Crash Course in Python Part 2: Flow Control, Loops, Functions, and Generators#

Flow Control#

There comes situations in real life when we need to make some decisions and based on these decisions, we decide what should we do next.

Similar situations arise in programming also where we need to make some decisions and based on these decisions we will execute the next block of code. Decision-making statements in programming languages decide the direction of the flow of program execution.

If statement#

  • It is used to decide whether a certain statement or block of statements will be executed or not

if condition:
   # Statements to execute if
   # condition is true

  • The condition after evaluation will be either true or false

  • If the statement accepts boolean values – if the value is true then it will execute the block of statements below it otherwise not.

  • Python uses indentation to identify a block. So the block under an if statement will be identified as shown in the below example:

if condition:
   statement1
statement2

# Here if the condition is true, if block
# will consider only statement1 to be inside
# its block.

Example if Statement#

# python program to illustrate If statement

i = 10

if i > 15:
    print("10 is less than 15")
print("I am Not in if")

I am Not in if

If-else#

The if statement alone tells us that if a condition is true it will execute a block of statements and if the condition is false it won’t. But what if we want to do something else if the condition is false? Here comes the else statement. We can use the else statement with the if statement to execute a block of code when the condition is false.

if (condition):
    # Executes this block if
    # condition is true
else:
    # Executes this block if
    # condition is false

Example of an if-else statement#

i = 20
if i < 15:
    print("i is smaller than 15")
    print("i'm in if Block")
else:
    print("i is greater than 15")
    print("i'm in else Block")
print("i'm not in if and not in else Block")

i is greater than 15
i'm in else Block
i'm not in if and not in else Block

If-nested#

If statements can be nested inside other if statements

Example Nested if statements#

i = 10
if i == 10:

    #  First if statement
    if i < 15:
        print("i is smaller than 15")

    # Nested - if statement
    # Will only be executed if statement above
    # it is true
    if i < 12:
        print("i is smaller than 12 too")
    else:
        print("i is greater than 15")

i is smaller than 15
i is smaller than 12 too

if-elif-else ladder#

  • It is possible to have a flow of if-else-statements

i = 20
if i == 10:
    print("i is 10")
elif i == 15:
    print("i is 15")
elif i == 20:
    print("i is 20")
else:
    print("i is not present")

i is 20

For Loops#

  • Python For loop is used for sequential traversal i.e. it is used for iterating over an iterable like String, Tuple, List, Set or Dictionary.

for var in iterable:
    # statements

Example: Using for Loops in a Python List#

# Python program to illustrate
# Iterating over a list
l = ["Drexel", "is", "Engineering"]
for i in l:
    print(i)

Drexel
is
Engineering

Example: Using for Loops in Dictionaries#

# Iterating over dictionary
print("Dictionary Iteration")
d = dict()
d["xyz"] = 123
d["abc"] = 345
for i in d:
    print(f"{i} {d[i]}")

Dictionary Iteration
xyz 123
abc 345

Example: Using for Loops in Strings#

# Iterating over a String
print("String Iteration")
s = "Drexel"
for i in s:
    print(i)

String Iteration
D
r
e
x
e
l

Loops Controlled with Range#

  • The range() function can be used to execute a loop a certain number of times

In-class exercise:#

Use the range function to print the values from 1 to 9

# Python Program to
# show range() basics

# printing a number
for i in range(1, 10):
    print(i, end=" ")

1 2 3 4 5 6 7 8 9

In class exercise#

Compute the sum of the numbers from 1-9 using the range() function and a loop

# performing sum of first 10 numbers
sum = 0
for i in range(1, 10):
    sum += i
print("\nSum of first 10 numbers :", sum)

Sum of first 10 numbers : 45

Example: Loops with if statements#

# Python program to demonstrate
# for-else loop

for i in range(1, 4):
    print(i)
else:  # Executed because no break in for
    print("No Break")

1
2
3
No Break

Loop Control Statements#

  • Loop control statements change execution from its normal sequence

  • When execution leaves a scope, all automatic objects that were created in that scope are destroyed

Continue Statement#

  • Returns the control to the beginning of the loop

Example: Use of Continue Statement#
# Prints all letters except 'e' and 'g'
for letter in "Drexel Engineering":
    if letter == "e" or letter == "g":
        continue
    print("Current Letter :", letter)

