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Understanding Slices in Sequences: Principles of Programming (COMPSCI 101)

Lecture 22 of COMPSCI 101 explores the concept of sequences in programming, focusing particularly on slices. By the end of this lecture, students will learn how to effectively use slices of sequences and apply step parameters in slicing. The lecture covers mutable lists and immutable strings, providing examples of list slicing and assigning to slices. Additionally, the session introduces the concept of extended slices using step parameters, compares string and list slicing, and presents a programming exercise on substring retrieval.

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Understanding Slices in Sequences: Principles of Programming (COMPSCI 101)

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  1. COMPSCI 101Principles of Programming Lecture 22 –Slices of Sequences Dr. Patricia J. Riddle

  2. Learning outcomes • At the end of this lecture, students should be able to: • Use slices of sequences • Use step parameters on slices COMPSCI 101 - Principles of Programming

  3. Programs vs Functions • If you put a call to the function at the bottom of the file it will call it automatically. • Where does the return value go? • What happens with prints? COMPSCI 101 - Principles of Programming

  4. Review of Sequences • Lists (mutable) >>> mylist = [1, 2, 3, 4, 5, 6] >>> mylist[0] 1 >>> mylist[1] 2 >>> mylist[-1] 6 >>> mylist[-2] 5 >>>mylist[2] = 7 >>>mylist [1, 2, 7, 4, 5, 6] COMPSCI 101 - Principles of Programming

  5. Review of Sequences Continued • Strings (immutable) >>> mystring = "samuel” >>> mystring[0] 's' >>> mystring[1] 'a' >>> mystring[-1] 'l' >>> mystring[-2] 'e' >>>mystring[2] = "f" ERROR COMPSCI 101 - Principles of Programming

  6. Slices • List slicing • Often want sublists rather than individual items • Done by extended indexing of the form “start:end+1” • Note that slice is up to but not including the second subscript • Missing first subscript defaults to 0 • Missing second subscript defaults to len(list) COMPSCI 101 - Principles of Programming

  7. Examples mylist = ["sally", "mary", "sue", "fred", "sam", "mike"] >>> mylist[2:4] ["sue", "fred"] >>> mylist[:4] ["sally", "mary", "sue", "fred"] >>> mylist[3:] ["fred", "sam", "mike"] COMPSCI 101 - Principles of Programming

  8. Assigning to Slices my_list[start:end] = another_list replaces the elements my_list[start] up to but not including my_list[end] with the elements from another_list Can do insertion too (but insert method is easier to read): but insert method will only insert a single item (see next page) my_list.insert(position,element) COMPSCI 101 - Principles of Programming

  9. Examples >>> my_list= [1, 3, 5, 7, 9, 11] >>> my_list[2:4] = [-3, -9, -11, -13] >>> my_list [1, 3, -3, -9, -11, -13, 9, 11] >>> my_list = [1, 3, 5] >>> my_list[1:1] = [-9] >>>my_list [1, -9, 3, 5] COMPSCI 101 - Principles of Programming

  10. More Examples >>> my_list = [1, 3, 5] >>> my_list .insert(1,-9) >>>my_list [1, -9, 3, 5] >>> my_list = [1, 3, 5] >>> my_list.insert(1,[-9]) >>>my_list [1, [-9], 3, 5] >>> my_list = [1, 3, 5] >>> my_list[1:1] = [-3, -9] >>>my_list [1, -3, -9, 3, 5] >>> my_list = [1, 3, 5] >>> my_list.insert(1,[-3,-9]) >>> my_list [1, [-3, -9], 3, 5] COMPSCI 101 - Principles of Programming

  11. Boundary Examples Boundaries larger than the list will work But better style to use “missing boundary” >>> my_list[2:38] = [8, 12] (use my_list[2:] = [8,12] instead) >>> my_list [1, 3, 8, 12] >>> my_list[-28:2] = [7, 6] (use my_list[:2] = [7,6] instead) >>> my_list [7, 6, 8, 12] COMPSCI 101 - Principles of Programming

