Writing Solid Code

1. Writing Solid Code Introduction to Python

2. Program 1 • python -c "print 'Hello World' “ • python -c "import time; print time.asctime()“ • Start an interactive session: • python –v • help(command/expression)

3. Unix Executable Scripts • Most portable version: • #!/usr/bin/env python $ cat first.py #!/usr/bin/env python # A comment print 'Hello World‘ print 2**100 $ chmoda+x first.py $ ./first.py Hello World 1267650600228229401496703205376

4. Basic Elements • Keywords: and assert break class continue def del elif else except exec finally for from global if import in is lambda not or pass print raise return try while with yield • Operators: + - * / % ** // << >> & | ^ ~ < <= > >= <> != == • Delimiters: ( ) [ ] { } , : . ' = ; += -= *= /= //= %= &= |= ^= >>= <<= **=

5. Data Types • All data values are objects • type(obj) returns the type. • Numbers • Integer: 23, 027 (octal), 0xDA5 (hex) • Floating points : 1.00, 1.0e2 • Complex numbers: 5+6j

6. Data Types • Sequences • Iterables: All sequences are iterable. (for) • Strings: • Can use “” or ‘’ to specify. “abc” ‘abc’ (Same thing.) Unmatched ones can occur within the string. “matt’s” • Use triple double-quotes for multi-line strings or strings than contain both ‘ and “ inside of them: “““a‘b“c””” • Many Methods built into the string, for example: “hello”.upper() gives ‘HELLO’ • Tuples • (x,y) • (100,200,300)

7. Sequences • Lists • [42, 3.14, ‘hello’] • list(‘wow’) gives [‘w’,’o’,’w’] • Dictionaries ( key:value pairs ) uses Hash. • D = { ‘x’ : 42, ‘y’:3.14, ‘z’:7} • {1:2 , 3:4} • A single dictionary can store values of different types • D[‘x’] is 42. • del D[‘x’] removes the key from D.

8. Sequences • Concatenation: • S1 + S2 • S1 * n gives n copies of S1 concatenated. • Membership • x in S : tests to check whether x is in S. • x not in S : Guess? • For strings: x in S means if x is a substring of S • Indexing • x = [1,2,3,4] then x[1] is 2 and x[-1] is 4

9. Sequences • Slicing a sequence: • S[i:j]: from item i (included) to item j (excluded) • x = [1,2,3,4] • x[1:3] # [2, 3] • x[1:] # [2, 3, 4] • x[:2] # [1, 2]

10. List Methods

11. List Methods

12. List Methods

13. List Methods • >>> a + [‘whites'] [‘blend','eggs‘,2,234,‘whites'] • >>> a.append('!') [‘blend','eggs‘,2,234,'!'] • >>> 2*a [‘blend','eggs',2,234,'!',‘blend','eggs',2,234,'!']

14. Dictionary Methods

15. Dictionary Methods

16. Control Flow • if expression: statement(s) elif expression: statement(s) elif expression: statement(s) ... else: statement(s)

17. Control Flow • if x < 0: print "x is negative" elif x % 2: print "x is positive and odd" else: print "x is even and non-negative"

18. Control Flow : while x = 64 count = 0 while x > 0: x = x // 2 # truncating division count += 1 print "The approximate log2 is", count # if count == 6: break

19. Control Flow : for for target in iterable: statement(s) for letter in "ciao": if letter == ‘c’: continue print "give me a", letter, "...“ for key, value in d.items( ): # cannot use iteritems if not key or not value: # keep only true keys and values del d[key] for x in range(1,5): print x # output: 1 2 3 4

20. Sample Code #!/usr/bin/env python import string, sys # If no arguments were given, print a helpful message if len(sys.argv)==1: print 'Usage: celsius temp1 temp2 ...' sys.exit(0) # Loop over the arguments for i in sys.argv[1:]: fahrenheit=float(string.atoi(i)) celsius=(fahrenheit-32)*5.0/9.0 print '%i\260F = %i\260C' % (int(fahrenheit), int(celsius+.5))

21. Functions def function-name(parameters): statement(s) def double(x): return x*2 Calling Functions in python function-object(arguments) print double(432) def f(x, y): x = 23 y.append(42) a = 77 b = [99] f(a, b) print a, b

22. Import statement • A Typical python program is made up of several source files. • Each source file corresponds to a module. • “import” keyword allows to include other modules into a python program. • Modules • sys: stdin, stderr, argv • os: system, path • string: split • re: match compile • math: exp, sin, sqrt, pow

23. Calling External programs • import os • subprocess.Popen(["ls", "-la"]).wait()

24. Sample Program # average4.py # A program to average a set of numbers # Illustrates sentinel loop using empty string as sentinel def main(): sum = 0.0 count = 0 xStr = raw_input("Enter a number (<Enter> to quit) >> ") while xStr != "": x = eval(xStr) sum = sum + x count = count + 1 xStr = raw_input("Enter a number (<Enter> to quit) >> ") print "\nThe average of the numbers is", sum / count

25. Output Enter a number (<Enter> to quit) >> 34 Enter a number (<Enter> to quit) >> 23 Enter a number (<Enter> to quit) >> 0 Enter a number (<Enter> to quit) >> -25 Enter a number (<Enter> to quit) >> -34.4 Enter a number (<Enter> to quit) >> 22.7 Enter a number (<Enter> to quit) >> The average of the numbers is 3.38333333333

26. Sample Program # average5.py # Computes the average of numbers listed in a file. def main(): fileName = raw_input("What file are the numbers in? ") infile = open(fileName,'r') sum = 0.0 count = 0 for line in infile.readlines(): sum = sum + eval(line) count = count + 1 print "\nThe average of the numbers is", sum / count

27. Sample Program # average6.py # Computes the average of numbers listed in a file. def main(): fileName = raw_input("What file are the numbers in? ") infile = open(fileName,'r') sum = 0.0 count = 0 line = infile.readline() while line != "": sum = sum + eval(line) count = count + 1 line = infile.readline() print "\nThe average of the numbers is", sum / count

28. Assignment for today • Implement: closest_pair([(0,0),(7,6),(2,20),(12,5),(16,16),(5,8),(19,7),(14,22),(8,19),(7,29),(10,11),(1,13)]) returns: (7,6),(5,8) Should run in O(nlogn)

29. Pointers • Learn Python in 10 minutes: • http://www.poromenos.org/tutorials/python • Dive into Python • http://www.diveintopython.org/