perl basic

by dan 10/05/06 12:00:00 Article perl.

"Notes from my early perl days. Life was simple back then. No readability, no named variables, no problem.

Contents

Variables
Literals
STRING
- string operator
ARRAY
HASHES
FUNCTIONS
- print()
CONTROL STRUCTURE
- Boolean
- For-Loop
REFERENCE

Variables

Variables are Case-Sensitive

$ScalarVar=...

@Array=(...);

%Hash={...}

To separate Variables from a scalar value: ${Var1}Go instead of $Var1Go.

Literals

' ': single quote- anything, incl. NL var not expanded, no formatting code

ex:

'hello
there' # same as ""hello\nthere""

"" "": double quote- var expanded

FORMATTING CODE:

\l= lowercase next letter
\L..\E= lowercase (""\LJOE""=""joe"")
\u=uppercase next letter
\U..\E=uppercase (""\Ufred"" = ""FRED"")
ex: ""\u\LDAVE"" = ""Dave""
\cX  = ctrl-X, X can be any char
\007 = octal, \0x7f=hex code
\Q ..\E=backslash quote
\$= ""$"" ie dollar sign

numeric operator:

<,<=,==,>,>=,!=,++,--,*=,+=
**=power()

STRING

Index(str,substr, [startPosition]);

Finds 1st occurrence of substr in str and returns Position.
Returns -1 if not found.
StartPosition: search starts at startPosition, not 0.
$result=Index('abcd','c'); # $result = 2;

rindex(str,substr, [startPosition]);

Finds 1st occurrence from backward order (rightmost).
StartPosition starts from that position and goes backward.

$I=Substr(str,start,length);

Returns substring based on position and length.
$a=substr(""abcd"",2,2) ; # $a=""cd"";
If $start is minus, it starts from end of string.
$a=substr(""abcde"",-2,2); # $a='de';

substr($strVar,start,length)=""new string"";

To replace part of string

ex:

$a=""hello world"";
substr($a,0,5)=""abcde""; #$a='abcde world';
substr($a,0,5)='hi; # $a='hi world'; Automatically resizes.
Substr($a,-6,5)='pppppppp'; # replaces 'world' and longer
#   ie 'hello pp

$str=sprintf("".."",$var);

printf in a var

chomp($name);

removes only the NL if it exists
chomp(@texts); # removes NL from all array elem.

chomp($a=<STDIN>);

shortcut for $a=<STDIN>;chomp($a);

chop($name);

removes any last char

undef

undefined type, when var is not assigned

string operator

""hello"" . ""world""

concat

eq,ne,lt,gt,le,ge

comparison

""fred"" x 3

string repetition, ""fredfredfred""

""5"" x 3 # ""555""

$str++

Magic increment. Increments a string containing number.

""000099"" -> ""000100"".
There is NO magical decrement.

ARRAY

Assignment of entire array

ex:

@words=(""camel"",""bat"",""apple"");
@words=qw(camel bat apple); #separated by whitespace
@a=(1,2,3,""fred"",""dave"");
@a=();  #Empty array
@a=(1..4); # =(1,2,3,4)
@a=(1.2..4.2);  # =(1,2,3,4)
@a=(2..4,7); #(2,3,4,7)
@a=($a..$b); #(whatever $a and $b ranges)
@a=1;  #assigns array ie @a=(1);
@a=@b;  #copy
print ""@a""; #prints all elements of array @a

Multiple Assignment:

($a,$b)=($b,$a);  #swap a,b
($a,$b,$c)=(1,2,3); #a=1,b=2,c=3
($a,@b)=($c,$d,$e); $a=$c;@b=($d,$e)

SCALAR OF ARRAY:

$i=@a; # i=size of array a
($i)=@a;  # i=1st element of array a
$#a= index value of last element of @a

@a=qw(a b c); $#a contains 2
$a[0]=5; #set element. 1st element is now set to 5

SLICE:

@a[0,1]=@[1,0]; #swap 0,1st element
@a[1,2] =('a','b');# a[1]='a',a[2]='b'
@a[0,1,2] # same as @a[0..2]
@b=(3,4,5);@a[@b]... #=@a[3,4,5]

OUT OF RANGE

out of range of array => returns undef

push(@a,$newValue);

same as @a=(@a,$newValue);

ex:

push(@a,@b);
push(a,4,5,7);  #push multiple items

$oldValue = pop(@a);

removes last element

$x=shift(@a);

($x,@a)=@a; # ie 1st elem->$x, @a gets rest, ie Queue
$x=shift; # $x=shift @_; for param

unshift(@a,7); # same as @a=(7,@a);

@a=reverse (@a);

reverse array order
reverse(7,8,9); #-> (9,8,7)

@a=sort('b','c','a'); # ('a','b','c'), only alphabetical sort, not numerical

@a=<stdin>; #each line is stored in an element of @a. CTRL-D to end.

%password=qx(fred a1 jack b2 tom c3); #???

