Control Flow - Loops

Control Flow - Loops#

while
for
range
continue
break

SideNote: counters#

# Initialize a counter variable
counter = 0 
print(counter)

counter = counter + 1
print(counter)

counter = counter + 1
print(counter)

The idea here…is that as the code executes, the value of the counter increases. We’ll use these a lot in loops!

Loops#

A loop is a procedure to repeat a piece of code.

Avoid copy + pasting#

For repetitive actions, if you find yourself copying + pasting, rethink your strategy.

Loops are one way to avoid this.

# this is an example of repetitive code without loops
lst = ['you@yahoo.com', 'them@bing.com', 'otherpeople@gmail.com']

email = lst[0]
print('the user\'s email address is: ' + email)

email = lst[1] 
print('the user\'s email address is: ' + email)

email = lst[2]
print('the user\'s email address is: ' + email)

# the same thing is much easier, cleaner, more efficient, and easier to maintain with a loop:
for email in lst:
    print('the user\'s email address is: ' + email)

`while` Loops#

A while loop is a procedure to repeat a piece of code while some condition is still met.

while loops always have the structure:

while condition:
    # Loop contents

While condition is true, execute the code contents.

Repeat until condition is no longer True.

`while` Loop Example I#

number = -5

while number < 0:
    print(number)
    number = number + 1  # must have code to make condition evaluate as False at some point

`while` Loop Example II#

shopping_budget = 20
bill = 0
index = 0
prices = [3, 4, 10, 3, 2, 15, 7]

while bill < shopping_budget:
  
    # add cost of item (prices) to bill
    bill = bill + prices[index]
    
    # increment index each time through the loop
    index = index + 1
    
    #print bill so we can see what's going on
    print(bill)

Class Question #1#

How many temperature values will be output from this while loop before “The tea is cool enough.” is printed?

temperature = 115
 
while temperature > 112: 
    print(temperature)
    temperature = temperature - 1
    
print('The tea is cool enough.')

A) 1
B) 2
C) 3
D) 4
E) Infinite

Class Question #2#

What will be the value of counter after this loop is run:

keep_looping = True
counter = 0

while keep_looping:

    if counter > 3:
        keep_looping = False

    counter = counter + 1
    
print(counter)

A) 0
B) 3
C) 4
D) 5
E) Infinite

IMPORTANT NOTE:#

You must make sure that the code block in the while loop can change the while condition, or break out of the loop in some other way. Double check that this happens within the code block of the loop, not outside it. Infinite loops cause all sorts of mayhem and they are to be avoided at all costs.

number = -5

while number < 0:
    print(number)
    number = number + 1  # must have code to make condition evaluate as False at some point
    
    
    
# DO NOT ACCIDENTALLY DO THIS:
# while number < 0:
#     print(number)
    
# number = number + 1  # this line is not indented with the rest of the loop code, so it will not run in the loop

`for` Loops#

A for loop is a procedure a to repeat code for every element in a sequence.

For Loop Example I#

Looping through a list of items

# Define a list of items
list_of_items = ['A', True, 12]

# Loop across each element
for item in list_of_items:
     print(item)
    
# difference between printing within and outside loop
print(item)

For Loop Example II#

Looping through a string

# Loop across items in a string
vowels = ['A', 'E', 'I', 'O', 'U', 'a', 'e', 'i', 'o', 'u']
my_string = 'python'

for char in my_string:
    if char not in vowels:
        print(char)

Class Question #3#

How many values will be printed from this for loop before it first prints “The tea is too hot!”?

temperatures = [114, 115, 116, 117, 118]

for temp in temperatures: 
    print(temp)
    
    if(temp > 115):
        print('The tea is too hot!')

A) 1
B) 2
C) 3
D) 4
E) Infinite

IMPORTANT NOTE:#

Be very careful not to change the length of the collection you are looping through inside the for loop. This can lead to errors or infinite loops.

my_list = [1, 2, 3, 4, 5, 6]

for num in my_list:
    
    print(num)
    if num % 2 == 0:
        my_list.append(num)  # adding an item to the list inside the for loop makes the for loop keep going
        
    # this code is here to stop it from actually being an infinite loop
    if len(my_list) > 100000:
        break
        

`range`#

range is an operator to create a range of numbers, that is often used with loops.

`range` Examples#

for ind in [0, 1, 2, 3, 4]:
    print(ind)

# the asterisk here unpacks the range
# don't worry about this syntax now
print(*range(0, 5))

# Loop across a sequence of numbers, using range
for ind in range(0, 5):
    print(ind)

# Range, like indexing, is defined by 'start', 'stop', 'step', but they are individual parameters, not separated by a :
for ind in range(2, 16, 2):
    print(ind)

# using range in example above
for temp in range(114, 119): 
    print(temp)
    
    if(temp > 115):
        print('The tea is too hot!')

Class Question #4#

How many values would this loop print and what would be the last value printed?

for ind in range(1, 10, 3):
    print(ind)

A) values printed: 3; last value: 7
B) values printed: 3; last value: 9
C) values printed: 4; last value: 9
D) values printed: 7; last value: 7
E) values printed: 7; last value: 9

`continue`#

continue is a special operator to jump ahead to the next iteration of a loop.

