Programming for Engineers in Python. Recitation 2

Transcription

1 Programming for Engineers in Python Recitation 2

2 Plan Range For loop While loop Lists Modules

3 Operations Arithmetic Operations: + plus - minus * multiply / divide (int / float) % modulo (remainder) ** power Comparison Operators (Return True or False): == equal!= not equal > greater than < less than >= greater than or equal to <= less than or equal to

4 Range Creates an ordered collection of all integers in a given range. range(a, b) contains all integers k satisfying a k < b. range(b) is a shorthand for range(0, b). >>> range(0,10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(-2,2) [-2, -1, 0, 1] >>> range(4,2) [] >>> type(range(3)) <type 'list > # coming soon

5 Range Cont. range(a,b,d), returns the list: a, a+d, a+2d,, a+id until b is reached. >>> range(0,10,2) [0, 2, 4, 6, 8] >>> range(10,0,-2) [10, 8, 6, 4, 2] >>> range(0, 10, -1) [] >>> range(0,10,0) Traceback (most recent call last): File "<pyshell#21>", line 1, in <module> range(0,10,0) ValueError: range() step argument must not be zero

6 For Loop for elem in list (TBD): statement1 statement2 Compute : partialsum = 0 for i in range(1,101): partialsum = partialsum + i print partialsum 5050 Cheat : sum(range(1,101)) determines the scope of the iteration.

7 For Loop Cont. Iterate over strings: name = Assaf for letter in name.upper(): print 'Give me', letter print 'What did we get?', name Give me A Give me S Give me S Give me A Give me F What did we get? Assaf

8 While Loop # Find the smallest divisor n = 9 div=2 while n % div!= 0: div = div+1 print "Smallest divisor of,n,"is",div is the while loop above infinite?

9 While Loop Cont. # Find prime if n < 2: print Out of range" If div!= n: print div," is a divisor of,n else: print n," is prime"

10 While Loop while [expression]: statement1 statement2 true expr statement(s) false Infinite loop: i = 1 while i < 4: print i

11 For / While Equivalence # Find smallest divisor using For: for div in range(2, n+1): if n % div == 0: break print div

12 Break - Aborting Iteration break terminates the nearest enclosing loop, skipping the code that follows the break inside the loop. Useful for getting out of loops when some predefined condition occurs. >>> for i in range(1,10): print(i) if i % 3 == 0: break 1 2 3

13 Lists A list is an ordered sequence of elements. The simplest way to create a list in Python: Enclose its elements in square brackets. >>> my_list = [2,3,5,7,11] >>> my_list [2,3,5,7,11]

14 Lists are Indexable Remember this? H e l l o The same indexing + slicing works for lists!

15 Slicing >>> my_list = [1,2,3,4,5,6,7,8,9,10] >>> my_list[1:5] # slicing ] 2,3,4,5[ >>> my_list[0:-1:2] # slicing an arithmetic progression ] 1,3,5,7,9[ >>> my_list[::2] # shorthand for previous slicing ] 1,3,5,7,9[

16 Slicing Cont. >>> my_list[::-1] # going backwards ] 10,9,8,7,6,5,4,3,2,1[ >>> my_list[3:8:-2] [ ] # output is an empty list. This is NOT an error >>> my_list [1,2,3,4,5,6,7,8,9,10] # slicing does NOT change original list

17 Lists are Indexable Cont >>> my_list = [2, 3, 5, 7, 11] >>> my_list[0] 2 >>> my_list[4] 11 >>> my_list[5] Traceback (most recent call last): File "<pyshell#7>", line 1, in <module> my list[5] IndexError: list index out of range >>> my_list[2:4] [5, 7]

18 Lists Cont. Iterate: >>> sum_list = 0 >>> for k in my_list: sum_list += k # shortcut for sum_list = sum_list + k >>> sum_list 28 Create lists: range( ) creates a list >>> new_list = [i**2 for i in my_list] >>> new_list [4, 9, 25, 49, 121]

19 Length of Lists len returns the length (number of elements) of a list. >>> len(my_list) 5 >>> len([]) 0 Quite often, the length is a useful quantity to have.

