COMPSCI 101 Principles of Programming

COMPSCI 101Principles of Programming Lecture 19 – Strings as Sequences Dr. Patricia J. RIddle

Who am I? • My research area is Artificial Intelligence • Girls are as good as boys at Computer Science research • If you want to talk about either of these topics (or just talk about Computer Science 101) my office hours are: • Friday 1-3pm COMPSCI 101 - Principles of Programming

Learning outcomes • At the end of this lecture, students should be able to: • understand that strings are just sequences • use an index to obtain a character from a string • use loops to iterate through a string • make good test cases COMPSCI 101 - Principles of Programming

Review of return from test Do these two functions print the same output? Why or why not? deffunction1(): print(["fred"]) print(["sally"]) return(["mary"]) def function2(): print(["fred"]) return(["mary"]) print(["sally"]) COMPSCI 101 - Principles of Programming

Review of Strings • Strings and Characters • A string is a SEQUENCE of characters. • String literals can be enclosed in matching single quotes (') or double quotes ("). • Python does not have a data type for characters. • A single-character string represents a character COMPSCI 101 - Principles of Programming

Lists as Sequences • List of strings • fruit_list = ["apple", "banana", "orange"] • fruit_list[3] • len(fruit_list) • List of strings • my_word_list= ["s", "t", "r", "I", "n", "g"] • my_word_list[3] • len(my_word_list) COMPSCI 101 - Principles of Programming

Strings as Sequences • String • my_word= "string” • my_word[3] • len(my_word) COMPSCI 101 - Principles of Programming

Sequences • Lists and strings are examples of a more basic Python type: the sequence • Sequences can be: • Indexed • Iterated over with for loops COMPSCI 101 - Principles of Programming

How to get the first letter of a string? • Just like a list • Which one will work? • my_word[0] • my_word[1] COMPSCI 101 - Principles of Programming

How to get the last letter of a string? • Just like a list • Can use len(my_word) • What does my_word[ len(my_word) ] return? • Can use my_word[ len(my_word)-1 ] • Or can use my_word[-1] COMPSCI 101 - Principles of Programming

Testing • isinstance(x,str) • Returns True if variable x is a string (i.e., is of type “str”) and False otherwise COMPSCI 101 - Principles of Programming

Exercise Write a function named substring() that accepts 2 strings as parameters and prints True if the first string is a substring of the second string and False otherwise. >>> substring("ab", "ab fab") True >>> substring("ab", "fact") False COMPSCI 101 - Principles of Programming

Answer COMPSCI 101 - Principles of Programming

Exercise Write a function named palindrome() that accepts a string as a parameter and prints True if the string is a palindrome and False otherwise. A palindrome is a word, phrase, number, or other sequence of symbols or elements, whose meaning may be interpreted the same way in either forward or reverse direction. Wikipedia >>> palindrome("otto") True >>> palindrome("fact") False COMPSCI 101 - Principles of Programming

Exercise Write a function named heterogram() that accepts a string as a parameter and prints True if it is a heterogram and False otherwise. • A heterogram is a word, phrase, or sentence in which no letter of the alphabet occurs more than once. • Wikipedia >>> heterogram("otto") False >>> heterogram("fact") True COMPSCI 101 - Principles of Programming

More Test Cases heterogram("i am") heterogram("iam boy") heterogram("iam a boy") COMPSCI 101 - Principles of Programming

Summary • Learned that Strings and Lists are analogous in many ways, both are the basic Python type Sequence • Learned how to iterate through strings • Learned how to make good test cases COMPSCI 101 - Principles of Programming

Tomorrow • More fun with Boolean Conditions COMPSCI 101 - Principles of Programming

COMPSCI 101 Principles of Programming