  12. Extended Slices using Step Parameter • Step parameter • Default value is 1 • When the step parameter is not 1, it is called striding, sliding or stepping in different sources. I do not think they have settled on a single name. Format is [start: end: step] >>> s='abcd' >>> s[::2] 'ac' >>> s[::-1] 'dcba' COMPSCI 101 - Principles of Programming

  13. What about strings? • Lists are mutable • Strings are immutable • String slices perform the same as list slices (except assignment) • So mylist[2] = "f" OK mystring[2] = "f" FAILS mylist[2:3] = "f" OK mystring[2:3] = "f" FAILS COMPSCI 101 - Principles of Programming

  14. Assignment with Step Parameter >>> my_list = [1, 3, 5] >>> my_list[0::2] = [4] ValueError: attempt to assign sequence of size 1 to extended slice of size 2 >>> my_list[0::2] = [4, 6] >>> my_list [4, 3, 6] COMPSCI 101 - Principles of Programming

  15. Exercise • Write a function named get_substring() that returns the substring from location i to j or an empty string if given an invalid range >>> get_substring(2,3,"fred") 'ed' >>> get_substring(2,3, "al") '' COMPSCI 101 - Principles of Programming

  16. Answer defget_substring(i,j,string): if i <= j and j < len(string): return string[i:j+1] return "" COMPSCI 101 - Principles of Programming

  17. Pig Latin • In words that begin with consonant sounds, the initial consonant is moved to the end of the word, and "ay" is added, as in the following examples: • beast ? east-bay • dough ? ough-day • In words that begin with vowel sounds or silent consonants, the syllable "way" is simply added to the end of the word • another? another-way • if? if-way COMPSCI 101 - Principles of Programming

  18. Exercise • Write a function named pig_latin()that accepts a string and returns the string with the words transformed into Pig Latin. >>> pig_latin("If I am away.") 'If-way I-way am-way away-way.' >>> pig_latin("Here is another sentence") 'ere-Hay is-way another-way entence-say' COMPSCI 101 - Principles of Programming

  19. Answer defpig_latin(sentence): last = 0 new_sentence = "" if sentence == "": return '' for i in range(0,len(sentence)): if sentence[i] == " " or sentence[i] == ".": new_sentence = new_sentence + pig_latin_word(sentence[last:i]) + sentence[i] last = i + 1 if not last == len(sentence): new_sentence = new_sentence + pig_latin_word(sentence[last:len(sentence)]) return new_sentence COMPSCI 101 - Principles of Programming

  20. Helper Function defpig_latin_word(word): vowels = “aeiouy” #can use string or list capital_vowels = ["A","E","I","O","U","Y"] if word[0] in vowels or word[0] in capital_vowels: return word + "-way" return word[1:len(word)] + "-" + word[0] + "ay” COMPSCI 101 - Principles of Programming

  21. Exercise • Write a function named rotate_matrix()that accepts a list and two integers representing the number of columns and rows in the initial matrix respectively and prints out the original matrix, and prints out the matrix in a rotated form. >>> rotate_matrix(["0","1","2","3","4","5","6","7","8","9","10","11"],4,3) ['0', '1', '2', '3'] ['4', '5', '6', '7'] ['8', '9', '10', '11'] ['0', '4', '8'] ['1', '5', '9'] ['2', '6', '10'] ['3', '7', '11'] >>> rotate_matrix(["0","1","2","3","4","5","6","7","8","9","10","11"],3,4) ['0', '1', '2'] ['3', '4', '5'] ['6', '7', '8'] ['9', '10', '11'] ['0', '3', '6', '9'] ['1', '4', '7', '10'] ['2', '5', '8', '11'] COMPSCI 101 - Principles of Programming

  22. Answer defrotate_matrix(matrix,size1,size2): start = 0 for i in range(0,size2): print(matrix[start:start+size1]) start = size1+start print("") start=0 for i in range(0,size1): print(matrix[i:len(matrix):size1]) COMPSCI 101 - Principles of Programming

  23. Summary • Slices can be used in any Sequence (strings or lists) • Slices can only be assigned in Lists, not strings • Because strings are immutable • Step parameter can be used to do Sliding/Striding COMPSCI 101 - Principles of Programming

  24. Tomorrow • Nested Loops COMPSCI 101 - Principles of Programming

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