Week 2

Variables

A variable is a descriptive name to label and store data in memory. Overall, variables can be thought of as the “data” that we use in our program

Variables can be assigned to be either values or expressions using the = operator

• Assigning a variable to a value
  • motto = keviniscool
  • x = 69
• Assigning a variable to an expression
  • product = 420 * 69
  • length = len(motto)

Variables help programmers more clearly understand and read the meaning of code. They also allow for the reuse of common values and in turn, the reusability of code

Practice:

• Create two variables called length and width with the values 12 and 17 respectively
• Create a variable called area that is equal to the area of a 12 x 17 rectangle
• Create a variable called area using an expression instead of individual values

Functions

A function is a self contained module of code that accomplishes a specific task by transforming a particular input into a corresponding output. We write functions to generalize a given task so the function can be “called” over and over again in the future

The general form of a function is as follows:

def function_name(parameters):
    body

• Function Name - the name to use in order to “call” the function
• Body - the code that the function will execute when called
• Parameters - any (optional) input values a function will use

Practice:

• Define a function called calculate_area that takes in two parameters length and width and calculates the area of a rectangle defined by such values
• Define a function called triangle_area that takes in two parameters base and height and calculates the area of a triangle defined by such values

Modules

Most programming languages (Python included) contain many libraries of pre-built functions for your own convenience called modules

To get a list of which functions a module contains, in the shell, you can use the command dir(module) where module is the name of the module you want to check

To find out more about a particular function, in the shell, you can use the command help(module.function) where module is the name of the module the function resides in and function is the name of the function you want to check

To use a module’s functions, you must first import the module by writing import module at the top of your program replacing module with the module you wish to import

Example:

>>> python
>>> import math
>>> dir(math)
['__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', ...
>>> help(math.floor)
...

Practice (using the module math):

• Calculate 2 * pi
• Take the floor and ceiling of 56.4
• Calculate the square root of the logarithm of 244
• Calculate the base 2 logarithm of 256

Strings

Python uses a built-in type called str which stands for “string”. We can iterate and index the characters of a string but the string itself is immutable (we cannot modify the values of a string once created, but we can construct new strings)

For the following print statements, recall that the + operator concatenates (combines) strings and the , operator can be used to separate items in a print statement

String Iteration Practice:

• Assign a string (perhaps keviniscool) to the variable s
  • Write a loop that prints each character in s on a separate line
  • Write a loop that prints each character in s on the same line with a space after each character
    • Example - k, e, v, i, n, i, s, c, o, o, l,
  • Write a loop that prints each character in s on the same line with a comma after each character but without the space after each comma
    • Example - k,e,v,i,n,i,s,c,o,o,l,
  • Bonus: Redo the last two tasks but without the trailing comma after the last character

A method is a special type of function that belongs to a specific data type and is designed to make use of a specific value it is “called on”. These functions cannot be called on their own and must be attached to a type in order to be called

For example, the str method upper() uses the value of the string it is attached to and returns its uppercase equivalent

>>> 'keviniscool'.upper()
'KEVINISCOOL'

String Methods Practice:

• Check whether or not a string ends with the letter h
• Check whether or not a string contains only numbers
• Replace every instance of the character l (lowercase “L”) in a string with a 1 (the number “one”)
• Given a string composed of only numbers, pad it with zeroes so that it is exactly 6 digits long
• Check whether or not a string contains only lowercase letters
• Remove all leading zeroes (0s at the beginning of the string) from a string composed of only numbers

For the following practice, you are NOT allowed to use any of Python’s str methods. However, you may use functions from __builtins__

String Functions Practice:

• longer(str, str): given two strings, return the length of the longer string
• earlier(str, str): given two strings made up of lowercase letters, return the string that would appear earlier in the dictionary
• count_letter(str, str): given a string and a single-character string, count all occurrences of the second string in the first and return the count
• remove_digits(str): return a new string that is the same as the given string, but with the digits removed
• repeat_character(str, str): given a string and a single-character string, return a string consisting of the second, single character string repeated as many times as it appears in the first string
  • Example:
    • repeat_character("keviniscool", "i") should return ii because there are two is in the first string
• where(str, str): given a string and a single-character string, return the index of the first occurrence of the second string in the first; if the second string is not in the first, return -1
  • Examples:
    • where("abc", "b") should return 1
    • where("abc", "z") should return -1

