LING/C SC/PSYC 438/538 Lecture 5 Sandiway Fong

Administrivia Terminal log from last lecture Arrays in Perl Homework 5

Terminal log from previous class ~$ cd Desktop Desktop$ perl ex.perl My name is Global symbol "$name" requires explicit package name (did you forget to declare "my $name"?) at ex.perl line 2. Execution of ex.perl aborted due to compilation errors. My name is John My name is $name Desktop$ perl ex.perl My name is \$name\nDesktop$ perl test.perl b a b c d e Desktop$ perl test.perl abcde a-b-c-d-e 4 5 Desktop$

Terminal log from previous class ~$ python3 Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 16:52:21) [Clang 6.0 (clang-600.0.57)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> print("My name is", name) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'name' is not defined >>> name = "Mary" My name is Mary >>> print("My name is ", name) My name is Mary >>> print("My name is" + name) My name isMary >>> @a = ('a', 'b', 'c', 'd', 'e') File "<stdin>", line 1 @a = ('a', 'b', 'c', 'd', 'e') ^ IndentationError: unexpected indent >>> a = ('a', 'b', 'c', 'd', 'e') >>> a[0] 'a'

Terminal log from previous class >>> a[1] 'b' >>> a[-1] 'e' >>> a[-2] 'd' >>> b = ['a', 'b', 'c', 'd', 'e'] >>> b[-2] >>> b[1] >>> b[1] = 'B' >>> b ['a', 'B', 'c', 'd', 'e'] >>> a[1] = 'B' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment >>> b ['a', 'B', 'c', 'd', 'e'] >>> b[-1] 'e' >>> len(b) 5 >>> len(b)-1 4

Shell quoting Bash shell (MacOS, Linux): cf. Windows 10 slide earlier manual: http://www.gnu.org/software/bash/manual/ can't write: perl -e '@a=(\'a\',\'b\',\'c\'); print "@a\n"'

Shell quoting Bash shell (MacOS, Linux): can write: perl -e "@a=(\"a\",\"b\",\"c\"); print \"@a\n\""

perlintro: Perl Arrays like a simple ordered list… (in Python, we use a list) Coercion @ARRAY = number of elements in scalar context Python (no coercion): len(array) Built-in functions: sort @ARRAY; reverse @ARRAY,; push @ARRAY, $ELEMENT; pop @ARRAY; shift @ARRAY; unshift @ARRAY, $ELEMENT, splice @ARRAY, $OFFSET, $LENGTH, $ELEMENT $ELEMENT above can be @ARRAY Python: array.sort(), array.reverse(), NO push (use array.append() instead), array.pop(), No shift/unshift etc… Built-in arrays: @ARGV (command line arguments) @_ (sub(routine) arguments)

perlintro: Perl Arrays Similar to pop/push, but operates at the left end of the array Python doesn't have these defined but can be simulated via slicing and concatenation: array[1:] list + array

perlintro: Perl Arrays

Homework 5: introduction Palindrome word: a word that spells the same backwards or in reverse (case insensitive) e.g. Dad, radar, racecar from Herculaneum (buried in AD 79): Word English POS/Features Root SATOR sower NN AREPO Arepo NNP TENET holds VBZ (3SG.PRES) tenere OPERA work NNS (NOM/ACC.PL) opus ROTAS wheels NNS (ACC.PL) rota "The farmer Arepo has [as] works wheels [a plough]" https://en.wikipedia.org/wiki/Sator_Square

Homework 5: introduction Suppose we represent a word as a string (Python or Perl). How do we check whether a word is a palindrome? Two ideas: Compare a word with its reverse for identity. An empty string or string with one letter is a palindrome. A string (> 1 letter) is a palindrome if the first and last letters match, and the rest of the string is also a palindrome. Example: racecar 'racecar' == reverse('racecar') (='racecar') r[aceca]r -> a[cec]a -> c[e]c -> e ok!

Homework 5: introduction Using Python reversed() and Perl reverse(): Notes: eq compares strings; == compares numbers @ARGV are the arguments passed on the command line to the program @ARGV is an array (historical naming convention) $ARGV[0] is the first argument $ARGV[1] would be the second, etc. Booleans in Perl: 0 or '' or '0' is False, any non-zero number, e.g. 1, is True.

ARGV ARGV: argument vector Python: sys.argv first popularized through the C programming language (Unix). command line: program_name arg1 arg2 arg3 … some differences in what ARGV[0], ARGV[1], etc. map to, in some cases ARGV[0] = program_name, in others ARGV[0] = arg1. Python: sys.argv import sys sys.argv[0] sys.argv[1] python3 prog.py arg1 arg2

ARGV prog.py: import sys print(len(sys.argv)) print(sys.argv) Desktop$ python3 prog.py arg1 arg2 3 ['prog.py', 'arg1', 'arg2'] Desktop$

ARGV prog.perl: Desktop$ perl prog.perl arg1 arg2 1 arg1 arg2 Desktop$ use warnings; use strict; print "$#ARGV\n"; print "@ARGV\n" Desktop$ perl prog.perl arg1 arg2 1 arg1 arg2 Desktop$

Homework 5: introduction Using Python reversed() and Perl reverse(): Notes: reversed() creates an iterator/iterable, ''.join(iterable)

Homework 5: introduction Using Python reversed() and Perl reverse(): Notes: reversed() creates an iterator/iterable, ''.join(iterable)

Homework 5: introduction Python also has a (non-transparent) slicing operator:

Homework 5: introduction Python also has a (non-transparent) slicing operator:

Homework 5 Your homework is to implement idea #2 in Perl: Two ideas: Compare a word with its reverse for identity. An empty string or string with one letter is a pallindrome. A string (> 1 letter) is a pallindrome if the first and last letters match, and the rest of the string is also a pallindrome. Example: racecar 'racecar' == reversed(racecar)='racecar' r[aceca]r -> a[cec]a -> c[e]c -> e ok! d[ad]a

Homework 5 palindrome.py Desktop$ python3 pallindrome.py dad True Desktop$ python3 pallindrome.py dada False Desktop$ python3 pallindrome.py racecar Desktop$

Homework 5 Your Perl program should behave similarly… Note: in Perl, 0 is False, non-zero True. @_ is the array used in subroutine parameter passing. Therefore $_[0] is the 1st argument passed to sub palindrome

Homework 5 Perl Resources: sub https://perldoc.perl.org/perlsub.html substr https://perldoc.perl.org/functions/substr.html length https://perldoc.perl.org/functions/length.html if-then-else https://perldoc.perl.org/perlsyn.html#Compound-Statements

Homework 5 Start with: Map the Python code line-by-line into Perl:

Homework 5 Q1: submit your Perl code and sample runs. If necessary, e.g. if your mapping is not straightforward, explain your Perl code. Q2: modify your Perl code to check for the presence of $ARGV[0], i.e. it should print an error message and exit gracefully in this situation: Q3: modify the Python code also to gracefully check for sys.argv[1]:

Homework 5 Due date next Monday midnight Usual rules: ONE PDF file We'll review the code in class next week this time