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5.21. Group Work: Functions and Strings

It is best to use a POGIL approach with the following. In POGIL students work in groups on activities and each member has an assigned role. For more information see https://cspogil.org/Home.

Note

If you work in a group, have only one member of the group fill in the answers on this page. You will be able to share your answers with the group at the bottom of the page.

Learning Objectives

Students will know and be able to do the following.

Content Objectives:

• Recognize strings.

• Use string indices.

• Understand how slice works for both positive and negative indices.

• Use input and convert between strings and numbers.

• Recognize common string methods.

Process Objectives:

• Predict the output from code.

• Modify code to produce the correct output.

A string is a sequence of characters enclosed in quotes. In Python you can use pairs of single or double quotes to enclose a string. You can even use tripe quotes when a string covers more than one line.

5.21.1. String Indices

Q-1: What is the last thing that will be printed when the code below runs?

Run the code below to see what it prints. Then fix it to pass the given test. It should return a string with the first character of the first name and first character of the last name.

# function definition def get_initials(first, last): return first[0] + last[-1] # function definition def main(): print(type("Hello")) print(type('Class')) print(type(42)) print(get_initials("J'Quan",'Alik')) # function call main() from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(get_initials("J'Quan",'Alik'),"JA",'''get_initials("J'Quan",'Alik')''') myTests().main()

Note

Use [index] to get a character from a string. The first character in a string is at index 0 and the last is at the length of the string minus 1 (also know as index -1 in Python). Also note that you can use pairs of single quotes or double quotes to create a string. This is useful when the string includes a single quote.

Fix the function get_initials above to return a string with the first letter of the first name followed by the first letter of the last name.

Q-3: What type of thing is “Hello”, i.e. what class does it belong to?

Q-4: What type of thing is 42, i.e. what class does it belong to?

Q-5: What is the symbol that is used to append (concatenate) strings together?

5.21.2. String Slices

Q-6: What is the last thing that will be printed when the code below runs?

Run the code below to see what it prints.

# function definition def get_short_name(first, last): print(len(first)) print(len(last)) return first[:2] + last[-2:] # function definition def main(): print(get_short_name('Simona',"Jacobs")) # function call main()

Note

Use the slice [start:end] operator to get a slice (substring) from a string. It will return a new string starting at the start and including all the characters up to just before the end (end - 1).

Q-8: Use the slice operator to return the first three characters from the variable dna?

Q-9: Use the slice operator to return a new string with just the last three characters from the variable named dna using a negative index?

Q-10: What index will the slice (substring) start with if the start index isn’t specified?

Q-11: What built-in function tells you the number of characters in a string?

Write a function join_no_first that takes two strings a and b and returns a new string with all the characters in string a except the first one followed by all the characters in b except the first one. For example, join_no_first('hi', 'bye') would return 'iye'.

def join_no_first(a, b): ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(join_no_first('hi', 'bye'), 'iye', "join_no_first('hi', 'bye')") self.assertEqual(join_no_first('abc', 'cba'), 'bcba', "join_no_first('abc', 'cba')") self.assertEqual(join_no_first('Watch out', 'bye'), 'atch outye', "join_no_first('Watch out', 'bye')") self.assertEqual(join_no_first('123', '456'), '2356', "join_no_first('123', '456')") self.assertEqual(join_no_first('!Hi', 'Bye!'), 'Hiye!', "join_no_first('!Hi', 'Bye!')") self.assertEqual(join_no_first('a', 'x'), '', "join_no_first('a', 'x')") myTests().main()

5.21.3. Input and Converting Between Strings and Numbers

Run the code below to see what it prints.

from datetime import datetime # function definition def get_name(): first = input("What is your first name?") print("Hello " + first) age_str = input("What is your age?") today = datetime.today() age = int(age_str) birth_year = today.year - age print("You were born in " + str(birth_year) + " or " + str(birth_year - 1)) # function call get_name()

Q-14: What is the name of the built-in function that will convert an integer to a string?

5.21.4. String Methods

Strings have methods (functions) that operate on a string object using dot-notation as shown in the example code below.

Q-15: What will be returned from the get_user_name function below?

Run the code below to see what it prints.

# function definition def get_user_name(first, last): print(first.lower()) print(last.upper()) print(last.replace("f", "1")) user = first.lower() + "_" + last.lower() return user # function definition def main(): print("run away!".capitalize()) print(get_user_name("Malana", "Coffy")) # function call main()

Note

Strings are immutable, which means that they do not change. String methods that appear to change a string return a new string.

Q-17: Drag each string method to the description of what it does. Read the chapter on strings and try again
• replace
• Returns a new string with all instances of the first specified character replaced with the second specified character.
• lower
• Returns a new string with all of the characters in lowercase.
• upper
• Returns a new string with all of the characters in uppercase.
• capitalize
• Returns a new string with the first letter capitalized (uppercase).

Run the code below to see what it prints.

# function definition def test(the_string): print(len(the_string)) s1 = the_string.strip() print(s1) print(len(s1)) print(the_string.find('H')) # function definition def main(): test(" Help! ") test(" how are you? ") # function call main()

Q-19: Drag each string method to the description of what it does. Read the chapter on strings and try again
• strip
• Returns a new string with leading and trailing spaces removed.
• find
• Returns the starting index of a target string if found or -1 if not found.

For more information on string methods see https://www.w3schools.com/python/python_ref_string.asp.

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