COMS 6100 Class Notes

Transcription

1 COMS 6100 Class Notes Daniel Solus November 1, General Remarks We will start with a small overview of the material from the end of last class. Since we were interrupted by the fire alarm. 2 Review 2.1 List Operators push/pop shift/unshift reverse split/join grep map Note: split is very useful for parseing text. You can split on multiple levels. I use this all the time! We looked at a few basic examples of list operators in action. In particular sort, reverse sort, split and join of starters. Then we looked at the more complex operators. 1

2 2.2 List Operators Examples: p r i n t \ n L i s t Operators \n ; = qw( cat f i s h dog cow ) ; = r e v e r s e ( s o r ) ; p r i n t I have the f o l l o w i n g p e t s \n ; p r i n t I have the f o l l o w i n g p e t s \n ; my $word = s t r e s s e d ; my $mirror = r e v e r s e $word ; p r i n t $word backwards i s $mirror \n ; my %wheels = ( u n i c y c l e => 1, bike => 2, t r i c y c l e => 3, car => 4, atv => 4, semi => 18, ) ; my %c o u n t 2 v e h i c l e = r e v e r s e %wheels ; # b e c a r e f u l o f key value p a i r s that have d u p l i c a t e v a l u e s being l o s t s p r i n t \%c o u n t 2 v e h i c l e :. j o i n (,, %c o u n t 2 v e h i c l e ). \n ; my $sentance = The quick brown fox... ; = s p l i t (, $sentance ) ; p r i n t ( Found the f o l l o w i n g words : \. j o i n ). \ \n ) ; j u i c e s = qw( apple cranapple orange grape apple c i d e r ) ; = grep ( / apple j u i c e s ) ; p r i n t ( These j u i c e s c o n tain apple : \ n ) ; f o r e a c h my $ j ) { p r i n t ( $ j. \n ) ; Question: Answer: What if we have more than one value? (when using.join) join only puts a, in between (be careful of duplicate values!) Note: $ is the fill in the blank variable Example: CHOMP, removes new line if they exist. but CHOMP alone will operate on $ 2

3 2.3 List Operators Examples: Continuing with list operator examples we looked at three blocks using grep, map and map again. The last block shows that we can use map instead of a loop to get all the primes less than 10. = ( 2, 3, 5, 7, 1 1, 1 3, 1 7, 1 9 ) ; = grep { $ < ; # $ i s the f i l l i n the blank v a r i a b l e p r i n t ( \nthese are the s m a l l e r primes : \ n ) ; f o r e a c h my $ i ) { p r i n t ( $ i. \n ) ; perfectsquaresofprimes = map { $ ; p r i n t ( \nthese are the p e r f e c t s q u a r e s o f the f i r s t primes : \ n ) ; f o r e a c h my $ i ) { p r i n t ( $ i. \n ) ; = map { $ < 10? $ : ( ; # same as using grep p r i n t ( \nthese are the s m a l l e r primes : \ n ) ; f o r e a c h my $ i ) { p r i n t ( $ i. \n ) ; 3 Flow Control We will take another look at flow control, which we have seen multiple times now, in Perl. Perl is much more expressive with flow control and conditional statements. This allows for greater flexiablity and efficiency. 3.1 Basics Conditional: if, elsif, else, unless - is the opposite of if, same as if (not $expression) Loop statements: While, until do, for, foreach Modifiers 3

4 3.2 Conditional Examples: my $a = 0 ; my $b = 1 ; i f (! $a &&! $b ) { p r i n t Neither \n ; # c o n v e n t i o n a l i f ( not $a and not $b ) { p r i n t Neither \n ; # same, but with e n g l i s h i f ( not ( $a or $b ) ) { p r i n t Neither \n ; # same, but with p a r e n t h e s e s u n l e s s ( $a or $b ) { p r i n t Neither \n ; # same, but maybe the c l e a r e s t Below is the basic outline for the loop structures while, until, and do while taken verbatim: Loop statements while( continuation_expression){ # loops while continuation_expression evaluates to true statement(s); until( expression){ # loops until expression evaluates to true statement(s); do{ # execute this block, at least once, #and then while continuation_expression is true statement(s); while( continuation_expression); while( continuation_expression); # either an infinite loop, or does nothing Question Who would like to see a loop example? 3.3 Loop Examples: 4

5 my $hungry = 1 ; while ( $hungry ) { $hungry = eat ( $banana ) ; my $answer ; my $ c o r r e c t ; do{ $answer = get answer ( ) ; $ c o r r e c t = check ( $answer ) ; while ( $ c o r r e c t ) ; 3.4 For Loop Outline: The following outlines how to execute a for loop in Perl explicitly. f o r ( i n i t i a l i z i a t i o n ; c o n t i n u a t i o n e x p r e s s i o n ; increment ) { # 1) i n i t i a l i z i a t i o n happens only once, at the very beginning # 2) then, the c o n t i n u a t i o n e x p r e s s i o n i s checked, i f i t true, then the block i s exectued # 3) then, increment, # 4) Go to 2 statement ( s ) ; 3.5 For Loop Examples: Let s do a countdown! p r i n t \nloops : \ n ; p r i n t \ncountdown : \ n ; f o r ( my $ i = 1 0 ; $ i >= 0 ; $i ){ p r i n t $ i... \ n ; p r i n t \ncountdown : \ n ; f o r e a c h my $ i ( r e v e r s e ) { p r i n t $ i... \ n ; 5