Lists

Python uses a built-in type called list which represents a list. A list is similar to the concept of “arrays” seen in other programming languages with the differences being that a Python list can contain any collection of types (not just a single type) and that when instantiating (creating) a list, you do not need to initially specify the length of the list

We process lists in a similar manner to strings. Suppose we have the list:

fruits = ['apple' ,'banana', 'orange', 'cherry', 'grape', 'onion']

Then fruits[1:4] would return a new list equal to ['banana', 'orange', 'cherry']

Practice:

• For the following steps, use names and slice notation:
  • Write an expression that produces this new list: ['orange', 'cherry', 'grape']
  • Write an expression that produces this new list: ['apple']
  • Write an expression that produces this new list: ['cherry', 'grape', 'onion']
  • Write an expression that produces a new list containing the only element that is not a fruit
• Given a list L and a value v, using list methods (i.e. not a loop), write an expression that removes the first occurence of v from L
• Write an expression that adds the string "Mr. Stark" in front of the list ["I don't feel so well"]
• Write code that transforms [2, 4, 99, 0, -3.5, 86.9, -101] into [99, 86.9, 4, 2, 0, -3.5, -101]
  • You should use just two method calls
  • Bonus: see if you can do the transformtion in a single statement
• Write a function every_third that takes a list as a parameter and returns a new list that contains every third element of the original list starting at index 0. Do not use slice notation
• Write a function every_ith that takes a list L and an integer i as parameters and returns a list consisting of every ith element of L starting at index 0. Do not use slice notation

While Loops

A while loop loops through an iterable object as long as a given condition is true. Use while loops to complete the following functions. Do not use for loops. The type of the parameter is given in parentheses. Do not actually use list as the name of a parameter or variable

Practice:

• display_list(list): print the elements of a given list
• display_list_even(list): print the elements of the given list that occur at even indicies
• display_list_reverse(list): print the elements of the given list from the end of the list to the front
• sum_elements(list): sum the elements of the given list of ints starting from the front of the list until the total is over 100 or the end of the list is reached and return the same at that point (as an int)
• duplicates(list): return True if the given list contains adjacent elements with the same value and return False otherwise

Nested Lists

Because a list can contain any type; it is possible for a list to contain an element that is a list in itself. When this happens, it is called a nested list or a list of lists

Example:

avengers = [["peter parker", "spiderman", True, 22], ["tony stark", "ironman", False, 53], ["stephen strange", "doctor strange", True, 42], ["steve rogers", "captain america", False, 37], ["bruce banner", "the hulk", False, 51], ["peter quill", "starlord", True, 39]]

We can access each element of the list avengers using its index:

>>> avengers[0]
["peter parker", "spiderman", False]

We can element each item of the inner lists using the approriate inner index:

>>> avengers[0][1]
"spiderman"

In the above example, we access the 0th element of the outer list and then the 1st element of the inner list to obtain the final value of "spiderman"

Practice:

• Write a loop that prints each list from avengers on a separate line
• Write a loop that prints the secret identity of each Avenger on a separate line
• Write a loop that examines the list avengers and computes the number of Avengers that were snapped by Thanos
  • An Avenger was snapped by Thanos if the 3rd element in the list is True
• Write a loop that examines the list avengers and computes the sum of their ages
  • An Avenger’s age ies the 4th element in the list
• Write a function nested_lengths that takes a list L as a parameter and returns a list of the lengths of the sublists
  • Formally, for each element e in L, the returned list contains a corresponding element c that represents the number of elements in e

Text Files

In this section, you will practice opening, reading, and writing text files. Use dir and help to get information on file methods provided by Python

Before you begin, download the text file data.txt and the starter code file file_basics.py. Implement the three function stubs according to their docstring descriptions

Resources

• Python Docs
• Function Definitions
• More on the OS module