Welcome to the HODP Python Bootcamp for Beginners

We assume no prior programming knowledge and give a brief introduction to Python, one of the most widely used programming languages. Python is incredibly useful for doing data analysis, especially for large data sets.

This bootcamp will make sure that you are comfortable with basic Python and equip you with the tools to Google and Stack Overflow your way through more projects!

Shortcuts and Commands

Some useful shortcuts are:

Esc & Enter keys are used to toggle between selecting a cell versus editing a cell. Up and down arrows are used to toggle between cells.

Shift + Enter runs your selected cell and moves to the next cell.

Ctrl / Cmd Enter runs your current cell.

Ctrl / Cmd + M lets you change a block of code into text.

Ctrl / Cmd + Y lets you change a block of text into code.

Ctrl / Cmd + A lets you insert a block of code above the current one.

Ctrl / Cmd + B lets you insert a block of code below the current one.

Variables & Operations

A variable is a way to represent data in Python. We declare a variable by giving it a name and a value. Values of the variable can change and can have different types (e.g. integers or strings).

Here is an example of declaring a variable:

y = 3.45

Here we have a variable y and have assigned it the value 3.45. Variable names must begin with letters.

To print the value of a variable, we pass the variable to a function called print.

print(y)

There are four types of values:

# Integer x = 1 print(x) # String (e.g. text) word = "a string of text" print(word) # Boolean (True or False, 1 or 0) b = True print(b) # Double or Float (e.g. a real number with decimals) doub = 3.14159 print(doub)

Additionally, you can use multiple operations on variables.

For operations on integers, doubles, and floats (e.g. the numbers),

x = 3 y = 2 # addition add = x + y print(add) # subtraction diff = x - y print(diff) # division div = x / y print(div) # modulus (remainder) mod = x % y print(mod) # exponent exp = x ** y # note that this means x^y print(exp)

To change the type of a variable, we have int(), str(), bool(), float()

# int(), floors the value print(int("2") + int(5.6)) # str() print("One is also " + str(1) + " and " + str(True)) # bool() print(bool("True")) # float() print(1.1 + float("1.1"))

Strings

Strings is the data type that is essentially text. They are treated as an array, or list, of individual characters.

To declare a string, you must use single or double quotation marks

my_str = 'Wow what a string.'
my_other_str = "Wow another string."

However, notice that you cannot add in a line break otherwise an error occurs. To write a multi-line string, you use triple quotation marks, e.g.

my_long_str = """
    This string
    takes up
    many lines.
"""

We will cover some simple operations with strings, including concatenation and parsing, but for more information, see here.

# concatenation print("a" + "b") # concatenate without spaces but can only concatenate strings print("a" "b") print("a", 1) # with commas, you can concatenate non-string values but it automatically adds spaces

# quotation marks print('"Hello," they said.') print("\"Hello,\" they said.") print("'Hello,' they said.")

# indexing through strings my_string = "hodp 1234" print(my_string[0]) # the first character print(my_string[-1]) # the last character print(len(my_string)) # the length of the string

# uppercase / lowercase my_string = "abcABC123" print(my_string.upper()) print(my_string.lower())

# splitting strings into an array or list by a delimiter my_string = "a. b. c." print(my_string.split(" ")) # split my string based on spaces, or " " print(my_string.split(".")) # split my string based on periods, or "."

# replacing all of one character by another character my_string = "bootcamp" print(my_string.replace('o', '0'))

Exercise 1

Define a variable x with value "2" (that is the string "2")
Define a variable s = "The square of 2.0 is 4.0.", without typing any numbers and only using x. (Hint: Use float(), str() to convert between types.)
Print s in all lower case, and then all upper case.
Print the length of s. (Hint: This should return 25)

# WRITE CODE HERE

Lists and List Methods

Lists are an easy and flexible way to store different types of information. Since lists are ordered, they can also be accessed via index (e.g. the first item of the list being at index 0).

To declare a list, use brackets [ ] to enclose the list and commas , to separate each item.

my_list = ["a string", 2, 1.5, True]

Notice how a single list can contain multiple data types.

To declare an empty list,

empty_v1 = []
empty_v2 = list()

The list() function creates an empty list, but also turns the input into a list.

Here, we will cover some basic info about lists and common list operations.

