
So, to be clear, if you absolutely have to have parentheses for your print, do this:
print($fh $stufftowrite)
Although personally I prefer to not use parentheses unless I have to, as they just add clutter.
Modern Perl book states in the Chapter 11 ("What to Avoid"), section "Indirect Notation Scalar Limitations":
Another danger of the syntax is that the parser expects a single scalar expression as the object. Printing to a filehandle stored in an aggregate variable seems obvious, but it is not:
# DOES NOT WORK AS WRITTEN
say $config->{output} 'Fun diagnostic message!';
Perl will attempt to call say on the $config object.
print, close, and say—all builtins which operate on filehandles—operate in an indirect fashion. This was fine when filehandles were package globals, but lexical filehandles (Filehandle References) make the indirect object syntax problems obvious. To solve this, disambiguate the subexpression which produces the intended invocant:
say {$config->{output}} 'Fun diagnostic message!';
Of course, print({$fh} $stufftowrite) is also possible.
It's how the syntax of print is defined. It's really that simple. There's kind of nothing to fix. If you put a comma between the file handle and the rest of the arguments, the expression is parsed as print LIST rather than print FILEHANDLE LIST. Yes, that looks really weird. It is really weird.
The way not to get parsed as print LIST is to supply an expression that can legally be parsed as print FILEHANDLE LIST. If what you're trying to do is get parentheses around the arguments to print to make it look more like an ordinary function call, you can say
print($fh $stufftowrite); # note the lack of comma
You can also say
(print $fh $stufftowrite);
if what you're trying to do is set off the print expression from surrounding code. The key point is that including the comma changes the parse.
I am somewhat a beginner at Perl (compared to people here). I know enough to be able to write programs to do many things with through the command prompt. At one point, I decided to write a command prompt game that constructed a maze and let me solve it. Besides quality graphics, the only thing that it was missing was the ability for me to use the WASD controls without pressing enter after every move I made in the maze.
To make my game work, I want to be able to have Perl take a single character as input from STDIN, without requiring me to use something to separate my input, like the default \n. How would I accomplish this?
I have tried searching for a simple answer online and in a book that I have, but I didn't seem to find anything. I tried setting $/="", but that seemed to bypass all input. I think that there may be a really simple answer to my question, but I am also afraid that it might be impossible.
Also, does $/="" actually bypass input, or does it take input so quickly that it assumes there isn't any input if I'm not already pressing the key?
IO::Prompt can be used:
#!/usr/bin/env perl
use strict;
use warnings;
use IO::Prompt;
my $key = prompt '', -1;
print "\nPressed key: $key\n";
Relevant excerpt from perldoc -v '$/' related to setting $/ = '':
The input record separator, newline by default. This influences Perl's
idea of what a "line" is. Works like awk's RS variable, including
treating empty lines as a terminator if set to the null string (an empty line cannot contain any spaces or tabs).
The shortest way to achieve your goal is to use this special construct:
$/ = \1;
This tells perl to read one character at a time. The next time you read from any stream (not just STDIN)
my $char = <STREAM>;
it will read 1 character per assignment. From perlvar "Setting $/ to a reference to an integer, scalar containing an integer, or scalar that's convertible to an integer will attempt to read records instead of lines, with the maximum record size being the referenced integer number of characters."
If you are using *nix, you will find Curses useful.
It has a getch method that does what you want.
Term::TermKey also looks like a potential solution.
IO::Prompt is no longer maintained but IO::Prompter
has a nice example (quoted from that site):
use IO::Prompter;
# This call has no automatically added options...
my $assent = prompt "Do you wish to take the test?", -yn;
{
use IO::Prompter [-yesno, -single, -style=>'bold'];
# These three calls all have: -yesno, -single, -style=>'bold' options
my $ready = prompt 'Are you ready to begin?';
my $prev = prompt 'Have you taken this test before?';
my $hints = prompt 'Do you want hints as we go?';
}
# This call has no automatically added options...
scalar prompt 'Type any key to start...', -single;