`continue` examples#

lst = [0, 1, 2, 3]

for item in lst:
    print(item)
    if item == 2:
        continue
    print('Luckily that wasn\'t a 2')

courses = ['cogs9', 'cogs18', 'cogs108']

for course in courses:

    if course == 'cogs18':
        print(course)
        continue
  
    
    print(course + '!')

string = "python"

for char in string: 
    
    if char == "p" or char == "y":
        continue
        
    print(char)

Counters within loops

A counter within a loop…

calories = [200, 300, 500]
foods = ['cake', 'pie', 'donuts']
counter = 0        # initialize a counter

for val in calories:
    # print val
    print(val)
    print(foods[counter])
    
    # increment by 1
    counter = counter + 1

# print after loop terminates
print("Final counter value:", counter)

Class Question #5#

What will be the value of counter after this code has run:

counter = 0
my_lst = [False, True, False, True]

for item in my_lst:
    if item:
        continue
    else:
        counter = counter + 1
        


print(counter)

A) 0
B) 1
C) 2
D) 3
E) 4

`break`#

break is a special operator to break out of a loop completely.

`break` examples#

lst = [0, 1, 2, 3]

for item in lst:
    
    if item == 2:
        break
    
    print(item)

courses = ["cogs9", "cogs18", "cogs108"]

for course in courses:

    if course == "cogs18":
        break
  
    print(course)

string = "love python"

for char in string: 
    if char == "p" or char == "y":
        break
        
    print(char)

# using range in example above
for temp in range(114, 119): 
    print(temp)
    
    if(temp > 115):
        print('The tea is too hot!')
        break

Class Question #6#

What will the following code print out:

number = 1

while True: 
    if number % 3 == 0:
        break
   
    print(number)
    
    number = number + 1

A) 1
B) 1 2
C) 1 2 3
D) Something else
E) This code prints forever

Class Question #7#

For how many temp will output be printed from this for loop?

(In other words, how many times in this for loop will something be printed out?)

# using range in example above
for temp in range(114, 119): 
     
    if(temp < 116):
        continue
    elif(temp == 116):
        print('The tea is too hot!')
    else:
        break 

A) 0
B) 1
C) 3
D) 5
E) 6

Class Question #8#

What will be the value of counter after this code has run:

counter = 0
my_lst = [False, True, False, True]


for item in my_lst:
    if item:
        continue
    else:
        counter = counter + 1
        
print(counter)

A) 0
B) 1
C) 2
D) 3
E) 4

Dictionaries: Indexing & Looping#

dictionary = {'key_1' : 'val_1', 'key_2' : 'val_2'}

# Dictionaries are indexed using their keys
dictionary['key_1']

# Loop over a dictionary loops across the keys
#   Inside the loop, you can use the key to access the associated value
for item in dictionary:
    print('Loop Iteration')
    print('\tKey:\t', item)
    print('\tValue:\t', dictionary[item])
    
    

# another approach that you will find if you Google
for key, val in dictionary.items():
    print('Loop Iteration')
    print('\tKey:\t', key)
    print('\tValue:\t', val)

Code Style: Loops#

for/while statement with a colon at the end on first line
all code within the loop inside a code block (indented)

Good Code Style

number = 5
while number < 0: 
    print(number)
    number = number + 1

Code Style to Avoid

number=-5
while number<0:print(number); number=number+1 # avoid all on a single line

Loops Practice#

Loops Practice #1#

Write a function count_odd() containing a loop that will add all the odd numbers for the input range together.

def count_odd(range_lower, range_upper):
                
count_odd(0,6)

A) I did it!
B) I think I did it.
C) I started but am now stuck.
D) I have no idea where to start.

Loops Practice #2#

Write a function count_vowels() containing a loop that will loop through all the letters in my_name (the input parameter) and count all the vowels in your name.

def count_vowels(my_name):
    
count_vowels('george')

A) I did it!
B) I think I did it.
C) I started but am now stuck.
D) I have no idea where to start.

Loops Practice #3#

Write a function create_dictionary that takes two input lists lst_1 and lst_2. Inside the function, join the two lists to form a dictionary joined_dictionary where the first element in lst_1 is the first key in joined_dictionary and the the first element in lst_2 is the first value in joined_dictionary and so on and so forth. Then return joined_dictionary as the output.

A) I did it! B) I think I did it. C) I tried but I am stuck. D) Super duper lost

# YOUR CODE HERE
def create_dictionary(lst_1, lst_2):
    

# TEST YOUR FUNCTION HERE
# Note that the two input lists must be the 
# same length for the function to work properly
random_lst_1 = ['a', 'b', 'c', 'd']
random_lst_2 = [1, 2, 3, 4]

create_dictionary(random_lst_1, random_lst_2)

Control Flow - Loops

Contents

Control Flow - Loops#

SideNote: counters#

Loops#

Avoid copy + pasting#

while Loops#

while Loop Example I#

while Loop Example II#

Class Question #1#

Class Question #2#

IMPORTANT NOTE:#

for Loops#

For Loop Example I#

For Loop Example II#

Class Question #3#

IMPORTANT NOTE:#

range#

range Examples#

Class Question #4#

continue#

continue examples#

Class Question #5#

break#

break examples#

Class Question #6#

Class Question #7#

Class Question #8#

Dictionaries: Indexing & Looping#

Code Style: Loops#

Loops Practice#

Loops Practice #1#

Loops Practice #2#

Loops Practice #3#

`while` Loops#

`while` Loop Example I#

`while` Loop Example II#

`for` Loops#

`range`#

`range` Examples#

`continue`#

`continue` examples#

`break`#

`break` examples#