# Indexing in lists # Lists of size n (e.g. have n elements) begin at index 0 and go to (n-1) lst = ["a", "b", "c", "d"] print(lst[0]) # the first item in the list is at index 0 print(lst[3]) # the fourth item of the list is at index 3 print(lst[-1]) # the last item of the list print(lst[-2]) # the second to last item of the list print(lst[2:]) # this prints the items in the list from index 2 and onwards print(lst[:2]) # this prints the items in the list up to, BUT NOT INCLUDING, index 2 print(lst[1:3]) # this prints the items in the list from index 1 up to (but not including) index 3

# list operations lst = ['a', 'b', 'c', 'd'] # concatenating lists print(lst + lst) print(lst * 2) # the length of a list print(len(lst)) # the first index a specific element appears in print(lst.index("a")) # add element to list lst.append("z") # this mutates the list print(lst) # remove element from list lst.remove("z") print(lst) # insert an element into a specific index in the list lst.insert(2, "zzz") print(lst) # remove an element at a specific index in the list del(lst[2]) print(lst) # reverse list lst.reverse() print(lst) # sort list lst.sort() print(lst)

# list operations for numbers lst = [1, 3, 9, 4, 8, 0] print(max(lst)) print(min(lst)) print(sum(lst))

Exercise 2

Define a variable lst which contains the strings "h", "a", "r", "v", "a", "r", "d", in that order. (Hint: Use the function list(x) which takes an iterable x and returns x as a list)
Print out the third letter in the list. (Hint: This should be "r")
Print the index of the first "a" in the list. (Hint: Use index())
Print the number of letters between the first and second "a" in the list. (Hint: Use lst[a:b] to only look at a sublist of lst, and index())
Append the list ['c', 'o', 'l', 'l', 'e', 'g', 'e','s'] to the end of lst, and set lst to be equal to this new list.
Remove the last element from the list (the character 's') and print out your lst.

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Dictionaries

Dictionaries are like named lists, in that they are mutable and can hold values. However, unlike lists, they attach a key, as opposed to an index, to each value. These key-value pairs make up the dictionary. Values can be any data-type (e.g. strings, ints, lists, dictionaries, etc) while keys must be unique and immutable (e.g. strings, ints, etc. but not lists).

An example of a declaration of a dictionary is:

grades = {"freshman": 9, "sophomore": 10, "junior": 11, "senior": 12}

where to get the value 9, we would access it with the key:

grades["freshman"]

We'll explore more dictionary functions below.

# declaring a dictionary grades = { "freshman": 9, "sophomore": 10 } # adding elements to the dictionary with a not-yet-used key grades["junior"] = 11 grades["senior"] = 12 print(grades)

Some useful functions include

# to get a list keys of a dictionary print(list(grades.keys())) # to get a list of values in the dictionary print(list(grades.values())) # to get a list of the items in the dictionary (e.g. a list of the key-value pairs as tuples) print(list(grades.items()))

Exercise 3

Create a dictionary words with keys "armadillo","armor","arm" whose values are the number of letters in each of the keys / words.
Print out the entire dictionary.
Print the number of keys in words.
Print out the value associated with the key "armadillo".

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Conditionals

Conditionals are if statements, which takes some boolean expression (e.g. an expression that evaluates to true or false) and "if" True, performs an action.

More concisely, in Python syntax, it is:

if (...):
    # do something
elif (...):
    # do something else
else:
    # do something else

We'll touch on the elif and else more below.

# we have integers x and y x = 5 y = 5 # this if statement prints a phrase if x > y if (x == y): print(x, "is equal to", y)

But what about when x is not equal to y? Below, we have a more comprehensive if statement using elif (short for "else if") and else.

# a more comprehensive if statement x = 8 y = 8 if (x > y): print(x, "is greater than", y) elif (x < y): print(x, "is less than", y) else: print(x, "is equal to", y)

Exercise 4

Define a variable age as an integer with value 65.
Write a conditional statement which prints "You are old." if age is greater than or equal to 30, "You are not old." if age is between 13 and 30, and "Child." if age is less than or equal to 13.
Change age to have value 20, and then 10 and see if what you expected to happen occurs!

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For Loops

In Python, whenever you want to iterate over a list, dictionary, string, etc. (any iterable), you can use for loops to perform an action on each element of the iterable.

An example of a for loop would be, if I had some list named students,

for student in students:
    # do something

where I chose student to refer to each element of the list students (but any valid variable name would have worked as well).

Below, we will delve into more examples of for loops.

# for loops with lists students = ["Anna", "Brian", "Chi", "Dilhan"] # for each element in list for student in students: print("My name is " + student + ".") # for each index and element in list for i, s in enumerate(students): print("My name is", s, "and I am in position", i) # for each index in list for i in range(len(students)): print(students[i])

One useful function is enumerate, which essentially pairs each element with an index.

print(enumerate(students)) print(list(enumerate(students)))

Another very useful (and common) function is range, which returns a sequence of numbers.

a = 5 b = 10 # range(a,b) returns a sequence of ints from a to b-1 (but not b) print(list(range(a,b))) # range(b) returns a sequence of ints from 0 to b-1 (but not b) print(list(range(b))) # range(a, b, 2) returns a sequence of ints from a to b-1, incrementing by 2 (e.g. every other step) print(list(range(a,b,2)))

for i in range(10): print (i**2)

# for loops with dictionaries grades = {"freshman": 9, "sophomore": 10, "junior": 11, "senior": 12} # for each key, value pair in the dicionary for name, year in grades.items(): print(name, "is grade", year) # for each key in the dictionary for key in grades.keys(): print(key, "is grade", grades[key])

Exercise 5

Define a variable nums as a list of every number from 0 to 19 (inclusive). (Hint: Use range.)
Define a new variable squares as the list of every number in nums squared (e.g. [0,1,4,...,361]).
Print out the sum of all the elements in squares. (*Hint: Should evaluate to 2470).