Current Letter : D
Current Letter : r
Current Letter : x
Current Letter : l
Current Letter :  
Current Letter : E
Current Letter : n
Current Letter : i
Current Letter : n
Current Letter : r
Current Letter : i
Current Letter : n

Break Statement#

  • A control statement to bring the process out of the loop

Example use of Break Statement#
for letter in "Drexel Engineering":

    # break the loop as soon it sees 'e'
    # or 's'
    if letter == "e" or letter == "s":
        break

print("Current Letter :", letter)

Current Letter : e

Pass Statement#

  • This is used if there is no code to execute

  • pass is different than continue as continue causes the loop to restart, pass just the loop.

  • pass is usually just used as a future placeholder

Example using Pass Statement#
# An empty loop
for letter in "Drexel Engineering":
    pass
print("Last Letter :", letter)

Last Letter : g
Example using Functions#
# Explicit function
def digitSum(n):
    dsum = 0
    for ele in str(n):
        dsum += int(ele)
    return dsum


# Initializing list
List = [367, 111, 562, 945, 6726, 873]

# Using the function on odd elements of the list
newList = [digitSum(i) for i in List if i & 1]

# Displaying new list
print(newList)

[16, 3, 18, 18]

While loops#

Python While Loop is used to execute a block of statements repeatedly until a given condition is satisfied. And when the condition becomes false, the line immediately after the loop in the program is executed.

while expression:
    statement(s)

Example While Loop#

# Python program to illustrate
# while loop
count = 0
while count < 3:
    count = count + 1
    print("Hello Drexel Dragon")

Hello Drexel Dragon
Hello Drexel Dragon
Hello Drexel Dragon

Example While Loop with a List#

# checks if list still
# contains any element
a = [1, 2, 3, 4]

while a:
    print(a.pop())

4
3
2
1

Example: While Loops with Control Statements#

# Prints all letters except 'e' and ' '
i = 0
a = "Drexel Dragons"

while i < len(a):
    if a[i] == "e" or a[i] == " ":
        i += 1
        continue

    print("Current Letter :", a[i])
    i += 1

Current Letter : D
Current Letter : r
Current Letter : x
Current Letter : l
Current Letter : D
Current Letter : r
Current Letter : a
Current Letter : g
Current Letter : o
Current Letter : n
Current Letter : s

Example: While Loop with a break statement#

# Breaks the loop as soon as it sees a 'e' and ' '
i = 0
a = "Drexel Dragons"

while i < len(a):
    if a[i] == "e" or a[i] == " ":
        i += 1
        break

    print("Current Letter :", a[i])
    i += 1

Current Letter : D
Current Letter : r

Example: While Loop with else Statement#

# Python program to demonstrate
# while-else loop

i = 0
while i < 4:
    i += 1
    print(i)
else:  # Executed because no break in for
    print("No Break")

1
2
3
4
No Break

i = 0
while i < 4:
    i += 1
    print(i)
    break
else:  # Not executed as there is a break
    print("No Break")

1

Functions#

  • An expression that takes some inputs \(\rightarrow\) does something \(\rightarrow\) returns an output

A very simple function#

  • Let’s make a function that squares a number

def square(x):
    y = x**2
    return y

Let’s test the function

print(square(10))

100

Let’s make a function that subtracts two values

def subtract(x, y):
    z = x - y
    return z

print(subtract(10, 5))

5

Importing Modules#

  • Python contains some basic functions but its power comes from the ability to import, create, share, and co-develop packages

  • Many modules are available that can be imported into python for use before you write your own code, make sure it does not already exist

  • If you think someone else might have built what you want … They probably have

Note

** Installing Packages ** It is very easy to install Python packages. Most packages that are well developed are listed on the Python Package Index (PyPI). Most packages can be installed using pip install <package_name>

# Imports a built in package random
import random

# Imports a built in function randrange from package random
from random import randrange

# instantiates a function
prob = random.random()

# prints the output of a function
print(prob)

0.37668223102913545

# two ways to instantiate the same function
diceThrow = random.randrange(1, 7)
diceThrow = randrange(1, 7)

# prints the output
print(diceThrow)

5

Iterators#

  • An iterator in python is an object that is used to iterate over iterable objects like lists, tuples, dicts, and sets

  • The iterator object is initialized using the iter() method

  • It uses the next() method for iteration

  • Iterators are used a lot in machine learning

  1. __iter(iterable)__ method that is called for the initialization of an iterator. This returns an iterator object

  2. __next__ method returns the next value for the iterable.

