Perl Learning - 9 (printf, filehandle, open, close)

发布时间：2020-12-15 21:02:39 所属栏目：大数据来源：网络整理

导读：printf is powerer to control the outputs,it's used with a format string,and a list of elements. ? printf "Hello,$s; your password expires in %d days!n",$user,$days_to_die; printf "in %d days!n",17.85;?# in 17 days! ? Format string has so

printf is powerer to control the outputs,it's used with a format string,and a list of elements.

printf "Hello,$s; your password expires in %d days!n",$user,$days_to_die;
printf "in %d days!n",17.85;?# in 17 days!

Format string has some kind of 'conversion',each format string starts with '%' and ends with a character.
The number of elements and format strings must be the same,otherwise printf doesn't work.

%d gets integer number,like %d ... 17.85 is 17.
%s gets characters,-15 means align by left.
%f gets fload number.

printf "%6dn",42;
printf "%2dn",2e3+1.95;
printf "%15sn","wilma";
printf "%-15sn","flintstone";

printf "%12fn",6*7+2/3;
printf "%12.3fn",6*7+2/3;
printf "%12.0fn",6*7+2/3;

$ ./printf.pl
Hello,Larry; your password expires in 5 days!
in 17 days!
??? 42
2001
????????? wilma
flintstone
?? 42.666667
????? 42.667
????????? 43

If you want output a '%',use '%%','%' don't work.

printf "Monthly interest rate: %.2f%%n",5.25/12;?# Monthly interest rate: 0.44%

Filehandle is the name of I/O connection in Perl program,is the bridge between perl and outside world.
It's name of connection,but not name of file.

Perl has six buildin filehandle: STDIN,STDOUT,STDERR,DATA,ARGV,ARGVOUT
STDIN=standard input
STDOUT=standard output
STDERR=standard error

If you need other filehandles,use operater 'open'.

open CONFIG,"dino";?# input from dino
open CONFIG,"<dino";?# input from dino
open BEDROCK,">fred";?# output to fred,cover the original contents of fred
open LOG,">>logile";?# append to fred

The upper-case are filehandles,the lower-case are filenames.
When opening a filehandle,the default action is get input from file,so the first two has same results.

We can use expressions with filenames.

my $selected_output="my_output";
open LOG,"> $selected_output";?# note the space after '>'

Use 'close' to close a filehandle.
close BEDROCK;

if(!open LOG,">>logfile"){
?die "Cannot create logfile:$!";
?}
?
'die' will end program when 'open' fails,and tell you it cannot open logfile.
$! is the system error messages that shell throws out.

$ ./die_open.pl
Cannot create logfile:Permission denied at ./die_open.pl line 4.

If you don't want the program name,just put a "n" at the end.

$ cat die_open.pl
#!/usr/bin/perl

if(!open LOG,">>logfile"){
??????? die "Cannot create logfile:$!n";
??????? }
??
$ ./die_open.pl
Cannot create logfile:Permission denied

After a filehandler opens,it can be used as STDIN to get input lines.

if(!open PASSWD,"/etc/passwd"){
?die "How did you get logged in?($!)";
?}
while(<PASSWD>){
?chomp;
?print "$_n";
?}
?
$ ./die_open.pl
root:x:0:0:RHEL5:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
......

Exercises:

1. Write a program,print the reversed lines from input.

#!/usr/bin/perl

while(<>){
??? push @lines,$_;
}

@lines=reverse @lines;
foreach(@lines){
??? print $_;
}
###########################################

2. Write a program,user input lines of characters,then print the formatted lines that right align by 20 bits.

#!/usr/bin/perl

print "1234567890" x 7 . "n";

while(<>){
??? chomp;
??? printf "%20sn",$_;
}
###########################################

3. Modify the above program,let user input the align width.

#!/usr/bin/perl

chomp($width=<STDIN>);
print "1234567890" x $width . "n";
while(<>){
??? chomp;
??? printf "%${width}sn",$_;
}

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!