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Example Scripts

Example Scripts. Lists. Script: Removing Duplicates in a List. Goal: Given a list L, construct a list K containing all the distinct elements of L maintaining the ordering in L. L = [2,8,1,2,3,6,3,8,6,1]  K = [2,8,1,3,6] One thought – convert to a set, then convert the set to a list:

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Example Scripts

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  1. Example Scripts Lists

  2. Script: Removing Duplicates in a List • Goal: Given a list L, construct a list K containing all the distinct elements of L maintaining the ordering in L. • L = [2,8,1,2,3,6,3,8,6,1]  K = [2,8,1,3,6] • One thought – convert to a set, then convert the set to a list: >>> K = list(set(L)) >>> K >>> [1,2,3,6,8] • Obviously, this does not preserve the order • Well, what shall we do, then? • How about forming the set, then for each element val doing the following: • find the first index j where val is found • remove all occurrences of val after that index • We will use functions for these steps

  3. File uniqueify.py # file: uniqueify.py defremove_all(K,val): while val in K: K.remove(val) defremove_repeats(K,val): j = K.index(val) if j == -1: # no occurrences return if j == len(K)-1: # only occurs at end of list return remove_all(K[j+1:],val) defuniqueify(K): S = set(K) for val in S: remove_repeats(K,val)

  4. File uniqueify.py # file: uniqueify.py continued# testing the uniqueify function L = [5,1,5,2,3,5,6,2,3,9,0] print('Original list L =',L) print('List order not preserved doing list(set(L)):', list(set(L))) uniqueify(L) print('Uniquefyingretains order:',L) # end of uniqueify.py Execution Output:>>> Original list L = [5, 1, 5, 2, 3, 5, 6, 2, 3, 9, 0] List order not preserved doing list(set(L)): [0, 1, 2, 3, 5, 6, 9] Uniquefying retains order: [5, 1, 2, 3, 6, 9, 0] >>>

  5. Another Solution • While the preceding was quite useful for illustrating a number of programming techniques, there is a much simpler way. • The sort method of the str class has a keyword parameter key • You can specify a function to be applied to the values being sorted before comparisons are done defuniqueify(K): J = list(set(K)) J.sort(key=K.index) #sort based on K.index(val) return J • One difference: K is not modified in place, so when you use it, you write L = uniqueify(L) not uniqueify(L)

  6. Python Script File Structure • The preceding is an example of a standard Python file structure. # imports import os, sys . . . # function definitions def func1(): . . . # Statements to be executed # “body” of the script # Some of the statements will involve functions from # modules and from the function definitions section above . . .

  7. Script: Earliest Date • We would now like to write a Python script to determine which of two dates entered by the user is the earlier date. • The user will enter a date of the form mm/dd/yyyy, where mm is the month (like 5), dd is the day (like 29) and yyyy is the year (2012) • So here is how out script would start. date1 = input('Please enter the first date (mm/dd/yyy): ') date2 = input('Please enter the second date (mm/dd/yyy): ') • So how do we determine which comes first? • First we compare the years; then if they are the same, • we compare the months; if they are the same, • we compare the days. • To get the components of a date we do the following: m1,d1,y1 = date1.split('/')m2,d2,y2 = date2.split('/')

  8. Script: Earliest Date • So here is the rest of the script. if y2>y1 or (y2==y1 and m2>m1) or (y2==y1 and m2==m1 and d2>d1): print(date1,'is earlier than',date2) elif y1>y2 or (y1==y2 and m1>m2) or (y1==y2 and m1==m2 and d1>d2): print(date2,'is earlier than',date1) else: print(date1,'is the same as',date2)

  9. Alternate Solution for Dates Problem • Here is another script for the same problem. date1 = input("Enter the first date: ") date2 = input("Enter the second date: ") m1,d1,y1 = date1.split('/') m2,d2,y2 = date2.split('/') L1 = [y1,m1,d1] L2 = [y2,m2,d2] if L1 < L2: print(date1,'is earlier than',date2) elif L2 < L1: print(date2,'is earlier than',date1) else: print(date1,'is the same as',date2)

  10. Script: Anagrams • We next build a script to test whether two strings are anagrams. • Strings are anagrams if they have exactly the same number of each alphabetic character, where case is ignored (as are all non-alphabetic characters) • The obvious tool here is the count method of the str class. s = input('Enter first string: ') t = input('Enter the second string: ') s = s.upper() t = t.upper() anagrams = True for ch in set('ABCDEFGHIJKLMNOPQRSTUVWXYZ'): if s.count(ch) != t.count(ch): anagrams = False break if anagrams: print('The strings are anagrams') else: print('The strings are not anagrams')

  11. Input Validation • It is often the case that user input must satisfy certain conditions • Input validation involves checking the conditions and asking the user to re-enter the values if the input does not satisfy the conditions. • The next example illustrates a way of doing this. h = float(input('Enter the height h (10<=h<=20): ')) while h < 10 or h > 20: print('Invalid; must be between 10 and 20, inclusive') h = float(input('Enter the height h (10<=h<=20): ')) print('You entered a height of ',h)

  12. Input Validation • A variation, with a Python feature that beginners often try (unsuccessfully) in other languages! h = float(input('Enter the height h (10<=h<=20): ')) while not(10 <= h <= 20): print('Invalid; must be between 10 and 20, inclusive') h = float(input('Enter the height h (10<=h<=20): ')) print('You entered a height of ',h) Sample run of the script: >>> Enter the height h (10<=h<=20): 25 Invalid; must be between 10 and 20, inclusive Enter the height h (10<=h<=20): 8 Invalid; must be between 10 and 20, inclusive Enter the height h (10<=h<=20): 15 You entered height 15.0 >>>

