1. LING/C SC/PSYC 438/538
Lecture 6
Sandiway Fong

2. Today's Topics Perl data type coercion, conditions and looping
More on Perl hashes and Python dicts
Homework 2

3. Implicit Coercion Example: Example:
Perl features implicit coercion of data types
Example:
the following program prints Equal!
== is the numeric equality comparison operator
my $one_string = "1";
my $one_number = 1;
if ($one_string == $one_number) {
print "Equal!\n"
} else {
print "Different!\n" }
Example:
the following program prints
3 is my number
. is the string concatenation operator
= qw(one, two, three);
my $string is my number";
print "$string\n";

4. Implicit Coercion print "4" * 4 16
print "4" x 4 (“x” is the repetition operator)
4444
@a = (4) x 4 (in list context)
(4, 4, 4, 4)

5. Conditionals and Looping
if < 10 ) { print “Small array\n” } else {print “Big array\n” }
here is a scalar = size of array
unless > 10) { print }
Note: if size of array a is ≤ 10, it prints the contents of array a
Ungraded Homework Exercise:
look up the equivalents in Python (
do they always exist?

6. Conditionals and Looping

7. Conditionals and Looping
Looping over arrays
%fruits = qw(apple green orange orange lemon yellow);
foreach $f (keys %fruits) { print $f, " => ", $fruits{$f}, "\n” }
gives output:
lemon => yellow
apple => green
orange => orange
Note: keys %fruits = ("lemon", "apple", "orange") is an array

8. ARGV
Command line:
perl argv.perl 1 2 3

9. General Looping

10. More on Perl hash tables
Example:
Output:
Unique key constraint:

11. Python dict Lists and tuples are indexed by whole numbers (from 0)
Dictionaries are indexed by a key (usually a string) and return some value associated with the key
Note: use of curly braces
Dictionaries are not ordered (like sets) – see next slide
Methods keys(), values(), items()
Refer to key + value as an item: encoded as a tuple

12. Python dict

13. Python dict
Dictionary order preservation depends on the version of Python used …
Python 3.6

14. Python dict
list comprehension

15. Python dict How to print the contents of a dictionary?
Use a for-loop and method items(): k,v is a tuple

16. Python dict All values are lists Advantage: simplifies the code
Trade-off
Values are lists or a string
Advantage: simpler-looking dict

17. Python dict Works too! Less transparent: relies on a
Python-particuar quirk …

18. Python dict
function zip() pairs up elements from two (or more) lists into an iterable

19. Python dict
How to get that Perl list data structure format into a Python dict?

20. Python list ranges Perl has a range operator: ..
less powerful in some ways, more powerful in others

21. Python dict
function zip() doesn't always work quite the same in all versions …

22. Homework 2 Q1: Perl. Q2: Perl. Q3: Python. Q4: Python.
What = 4 x 4 do?
Q2: Perl.
@l1 = ("a", "e", "i", "o" , "u");
@l2 = ("あ", "え", "い", "お", "う");
write a program that builds a hash that maps romaji to hiragana, e.g. $h{i} should be "い"
Give examples to show your program works
Hint: use a loop and shift (or pop)
Q3: Python.
Do the same for l1 and l2 in Python using zip.
Q4: Python.
Do the same, without zip, using list comprehensions

23. Homework 2 Instructions: email me submit everything in one PDF file!
subject of course number, your name, homework 2
cite any discussion or source
due date: Monday 10th by midnight