Example of an Iterator#

# Here is an example of a python inbuilt iterator
# value can be anything which can be iterate
iterable_value = "Drexel Dragons"
iterable_obj = iter(iterable_value)

while True:
    try:
        # Iterate by calling next
        item = next(iterable_obj)
        print(item)
    except StopIteration:
        # exception will happen when iteration will over
        break

D
r
e
x
e
l
 
D
r
a
g
o
n
s

Example: Iterating over Different Data Structures#

# Sample built-in iterators

# Iterating over a list
print("List Iteration")
l = ["Drexel", "for", "Engineering"]
for i in l:
    print(i)

List Iteration
Drexel
for
Engineering

# Iterating over a tuple (immutable)
print("Tuple Iteration")
t = ("Drexel", "for", "Engineering")
for i in t:
    print(i)

Tuple Iteration
Drexel
for
Engineering

# Iterating over a String
print("String Iteration")
s = "Drexel"
for i in s:
    print(i)

String Iteration
D
r
e
x
e
l

# Iterating over dictionary
print("Dictionary Iteration")
d = dict()
d["xyz"] = 123
d["abc"] = 345
for i in d:
    print("%s  %d" % (i, d[i]))

Dictionary Iteration
xyz  123
abc  345

Return and Yield Statements#

Return Statement#

The return statement immediately terminates a function execution and sends the return value back to the caller code

Example: Return Statement#

def return_42(number):
    if number == 42:
        return 42
    print("The number is not 42")


print(return_42(42))
print("Notice that the second print command did not show")

print("\n ")
return_42(24)

42
Notice that the second print command did not show

 
The number is not 42

Yield Statement#

  • The yield statement suspends a function’s execution and sends a value back to the caller, but retains enough state to enable the function to resume where it left off

  • When the function resumes, it continues execution immediately after the last yield run. This allows its code to produce a series of values over time, rather than computing them at once and sending them back like a list

  • The yield statement is an intrinsic generator

  • yield is useful when we want to iterate over a sequence but we do not want to save all the data in memory

Example: Yield Statement in a for loop#

# A Simple Python program to demonstrate working
# of yield

# A generator function that yields 1 for the first time,
# 2 second time and 3 third time


def simpleGeneratorFun():
    yield 1
    yield 2
    yield 3


# Driver code to check above generator function
for value in simpleGeneratorFun():
    print(value)

1
2
3

Generators#

  • A generator-function is defined like a normal function, but whenever it needs to generate a value, it does so with the yield keyword rather than return.

  • If the body of a def contains yield, the function automatically becomes a generator function.

Example of a Very Simple Generator#

# A generator function that yields 1 for first time,
# 2 second time and 3 third time
def simpleGeneratorFun():
    yield 1
    yield 2
    yield 3


# Driver code to check above generator function
for value in simpleGeneratorFun():
    print(value)

1
2
3

Example: A Python program to demonstrate the use of generator object with next()#

# A generator function
def simpleGeneratorFun():
    yield 1
    yield 2
    yield 3


# x is a generator object
x = simpleGeneratorFun()

# Iterating over the generator object using next
print(x.__next__())
print(x.__next__())
print(x.__next__())

1
2
3

In Class Example: A Python program to generate squares from 1 to 100 using yield and therefore generator#

Make a function nextSquare() that uses a while loop to print all of the squares (using yield) that are less than 100.

  • Try to think about a way to make it the most efficient

def nextSquare():
    # Your Code here

next_square = nextSquare()

for i in range(100):
    try:
        print() # Add the code the calls the iterator
    except StopIteration:
        break

  Cell In [1], line 4
    next_square = nextSquare()
    ^
IndentationError: expected an indented block after function definition on line 1

def nextSquare():
    i = 1

    # An Infinite loop to generate squares
    while True:
        if i * i > 100:
            break

        yield i * i
        i += 1  # Next execution resumes
        # from this point


next_square = nextSquare()

for i in range(100):
    try:
        print(next_square.__next__())
    except StopIteration:
        break

1
4
9
16
25
36
49
64
81
100