Perl Learning - 8 (I/O, <>, print(), @ARGV)
发布时间：2020-12-15 21:02:41 所属栏目：大数据 来源：网络整理
导读：InPut and Output ? STDIN can get user's input,in scalar context it returns the next line. ? $line=STDIN; chomp($line); chomp($line=STDIN)?# same as above two lines ? while(defined($line=STDIN)){ ?print "I saw $line"; ?} ? When the input en
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InPut and Output
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<STDIN> can get user's input,in scalar context it returns the next line.
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$line=<STDIN>;
chomp($line);
chomp($line=<STDIN>)?# same as above two lines
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while(defined($line=<STDIN>)){
?print "I saw $line"; ?}
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When the input ends(CTRL+D) with undef,the while loop will ends too.
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while(<STDIN>){
?print "I saw $_"; ?} ? Only in while or for(contidtion),the <STDIN> returns its line to $_,if there's something else in condition,it doesn't work. In fact <STDIN> has nothing to do with $_
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foreach(<STDIN>){???? # list context
?print "I saw $_"; ?} ? while(<STDIN>) is scalar context,once it gets one line the line be will be printed. Then gets another line,one line a time. foreach(<STDIN>) is list context,so it will gets all lines at a time,then print one line at a time going through elements.
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$ ./while_STDIN.pl
line 1 I saw line 1 line 2 I saw line 2
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$ ./foreach_STDIN.pl
line 1 line 2 line 3 I saw line 1 I saw line 2 I saw line 3
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Another way is <>,it's called "dismond operator".
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while(<>){
?chomp; ?print "It was $_ that I saw!n"; ?}
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<> gets arguments from array @ARGV,like sub &routine gets arguments from array @_
When the program starts to run,the arguments are already in @ARGV,we can modify it before <> comes,then the command line arguments are ignored.
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If @ARGV is empty,<> gets lines from user input (keyboard); otherwise it gets lines from files of arguments.
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@ARGV=qw#larry mor curly#;?# force to use these 3 files
while(<>){ ?chomp; ?print "It was $_ that I saw!n"; ?} ? Operater 'print' put everything it got to the standard device of output,typically your screen. If you need sapce or new line you have to put it in 'print'.
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$name="Larry Wall";
print "Hello there,$name,did you know that 3+4 is",3+4,"?n";
my @array=qw/hello there how are you/;
print @array;?# no space between elements print "@array";?# spaces in elements
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$ ./print_array.pl
hellotherehowareyou hello there how are you #!/usr/bin/perl
my @array=("hello
", "there ", "how ", "are ", "you ");
print @array;
print "n@array";
$ ./print_array.pl
hello there how are you
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hello
?there ?how ?are ?you
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?If 'print' has () with it,print() is a function,it returns true if succeeds false if fails.?print (2+3)*4;??# gets 5(print (2+3)) * 4;?# same as above
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