HASHES

# Hash value must be Scalar, not array/list. TO have arrays, use reference.

$I{""John""}=7; # creates key ""John"", and value 7

@a=%I; # @a gets all keys and value so hash %I

%b=@a;

%c=(""key1"",""val1"",""key2"",""val2"",...);

%a=%b; # copy

%a=reverse %b;

keys and values are reversed! But if there are any values that are identical, it will only store 1 set, not both.

('a',1,'b',2,'c',1) =>reverse=> (1,c,2,b,1,a)->(1,c,2,b)?

@a=keys(%I);

# returns @ of all keys

foreach $I (keys (%a)) { print ""$a{$I}n""}

$n= %a;: # returns 0/1 true/false if hash is defined or not
@a=values(%hash);: # lists of values
@a=each(%has);: # iterates, returns pair of key,value elem until end. # returns empty list () when finished. While ($first,$last)=each($name) {...}
delete $hash{""key""};: # removes element from hash

SLICE:

($a{a},$a{b},$a{c})=(1,2,3);
    # same as $a{a}=1, $a{b}='b',...

@a{'a','b','c'}=(1,2,3);
    print ""@a{'a','b','c'}; # output=1 2 3

Merging

@league{keys %score}=values %score;

slower merge: %league=(%league,%score);

FUNCTIONS

print()

print(""..""); print ""..."";

sub f { }

#all vars are global even w/in sub except private var

f ();

# call function f

return $a;

# func can return value

$i=someFunctionThatReturnsValue();

@_

# contains list of args, 1st arg=$_[0], 2nd arg=$_[1],...

my($arg1,$arg2)= @_; #faster

my($var1,$var2,..);

# declare var to be private, visible only within current scope

my $var1= ""hello"";

local($var1,$var2,...);

# declare var to be semi-private. Original var is restored when func returns

$a=""original"";
f(); #$a is restored to ""original"" after returning from func f()
sub f { local ($a); $a=7; }
sub f { local $_; } # used frequently.

use strict;

#forces all var to be declared (either via my() or local())

our($var1,$var2,...);

# declare global variable from within a package

*var; ('star-var')

# TypeGlobbing

CONTROL STRUCTURE

If (...) {...}: # if it is a 1-line code, statement doesn't need "";"" semi-colon.

If () {} elsif () {} else {}

$a=<TestExpr> ? <TrueExpr> : <FalseExpr>;: same as in C
Unless() { }: # same as if(!...) { }

While() { }

Until () { }

Do { } while ();

Do { } until ();

{exp2} if (exp1);

# backward if's: if (exp1) exp2;

a++ if (b<0);

{exp2} unless (exp1);

#backward unless : unless (exp1) exp2;

{exp2} while (exp1);

#backward while: while(exp1) {exp2}

{exp2} until (exp1);

#backward until: until(exp1) {exp2}

exp1 || exp2;

# Do exp1. If exp1 is false, do exp2.

DoThis || die(""failed"");

Exp1 && exp2;

# do exp1. If exp1 is true, do exp2.

Boolean

true=anything other than 0. 00, 0.00=TRUE!

False=0,"""" (empty str), undef

For-Loop

for ( ; ; ) { }

#same as C

foreach $I (@a) { } #$I is restored when loop exits.

$I is Alias/Reference, not just a copy. So assigning $I will alter @a

By default, $_ is assigned if $I is not present.

Foreach(@a) { print} # prints $_

Foreach(@a) {$_=7} # @a=(7,7,7,..)

Last;

# breaks out of loop

last LABEL; # breaks out of loop w/ label LABEL.

next;

# goes to end of loop and continue

next LABEL; # does next on loop labeled LABEL

redo;

#goes to beginning of loop and continue

redo LABEL; # does redo on loop labeled LABEL

REFERENCE

$aref=\$a;

# \ indicates reference. $aref contains ref to $a

$aref=\@a;
$aref=\%a;

$aref=[lists...]

# [] returns anonymous list and returns ref

$ar=[1 2 3]; #same as $a=(1,2,3);$ar=$a;

$aref={....}

# { } returns anonymous hash and returns ref

@{$aref}

@$aref

# dereference aref
# {} are optional: @$aref
$aref=\@array;
print @{$aref}; # print content of array
print ${$aref}[3]; # print 4th element of array
${$aref}[7]=34; #assign array

$aref->[n]: # shortcut to ${$aref}[n]

%{$href}

%$href

# dereference
# {} are optional.
$href=\%hash;
print %{$href}; # print hash
print ${$href}{""b""}; #print $hash{'b'}
${$href}{'app'}=34;

$href->{'ban'}: #shortcut to ${$href}{'ban'}

ex (PerlRefTut):

@a=([1,2,3],[4,5,6],[7,8,9]);
# @a is an array of 3 elem containing ref to arrays
# ie $a[0] -> ref to [1,2,3]
$a[0]->[2]=='3' #3rd elem of 1st array is '3'.

$aref[n][m]