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Functions

Oftentimes, we will want to perform the same action (with the same chunk of code) in multiple areas. To avoid copy & pasting everything multiple times, we define functions. Functions can take in inputs and return an output.

The syntax for defining a function is:

def square(x):
    return x**2

where square(x) takes in a variable and returns the value squared. To call this function, we can run

square(2)

which would then return 4. Notice how it is not actually another variable or x. This is because x is just what the function internally calls the input, and is not necessarily the name of the input outside of the function definition.

Below, we will go through a couple more examples of functions.

# greater(x,y) returns the greater of the two values x and y def greater(x,y): if x > y: return x else: return y print(greater(3,10)) print(greater(5,4)) print(greater(51,51))

Now, what happens when the input, lets call it x, is both defined outside the function and inside the function?

x = 10 def square(x): return x ** 2 square(5)

# extend takes the lst and appends element n to the end def extend_0(lst, n): lst = lst + [n] return lst def extend_1(lst, n): return lst.append(n) lst = [1,2,3,4] extend_0(lst, 5) print(lst) extend_1(lst,5) print(lst)

# let's explore the difference a bit more lst = [1,2,3,4] print(extend_0(lst, 5)) print(lst) print(extend_1(lst,5)) print(lst)

# because print performs and ACTION and does not have a return value # it returns None # in Python, if there is no specified return value, the function will return None print(print("x"))

Exercise 6

Define a function long(s) which takes in a string s and prints the string "long" if the length of s is greater than 10. Else, it prints "not long". Print the result of long("word") and long("universities").
Define a funcion multiple(k, n) which takes in an int n and returns a list of all multiples of k less than or equal to n. Print the result of multiple(3,9) and multiple(3,3).

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Imports

Oftentimes, there are functions that people want to use for different projects and don't want to copy over or rewrite the same function multiple times. This is where imports come in.

Python has an incredibly large amount of available libraries for doing almost anything you could possibly need, from making graphs, to analyzing data, to even implementing neural networks.

To access these libraries we use imports. For example, some common libraries for data analysis are Numpy and Pandas. Thus, to use these libraries, you would

import numpy as np
import pandas as pd

where we have imported the libraries Numpy and Pandas, and gave them a "nickname" so that we don't have to type the entire word each time we call on a function from the library.

To use functions from these libraries, e.g. Numpy's array function, which takes in a list and returns an ndarray (Numpy's own array), I would

lst = [1,2,3,4,5]
lst = np.array(lst)

If I do not want to type the np.array each time I call the function, or only want to import that function and not the whole library, I can specify that I just want to import array, e.g.:

from numpy import array

and then I can just type array() each time I call the function.

import numpy as np import pandas as pd lst = [1,2,3,4,5] print(lst) lst = np.array(lst) print(lst)

from numpy import array lst = [1,2,3,4,5] print(array(lst))

Practice Problems

Problem 1

Define a function place(lst, i, n) which takes in a list of ints lst and inserts at index i all numbers from 0 up to (but not including) n. This should edit the list lst in place and not return anything.

For example,

lst = [-2,-1,5]
place(lst, 2, 5)
print(lst)

should print out

[-2,-1,0,1,2,3,4,5]

as we insert numbers 0 through 4 starting at index i.

Hint: Check out the list operations which has a very handy function insert.

# WRITE CODE HERE

Problem 2

Define a function fib(x)which returns the xth Fibonacci number, with 1,1,2 being the first three Fibonacci numbers.

Then, using the function you just defined, print out a list of the first 20 Fibonacci numbers.

# WRITE CODE HERE

Problem 3

Define a function count_abc(s) which takes in a string s and returns the number of times the string "abc#" where the # represents any digit. Essentially, we want to know the number of instances of the strings "abc0",...,"abc9" in s.

For example,

count_abc("asdabckabc1")

should return 1, and

count_abc("abbc1abc0abc91")

should return 2.

Hint: Check out the string function count() which counts the number of times a substring appears in a string).

# WRITE CODE HERE

Welcome to the HODP Python Bootcamp for Beginners

Shortcuts and Commands

Variables & Operations

Strings

Exercise 1

Lists and List Methods

Exercise 2

Dictionaries

Exercise 3

Conditionals

Exercise 4

For Loops

Exercise 5

Functions

Exercise 6

Imports

Practice Problems

Problem 1

Problem 2

Problem 3

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