20 Lists, Strings, and Beyond Lists can contain strings: days = [ Sunday, Monday", "Tuesday, Wednesday, Thursday, Friday, Saturday ] >>> s =.join(days) 'SundayMondayTuesdayWednesdayThursdayFridaySaturday >>> s.lower() 'sundaymondaytuesdaywednesdaythursdayfridaysaturday >>> s.find('y') 5 >>> first = s[:s.find('y')+1] # what does first contain? >>> first = first[::-1] # and now?

21 Nested Lists >>> mat = [ [1, 2, 3], [4, 5, 6], [7, 8, 9] ] >>> mat[1] [4, 5, 6] >>> for row in mat: # swap rows and columns for col in row: print col, #, - continue the same line print What is len(mat)?

22 Lists Dynamic Mixing types: assume you want to maintain a list of the students in this course either by name or by id: students = ] Itay, , Alon, Zohar, ] print students[2] >>> Alon Michal decided to join the course, so we update the list: # append - add an element to the end of the list >>> students.append('michal') >>> students ['Itay', , 'Alon', 'Zohar', , 'Michal']

23 Lists Dynamic But Alon wants lo leave the course: >>> students.remove('alon') >>> students ['Itay', , 'Zohar', , 'Michal'] remove removes only the first occurrence of a value.

24 Functions def function_name(argument1, argument2,...): statement1 statement2 return result1, result2, # optional Calling func: [var1, var2, [ = function_name(val1, val2,...) A function is: A named code scope Can receive input parameters Can return output parameters

25 Palindromes Palindromes are read identically from right and left. Examples: ילד בדלי A Man, A Plan, A Canal, Panama Never odd or even ו נ ש ר נ ת ב ע ש ב ד ב ר ע ב ת פ ר ש נ ף ש ר נ ו Pseudo-code: Translate from definition to code - For every index in the string: - Check if word[i] == word[len i 1] # why -1? - False: this is not a palindrome - If all indices survived this is a palindrome

26 Palindromes Code def is_palindrome(word): middle = len(word)/2 for i in range(middle): if word[i]!= word[-i-1]: # no palindrome return False return True # matches all the way Is there a shorter code for identifying palindromes?

27 Functions Scope Consider the following function, operating on two arguments: def linear_combination(x,y): y=2*y return (x+y) The formal parameters x and y are local, and their life time" is just the execution of the function. They disappear when the function is returned. 3, 4 linear_combination 11 x, y

28 Functions Scope >>> a=3 >>> b=4 >>> linear combination(a,b) 11 # this is the correct value >>> a 3 >>> b 4 # b has NOT changed The change in y is local - inside the body of the function.

29 Functions Scope >>> x=3 >>> y=4 >>> linear_combination(x,y) 11 # this is the correct value >>> x 3 >>> y 4 The y in the calling environment and the y in the function are not the same variable!

30 Exercise Unit Testing Test (1) Run my code with some input (2) Check the output My Code

31 Exercise Unit Testing Use the given implementation: - Download hw1.py - Fill in the necessary code instead of the remarks # +++your code here+++ - Instead of printing the code to the shell, return it to the testing function. - Run Module Example s output: OK got: 'Number of donuts: 4' expected: 'Number of donuts: 4' OK got: 'Number of donuts: 9' expected: 'Number of donuts: 9' OK got: 'Number of donuts: many' expected: 'Number of donuts: many' OK got: 'Number of donuts: many' expected: 'Number of donuts: many'

32 Python Code Hierarchy Statement function Module Package

33 Modules All modules and their contents (functions, constants) can be found at >>> import math # mathematical functions >>> dir(math) [' doc ', ' name ', ' package ', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'hypot', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc'] >>> math.pi # a constant >>> math.sqrt(16) # a function 4.0 >>> math.log(8, 2) 3.0

34 Modules Cont. >>> import os #operating system interfaces >>> os.rename( my_file.txt, your_file.txt ) >>> os.mkdir( new_dir ) >>> for root, dirs, files in os.walk('lib/ '): print root, "consumes", print sum(getsize(join(root, name)) for name in files), print "bytes in", len(files), "non-directory files" >>> os.times()[0] # user time