6 Question: Answer: index Just wondering if i gets lost with the decrement? Perl keeps the scope of the variable within the loop. just a loop 4 Modifiers Modify simple statements Parenthese usually are not needed Works well to set default values when needed 4.1 Valid modifiers: if unless while until when for foreach 4.2 Modifier Example I: p r i n t \ nmodifier Examples... \ n ; my $c ; my $ d e f a u l t = DEFAULT ; $a = $ d e f a u l t u n l e s s d e f i n e d $a ; $b = $ d e f a u l t u n l e s s d e f i n e d $b ; $c = $ d e f a u l t u n l e s s d e f i n e d $c ; p r i n t a : $a, b : $b, c : $c \n ; $a was already set to 0; $b was already set to 1 and $c was unde- Note: fined 6

7 Note: Note: Use modifiers if they fit your style. A big part of this is to recognize modifiers used in other s code. 4.3 Modifier Example II: my $balance = ; my $ d e p o s i t = 0 ; my $withdrawal ; p r i n t balance : $balance ( i n i t i a l ) \n ; $balance += $ d e p o s i t i f $ d e p o s i t ; p r i n t balance : $balance \n ; $ d e p o s i t = ; $balance += $ d e p o s i t i f $ d e p o s i t ; p r i n t balance : $balance \n ; $withdrawal = ; $ b a l a n c e = $withdrawal i f $withdrawal and $withdrawal <= $balance ; p r i n t balance : $balance \n ; This examples increments (and decrements) the balance variable after the requirement variable is modified. This is a style of coding that is familiar and intuitive. Understanding modifier usage is important when reading other people s code. 5 Subroutines A subroutine is a group of related statements treated as a single task. Perl allows for named subroutines and anonymous subroutines. There are various ways of handling arguments and subroutines. 5.1 Declaring Subroutines Call a subroutine by it s name, usually followed by parentheses enclosing any arguments. Parentheses are not needed if the subroutines is declaring first (use the parentheses anyway). Subroutines can call themselves (recursive subroutines) 7

8 Subroutines are another datatype. Names may begin with and & character but is not needed to call. 5.2 Arguments 1. Pass by value 2. Pass by reference Note: arguments are copied into the fill in the blank 5.3 Returning 1. return command 2. last executed statement is returned if there is no return command 3. + scalar or a flat list. 5.4 Subroutine Structure 1. Declare subroutine 2. Copy arguments (before perl overwrites ) 3. Check arguments 4. Perform computation 5. Return results 5.5 Subroutine Example I: p r i n t ( f a c t o r i a l ( 5 ). \n ) ; # p a r e n t h e s e s are r e q u i r e d sub f a c t o r i a l { my ( $n ) ; # f i l l i n the blank array r e t u r n $n i f $n <= 2 ; $n f a c t o r i a l ( $n 1) ; # by d e f a u l t, a s u b r o u t i n e r e t u r n s the l a s t executed statement p r i n t ( f a c t o r i a l 5) ; p r i n t ( \n ) ; # p a r e n t h e s e s not r e q u i r e d p r i n t ( &f a c t o r i a l ( 5 ). \n ) ; # & i s o p t i o n a l 8

9 Note: Subroutine to calculate factorial of n with no final return command. 5.6 Subroutine Example II: sub ten { r e t u r n wantarray ( )? ( ) : 1 0 ; = ten ( ) ; # 1, 2, 3, 4, 5, 6 7, 8, 9, 1 0 p r i n t ( \n ) ; my $value = ten ( ) ; # 10 p r i n t ( \ $value : $value \n ) ; ( $value ) = ten ( ) ; p r i n t ( (\ $value ) : $value \n ) ; my ( $one, $two ) = ten ( ) ; p r i n t ( (\ $one, \$two ) : ( $one, $two ) \n ) ; Note: wantarray indicates datatype to be used. 5.7 Subroutine Example III: # 1) Declare s u b r o u t i n e sub f i b o n a c c i { # 2) Copy arguments ( b e f o r e p e r l o v e r w r i t e s ) my ( $n ) ; # 3) Check arguments d i e Number must be p o s i t i v e i f $n <= 0 ; # 4) Perform computation # 5) Return r e s u l t s r e t u r n 1 i f $n <= 2 ; r e t u r n ( f i b o n a c c i ( $n 1) + f i b o n a c c i ( $n 2) ) ; f o r e a c h my $ i ( ) { my $ f i b = f i b o n a c c i ( $ i ) ; p r i n t f i b o n a c c i ( $ i ) i s $ f i b \n ; Subroutine prints out the first seven Fibonacci numbers. 9