Compsci 06/101, Fall 2010 4.1 What will you do in Compsci 6 Today?  Learn about selection and if/else statements  In context of solving problems  Understanding how to alter flow-of-control in Python  Vastly increasing the kinds of problems we can solve  Learn about looping over sequences  See above! Sequences will be strings, lists, images  Learn about lists  Humongously powerful structure for storing data Compsci 06/101, Fall 2010 4.2 Language and Problems in Context  Convert Romeo and Juliet to Piglatin  What else could we do with Shakespeare's plays?  How do we convert HTML to text?  Remove all $$ from salary data in a file  How do we make an image larger, more red, …  What is an image? How do read it? Convert it? Access it?  How do get the parts of an IP address 152.3.250.1  What are the parts? How do we access them?  How would a router use these parts? Compsci 06/101, Fall 2010 4.3 How do you solve a problem like …?  Solution from lab last week always returns -1? def firstVowelIndex(word): ai = word.find('a') ei = word.find('e') ii = word.find('i') oi = word.find('o') ui = word.find('u') return min(ai,ei,ii,oi,ui)  There is more than one way to skin a cat, but we need at least one way Compsci 06/101, Fall 2010 4.4 Python if statements and Booleans  In python we have if: else: elif:  Used to guard or select block of code  If guard is True then, else other  What type of expression used in if/elif tests?  ==, <=, <, >, >=, !=, and, or, not, in  Value of expression must be either True or False  Type == bool, George Boole, Boolean,  Examples with if  String starts with vowel  Rock, paper, scissors (!aka Rochambeau) winnerCompsci 06/101, Fall 2010 4.5 Pragmatics of using if, else, elif  Indentation in Python determines how blocks of code execute  What's in a function, what's not  What's guarded by if/else statement  Preview: what's in control of for loop  Nested blocks are hard to develop and reason about, but powerful  Sometimes factor nested blocks into other functions, for example see Uppity.py  Complex boolean expressions difficult to deal with  When is a year a leap year? When divisible by 4, unless divisible by 100 except when divisible by 400 Compsci 06/101, Fall 2010 4.6 Grace Murray Hopper (1906-1992)  “third programmer on world’s first large-scale digital computer”  US Navy: Admiral “It’s better to show that something can be done and apologize for not asking permission, than to try to persuade the powers that be at the beginning”  ACM Hopper award given for contributions before 35 2004: Jennifer Rexford 2009: Tim Roughgarden Compsci 06/101, Fall 2010 4.7 READ THIS NOW!  How do we convert Romeo and Juliet to all uppercase: WHEREFORE ART THOU ROMEO?  See Uppity.py for details, note how many functions have loops  We start with loops over sequences for element in sequence: #Process element  Sequences: Strings, files, lists, (more in future)  Sequences queryable with boolean in operator Compsci 06/101, Fall 2010 4.8 Sequences: Strings, Files, Lists  String is a sequence of characters which are essentially strings of length 1  Python does not have a character type  In Python type("rocket"[0]) is ?  File is a sequence of lines, each line is a string  Can process files in other ways, e.g., read entire file or process a fixed number of bytes/chars at a time  List is a sequence of elements  For example, returned by line.split() in Uppity.py  Lists are powerful abstractions and they are mutable!Compsci 06/101, Fall 2010 4.9 Anatomy of a Python List  Lists are sequences and indexable/subscriptable  What does this mean in Python?  Lists created by using list(..) as a function, similar to str(..) and int(..)  What is list("apple") and what is list(123)?  Lists created and returned by other functions, e.g., str.split() or range(..)  We'll see range later, not a list in Python 3  See list idiom in Uppity.py, create [], then append  Grow the list when appropriate Compsci 06/101, Fall 2010 4.10 Python loops and strings  Smelly code and how to fix it: firstVowelIndex  If you're duplicating code it's smelly  If you think there's a better way, it could be smelly  Smelly code should NOT always be fixed, sometimes you can simply call it odorific! Running right is most important! def firstVowelIndex(word): mi = len(word)+1 for v in "aeiou": if v in word: mi = min(mi,word.find(v)) return mi Compsci 06/101, Fall 2010 4.11 Looping and other idioms  Initialize a variable before a loop, update in loop  See vowel indexing code on previous slide: min • What about maximal element in a list?  Create list of "long" words (from list, file, other source)  This is filtering: selecting according to some criteria  Could also remove elements from list, changing list  Create new list v. alter existing list: benefits, tradeoffs?  Can we add up the digits of a number?  How can we index elements of a number?  Can we add up divisors of a number?  How do we loop over possibilities? Compsci 06/101, Fall 2010 4.12 Donald Knuth (b. 1938, Hopper '71)  Scholar, practitioner, artisan  Art of Computer Programming  Began effort in 1962 to survey entire field, still going  Writes beautiful code  Developed TeX to help typeset his books, widely used scientific document processing program  Many, many publications  First was in Mad Magazine  On the Complexity of Songs  Surreal NumbersCompsci 06/101, Fall 2010 4.13 It’s all relative and it depends I make the best bread in the city I make the best bread in the world I make the best bread in the universe I make the best bread on the block Compsci 06/101, Fall 2010 4.14 Richard Stallman (b.1953, Hopper '90)  "World's Best Programmer"  Gnu/Linux: g++  Believes all software should be free, but like “free speech”, not “free beer”  Won MacArthur award for his efforts and contributions  League for Programming Freedom • It's about free, not