Bertelsmann Tech Scholarship Challenge, Python, Udacity

Python Control Flow: Loops

Unlike humans who get bored easily, computers are great at repetitive tasks. They do them fast. We have to have an easier way of having a computer do a repetitive task instead of writing the same line of code over and over and over. We use loops to do this.

For loop

The preferred way when working with lists, dictionaries, or tuples. In a for loop the code will execute a certain number of times.

Example of a for loop:

names = [“Sal”, “Mary”, “Charlie”, “Patty”]

for name in Names:

A for statement will have:

  • the for keyword
  • a variable name
  • The in keyword
  • The name of the list, dictionary or tuple
  • A colon
  • Starting on the next line an indented block of code

The code in the loop is ran 4 times, one time for each element in the list.

The for loop is also used with a built in range() function. This function helps us specify the number of times that we want a for loop to run.

Example of for loop with range() function

total = 0
for num in range(101):
   total += num


This loop will add all the numbers from 1 to 100 and then when it’s done going through all the numbers it will print the total.

The start, stop and step arguments to range()

We can change the number in our ranges. We can change the number that we start using and the number we end at (but not included).

for i in range(12, 16):
-> 12
-> 13
-> 14
-> 15

If we use the third argument. The first two arguments are the start and stop values and the third will be the step or the amount that the variable is increased by after each loop.

range(start, stop, step)

for i in range(5, 15, 5):

-> 5
-> 10
-> 15

The range() function is flexible and we can have negative numbers if we even wanted to count backwards

for i in range(5, -1, -1):

-> 5
-> 4
-> 3
-> 2
-> 1
-> 0


While Loop

A while loop the code while something is true and stops when it becomes false. This will make it easy print out all the numbers between 1 and 100

number = 1
while number < 101:
   number += 1


A while loop will have:

  • The while keyword
  • a variable name that we created before the loop
  • The condition to be met
  • A colon
  • Starting on the next line an indented block of code
  • Somewhere in the code block there should be a change made to the variable


Notice a couple of things. Our loop will run until number is less than 101. We didn’t use 100 because if we did it would only print to 99. The expression would no longer be true when number got to 100 so we used one larger

We could’ve done something like this

While number <= 100 and this would still run at 100 because 100 is less than or equal to 100.

Most of the time errors with loops happen because of logic (human error) as opposed to us syntax (writing something wrong).

The last line in our code increments number by one. If we were not careful and forgot to add this line to our code we would have what is known as an endless loop because 1 is always less than 101 and the loop would run forever until our computer crashed.

Before running loops make sure that the condition will turn false at some point and not crash our computers.

Looping through Dictionaries

We might have the need to loop through a dictionary to get some values or to count items. This can be done in different ways.

Loop through the keys

favorite_books = {
  "Harry Potter" : "J.K Rowling", 
  "The BFG": "Roald Dahl", 
  "Pet Sematary": "Stephen King"

for book in favorite_books: 

-> Harry Potter
-> The BFG
-> Pet Sematary

Loop through the values

favorite_books = {
  "Harry Potter" : "J.K Rowling", 
  "The BFG": "Roald Dahl", 
  "Pet Sematary": "Stephen King"

for author in favorite_books.values(): 

-> J.K Rowling
-> Roald Dahl
-> Stephen King

Loop through all keys and values

favorite_books = {
   "Harry Potter" : "J.K Rowling", 
   "The BFG": "Roald Dahl", 
   "Pet Sematary": "Stephen King"

for book, author in favorite_books.items(): 
   print(book,  ":",  author)

-> Harry Potter : J.K Rowling 
-> The BFG : Roald Dahl
-> Pet Sematary : Stephen King

Using get and loops to count objects in a dictionary

Let’s say we wanted to count how many people there were on one side of our family.


We have two lists of families one on our mom’s side and one on our dad’s side

mom = ["Charlie", "Martha", "Alexandra", "Stephanie"]
dad = ["Mary", "Joe", "Wally"]

We have a dictionary of the family sizes

family_sizes = {
  "Mary": 2, 
  "Charlie": 2, 
  "Joe": 4, 
  "Wally": 1,
  "Martha": 3, 
  "Alexandra": 1

We want to create a new dictionary of only our moms family size.

mom_family_size = {}

for family in family_size: 
   mom_family_size[family] = mom_family_size.get(family, 0)


Bertelsmann Tech Scholarship Challenge Course – Data Track Nanodegree Program. Lesson 23: Videos 10 – 21

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