  13. Circular Shift of Characters • We next look at one of the oldest cryptographic ciphers, which is to say method of encrypting a message • It supposedly goes back to the days of Julius Caesar and is called the caesarclipher. • The cipher key is a shift amount k and both the sender and receiver must know the key value. • Each letter is replaced by shifting it by the shift amount k, with wrap around at the end of the letters • That is, 'a' is considered the successor of 'z', etc. • Here is the translation table for a shift amount of 5: a b c d e f g h i j k l m n o p q r s t u v w x y z f g h i j k l m n o p q r s t u v w x y z a b c d e • Thus every 'a' would be replaced by 'f', every 'b' by 'g', … every z by 'e'. • Typically, spaces and punctuation would be omitted. • The message 'meet me at dawn' would be encrypted as 'rjjyrjfyifbs'

  14. Circular Shift of Characters • Here is the translation table for a shift amount of 5: a b c d e f g h i j k l m n o p q r s t u v w x y z f g h i j k l m n o p q r s t u v w x y z a b c d e • Suppose the first line represents the entries of a list L. How would we construct the list represented by the second line? >>> L = list('abcdefghijklmnopqrstuvwxyz') >>> L ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'] >>> L[5:] ['f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'] >>> L[:5] ['a', 'b', 'c', 'd', 'e'] >>> L ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'] >>> K = L[5:]+L[:5] >>> K ['f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'a', 'b', 'c', 'd', 'e']

  15. Circular Shift of Characters • We are now prepared to write out shift function. >>> def shift(ch,k): L = list('abcdefghijklmnopqrstuvwxyz') if ch.lower() not in L: return ‘’ if k not in range(26): return None K = L[5:]+L[:5] return K[L.index(ch)] • Testing: >>> shift('a',5) 'f' >>> shift('a',6) 'g'

  16. Circular Shift of Characters • And now for our caesar cipher encryption function. >>> defcaesar_cipher(s,k): if k not in range(26): return None t = '' for i in range(len(s)): if s[i].isalpha(): t += shift(s[i].lower(),k) return t • Testing: >>> caesar_cipher('hello',5) 'mjqqt' >>> caesar_cipher('mjqqt',21) 'hello'

  17. List Comprehensions • We now introduce a powerful method for constructing new lists from old lists (and more) • The construction is called a list comprehension. • Format: L = [ <expression> for item in <coll> [if <expression>]] • Simple Examples • L = [2*k for k in [1,2,3,4,5,6] ]sets L equal to [2,4,6,8,10,12] • L = [2*k for k in [1,2,3,4,5,6] if k%2 == 1] sets L equal to [2,6,10] • More complex examples: • For simple applications, a 2-dimensional matrix may be represented as a list of lists • M = [[1,2,3],[4,5,6],[7,8,9]] represents a 3x3 matrix of integers, where the entries in M are the rows of the matrix. 1 2 3 4 5 6 7 8 9 • So, how can we extract the columns? • col1 = [row[1] for row in M ] • [[1,2,3],[4,5,6],[7,8,9]]  [2,5,8]

  18. List Comprehensions • M = [[1,2,3],[4,5,6],[7,8,9]] represents a 3x3 matrix of integers, where the entries in M are the rows of the matrix. 1 2 3 4 5 6 7 8 9 • How about lists for the diagonal and counter-diagonal? • diagM = [M[i][i] for i in range(3)] 12 3[0][0] 4 56[1][1] 7 8 9[2][2] • cdiagM = [M[i][2-i]] for i in range(3)] 1 2 3 [0][2] 4 56 [1][1]7 8 9[2][0]

  19. List Comprehensions • M = [[1,2,3],[4,5,6],[7,8,9]] represents a 3x3 matrix of integers, where the entries in M are the rows of the matrix. 1 2 3 4 5 6 7 8 9 • How about lists for the diagonal and counter-diagonal? • diagM = [M[i][i] for i in range(3)] 12 3[0][0] 4 56[1][1] 7 8 9[2][2] • cdiagM = [M[i][2-i]] for i in range(3)] 1 2 3 [0][2] 4 56 [1][1]7 8 9[2][0]

  20. Magic Squares print('We have formed the following 4x4 matrix:\n') for row in M: print('\t'.join([str(val) for val in row])) print() columnsM = [] for i in range(4): col = [row[i] for row in M] columnsM.append(col) diagM = [M[i][i] for i in range(4)] cdiagM = [M[i][3-i] for i in range(4)] rowsums = [sum(row) for row in M] colsums = [sum(col) for col in columnsM] diagsum = sum(diagM) cdiagsum = sum(cdiagM)

  21. Magic Squares print('row sums: ',rowsums) print('column sums:',colsums) print('diagonal sum:',diagsum) print('contra-diagonal sum:',cdiagsum) print() S = ( set(rowsums) | set(colsums) | set([diagsum]) | set([cdiagsum]) ) if len(S) > 1: print('The matrix is not a magic square') else: print('The matrix is a magic square with common', 'value',diagsum)

  22. Magic Squares Enter on one line the integers from 1 to 16 in any order, separated by spaces. We will form a 4 by 4 matrix from your entries and tell you if it is forms a magic square. Enter the values: 16 3 2 13 5 10 11 8 9 6 7 12 4 15 14 1 We have formed the following 4x4 matrix: 16 3 2 13 5 10 11 8 9 6 7 12 4 15 14 1 The matrix is a magic square with common value 34

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